CHAPTER VI.
CHEMICAL AND MECHANICAL TESTS.
These tests may be necessary to confirm an optical determination or to assist in the differentiation of the closely related species of a group, as for example, the different plagioclases in the feldspar group. They may also be useful in the case of opaque substances.
The ordinary chemical methods employed in mineral analyses are often not applicable, on account of the minute size of the mineral under investigation and the lack of sharpness in the reactions. Those methods[132] are to be preferred which produce crystallizations independent of the relative proportions of the materials taking part in the reaction and also of the physical conditions invoked.
The tests can be made either on the crystal in a rock section or on the isolated crystal or fragment, the latter method being preferable when possible.
CHEMICAL TESTS MADE ON CRYSTAL IN SECTION.
The part of the section to be tested must be prepared by thorough washing with alcohol and benzole to remove all traces of balsam. If the section is covered, the cover-glass can be cut across with a diamond, and, after heating, the desired portion removed with a knife edge. Any portion of a section can be isolated by surrounding it with a rim of viscous balsam or by putting on a new cover-glass, in which a small hole has been made.[133] This hole can be accurately adjusted over the special portion of the section and the balsam removed by alcohol and benzole.
The treatment of rock sections with ordinary acids, such as hydrochloric, may show the presence of easily soluble minerals[134] and carbonates, or distinguish silicates that are soluble with jelly, or produce etched figures on the minerals.
_Test for Carbonates._ When only present in very minute grains the test can be made as follows: Cover the rock section with a drop of water and a cover-glass, then allow a drop of acid to slowly diffuse through the water film. The glass cover will prevent the escape of the gas bubbles[135] which will thus surely be detected.
If necessary the section can be warmed by heating the projecting part of a suitable stand, placed under the section on the stage of the microscope.
_Test for Gelatinizing Silica._ Cover the carefully cleansed section with a little dilute acid (commonly hydrochloric) and let stand. If too much acid is used the resultant gelatin will spread over the whole section and not appear simply on the gelatinizing silicate. Warm the section, if necessary, and finally rinse off the remaining acid thoroughly with water. Do not allow the action of the acid to continue too long, as it is desirable to obtain only a very thin film of gelatinous silica over the minerals attacked, so that the optical tests can still be made on these minerals. If after the first trial the action has not been pronounced enough, the test should be repeated.
The transparent film of gelatinous silica is made more visible by covering the section with a drop of water, containing a dilute solution of fuchsine. After standing for some time the section should be washed, when only those portions covered by the gelatinous film will show the color stain.
ETCHED FIGURES.[136]
The results of etching tests can not be regarded as very satisfactory in the case of sections of minerals in rock sections, on account of doubt as to the crystallographic orientation of these sections.
The symmetry of the etched figures depends essentially on the relation of the crystal faces on which they are obtained to the planes of symmetry.
The forms of the etched figures differ on the same face of a mineral, depending on the reagent used, but their degree of symmetry is independent of the reagent or its degree of concentration. The sharpest figures are produced on crystal and cleavage faces, the figures being less perfect on artificially prepared faces, even when polished. The etching tests may, however, be tried to prove the presence of twinning, or to distinguish between minerals of similar appearance but belonging to different systems.[137]
Various acids or alkalies are used to produce the etched figures, depending on the mineral to be tested. Different factors influence the formation of good etched figures, and that method must be used which seems to give most satisfactory results in the given case. The action of the reagent should be sufficiently pronounced to develop clearly the etched figures; at the same time the tests must be stopped before the solvent action has been too powerful.[138] After treatment for etching the section should be thoroughly washed and examined in some fluid of weak refraction (with _n_ lower than that of the crystal section), such as water or air.[139] The objective, of course, must be focused on the surface of the section.
HEATING SECTIONS TO REDNESS.[140]
The part of the section to be tested must be removed from the object glass, carefully cleansed of balsam, and held on platinum foil in the oxidizing blowpipe flame. After the test the fragment used may be remounted in Canada balsam for study.
As a result of heating:
Colorless, hydrous minerals (zeolites and chlorites) become cloudy in appearance.
Colorless silicates, containing protoxide of iron (as olivine, or faintly colored pyroxene or amphibole), become red or reddish-brown.
Colored minerals may change their color, chloritic substances becoming brown or black if heated enough.
Hornblende always becomes pleochroic, and olivine sometimes becomes so.
Members of the sodalite group may be turned blue, if not already of that color.
The dichroism (yellow to blue) of almost colorless iolite may be developed.
Carbonaceous particles may be distinguished from the iron oxides by being consumed.[141]
METHODS OF ISOLATING CRYSTALS OR MINERAL FRAGMENTS FOR TESTING.[142]
For the application of these methods the rock or aggregate of minerals should be reduced to homogeneous[143] grains of uniform size (preferably crystals or cleavages) and not to powder. This is best done by pounding in a metal mortar, avoiding all grinding motion.
The separations required may be made by specific gravity solutions or magnetic methods, alone or combined; and in some cases may be assisted by chemical action.
When other methods fail it may be necessary to separate single grains from a mixture by hand. A grooved piece of plate-glass, passed beneath the objective of the microscope, will be found useful. The desired grains in this groove can be picked out by means of a piece of fine waxed thread or a fine pointed stick moistened at the end.
The _hardness_ of the homogeneous grains may be obtained by pressing them firmly into the end of a lead stamp or holder and trying the effect of scratching upon the faces of minerals of known hardness.
_Specific Gravity Separation._[144] Accomplished by use of fluids of different specific gravity. These fluids can be made specifically lighter by dilution, and hence the fragments will fall to the bottom in order of decreasing density. Dilution of the heavy solutions to any specific gravity may be affected empirically until the solution will just suspend a fragment of a mineral having the desired specific gravity; or the exact specific gravity of the solution may be determined by the Westphal balance.
The following _indicators_ may be employed to determine the limits of the specific gravity of the solution to be used for separation purposes (V. Goldschmidt):
NO. NAME. LOCALITY. SP. GR. 1. Sulphur, Girgenti, 2.070 2. Hyalite, Waltsch, 2.160 3. Opal, Scheiba, 2.212 4. Natrolite, Brevig, 2.246 5. Pitchstone, Meissen, 2.284 6. Obsidian, Lipari, 2.362 7. Pearlite, Hungary, 2.397 8. Leucite, Vesuvius, 2.465 9. Adularia, St. Gotthard, 2.570 10. Elæolite, Brevig, 2.617 11. Quartz, Middleville, 2.650 12. Labradorite, Labrador, 2.689 13. Calcite, Rabenstein, 2.715 14. Dolomite, Muhrwinkel, 2.733 15. Dolomite, Rauris, 2.868 16. Prehnite, Kilpatrick, 2.916 17. Aragonite, Bilin, 2.933 18. Actinolite, Zillerthal, 3.020 19. Andalusite, Bodenmais, 3.125 20. Apatite, Ehrenfriedersdorf, 3.180
Among the heavy solutions employed may be mentioned:
Thoulet’s solution of potassium-mercuric iodide (KI: HgI_{2} = 1: 1.24), maximum specific gravity 3.196. Klein’s solution of cadmium borotungstate (2H_{2}O, 2CdO, B_{2}O_{3}, 9WoO_{3} + 16H_{2}O), maximum specific gravity 3.6. These two solutions can be mixed with water in any proportion without being decomposed. Solution of barium mercuric iodide, maximum specific gravity, 3.588, cannot be diluted with water. Methylene iodide (CH_{2}I_{2}), specific gravity 3.3243 at 16° C. varying with the temperature, can be diluted with benzole but not with water.
Nitrates of silver and thallium[145] (AgNO_{3}: TlNO_{3} = 1.1) fuse at about 75° C. to a clear mobile liquid with specific gravity over 4.5. Can be mixed while melted with water in all proportions; but cannot be used for separation of sulphides, as these nitrates are attacked by them.
Possible chemical action between the minerals and heavy solutions must not be overlooked in this method of separation.
The funnel-shaped apparatus for these separations must be so arranged, with stop-cocks, etc., that the heavier material collected at the bottom can be easily drawn off or removed at any stage of dilution.
These separations, for various reasons, are not always complete, but the best results are obtained when the processes are repeated several times.
_Electro-magnetic Separation._ All iron-bearing minerals may be separated from those free from iron by an electro-magnet.
The factors influencing the attraction of a mineral by an electro-magnet are not definitely known, and do not seem to depend only on the percentage of iron.
Minerals, such as amphibole, pyroxene, epidote, olivine and garnet (containing iron), may often be separated by an electro-magnet by regulating its magnetic intensity.[146]
_Separation by Chemical Means._ Very many different methods may be used, depending on the nature of the work to be accomplished; but they are generally only reliable in the hands of a good chemist. The material should be in the state of fine powder.
As an example may be mentioned the treatment with pure concentrated HFl, by which the minerals of a rock are attacked in a certain sequence, the feldspars and related minerals first, then the quartz and finally the ferro-magnesium silicates, such as amphibole, pyroxene, olivine, etc.
MICRO-CHEMICAL REACTIONS.
The first requisite is to bring the substance to be investigated into solution. This can be done in the case of non-silicates by the ordinary solvents, while silicates can be decomposed and investigated either by the methods of Borichy or Behrens. Both methods rest upon the recognition of the forms, etc., of artificially produced crystals.
The size of a fragment for testing may vary, according to circumstances, from that of a poppy seed to a pin head. Good results are recorded from fragments of not more than 0.2–0.7 sq. mm.
The substance to be tested is placed on glass, protected by a film of balsam, and covered with a spherical drop of the solvent,[147] which should be allowed to act until all the different elements composing the sample are in solution (in the case of a very small fragment until it has all dissolved). Transfer the solution to another protected object glass, and, after evaporation, the crystallizations characteristic of the different elements will be seen.
If the evaporation is too rapid and the crystallizations incomplete, the residue should be redissolved in water, or a very dilute solution of the solvent employed, transferred to a fresh glass and allowed to recrystallize.
Borichy’s Method.[148] (Hydrofluosilicic Acid.)
This method has the advantage of simplicity of manipulation and relative distinctness in results; but on the other hand these results are only obtained after several hours, and the temperature has an influence on the crystalline forms obtained. It is well to make the tests in a temperature of about 15° C.
Put a spherical drop of pure hydrofluosilicic acid[149] on the fragment and leave it for some hours in damp air until the action has been sufficient, then transfer it to a dry air bell-glass and allow evaporation and crystallization to take place.
For the microscopic examination the objective (200–300 diams. best for these observations) can be protected with glycerine and a mica disc or thin cover-glass, or the drop can be all evaporated and the crystals covered with liquid balsam and a cover-glass.
Crystallizations Obtained by Borichy’s Method.
_Potassium._ From hydrofluosilicic solutions,[150] isotropic, colorless crystals of K_{2}SiFl_{6}, in cubes, octahedra or combinations of these forms with the rhombic dodecahedron. Apparently orthorhombic crystals may form from acid solutions and at a low temperature, but if these crystals are dissolved in hot water and recrystallized they will assume the normal forms.
Platinic chloride will produce under proper conditions sharp, yellow octahedra of K_{2}PtCl_{6}.
[Illustration:
FIG. 78.—Fluosilicate of potassium.[151] ]
[Illustration:
FIG. 79.—Fluosilicate of sodium.[151] ]
_Sodium._ From hydrofluosilicic solutions, colorless, very weakly doubly refracting, hexagonal crystals of Na_{2}SiFl_{6}, which are generally longer the higher the percentage of sodium in the solution. This test is very certain even for small amounts.
[Illustration:
FIG. 80.—Fluosilicate of calcium.[151] ]
[Illustration:
FIG. 81.—Fluosilicate of magnesium.[151] ]
_Calcium._ From hydrofluosilicic solutions, monoclinic crystals of CaSiFl_{6} + 2H_{2}O of various forms, generally spindle-shaped, with not very strong double refraction. The crystals have seldom straight-edged boundaries and are often grouped in rosettes. The addition of dilute H_{2}SO_{4} decomposes the crystals, recrystallization yielding long prismatic crystals of gypsum (distinction from strontium).
Treatment with HFl and dilute H_{2}SO_{4} (in excess), producing on evaporation characteristic crystals of gypsum, furnishes a very delicate test for small percentages of calcium.
_Magnesium._ From hydrofluosilicic solutions, rhombohedral crystals of MgSiFl_{6} + 6H_{2}O with plane faces and sharp edges. The crystals are colorless and strongly doubly refracting with positive optical character, Fig. 81.
The formation of struvite crystals (NH_{4}MgPO_{4} + 6H_{2}O), of coffin-like forms, is very characteristic and takes place from very dilute solutions (rendered alkaline) on the addition of a grain of salt of phosphorus or a drop of sodium phosphate.
_Iron._ From hydrofluosilicic solutions, crystals of FeSiFl_{6} + 6H_{2}O, which are isomorphous with those of magnesium salts, with the same optical characters. They may be differentiated by moistening with potassium ferrocyanide or ammonium sulphide, in the first case by turning blue, in the second case black.
_Aluminium._ From hydrofluosilicic solutions, not satisfactory on account of the gelatinous formation.
When the gelatinous formation is obtained by the action of hydrofluoric acid on an _aluminous silicate_, the staining test can be used to distinguish between fine grains of feldspar and quartz or iolite and quartz.
Behrens’ Method.[152] (Hydrofluoric and Sulphuric Acids.)
This method depends on common reactions that can be made rapidly, but has the disadvantage of being rather complicated and requiring delicate manipulation.
The tests are best made upon about ½ mg. of powder with HFl (pure and fuming). As soon as the fluorides begin to dry treat with dilute H_{2}SO_{4} and warm until white fumes of SO_{3} appear. In this way the HFl and SiFl_{4} are driven off and the sulphates are left. This part of the test can conveniently be made on a piece of platinum foil. Add excess of water and concentrate.
Transfer a drop to a clean object glass and, while still liquid, examine it with the microscope. Do not use a cover-glass over the drop.
Crystallizations Obtained by Behrens’ Method.
_Potassium._ Add a little platinic chloride when octahedral crystals of K_{2}PtCl_{6} (size .18 to .30 mm.) will appear, which are clear, bright yellow in color with strong refraction.
[Illustration:
FIG. 82.—Potassium platinic chloride.[153] ]
[Illustration:
FIG. 83.—Sulphate of calcium (gypsum).[153] ]
_Sodium._ Use sulphate of cerium and allow a small amount of this reagent to act through a capillary pipette upon a drop of the solution. Very small aggregates of brown crystals (size .02 mm.) of the double sulphate of cerium and sodium are formed, which are clearly visible with 600 diams. If potassium is present the double sulphate of that alkali will appear in larger, grayish grains (size .05 to .06 mm.). An excess of H_{2}SO_{4} is to be avoided.
_Calcium._ After a few minutes little gypsum crystals (CaSO_{4} + 2H_{2}O) will appear. In strongly acid solutions the thin acicular crystals will be grouped in bushes or stars; in neutral solutions the crystals will have the normal shape of selenite crystals or form swallow-tailed twins.
_Magnesium._ Use salt of phosphorus dissolved in water and allow it to mix with the solution (to which has been added ammonium chloride and ammonia) through a capillary pipette. From a solution containing more than 5 per cent. of magnesium are first deposited X-shaped skeletons and rudimentary crystals of Mg.NH_{4}.PO_{4} + 6H_{2}O. If the solution is more dilute beautiful, sharp, hemimorphic crystals (.10 to .20 mm. in size) of the orthorhombic system will appear. These crystals often resemble the roof of a house. The formation of the crystals is assisted by heat. Iron and manganese phosphates yield crystals of the same type, but the iron is separated on the addition of ammonia.
_Aluminium._ Use chloride of cæsium. Take a drop of the solution, with excess of H_{2}SO_{4} driven off, and touch it with a platinum wire that has been dipped in the melted chloride of cæsium. Large crystals (.40-.90 mm. in size) of cæsium alum will form, which are octahedral and cubo-octahedral in shape. Iron does not interfere, as its crystallization would take place much more slowly. The solution should not be too concentrated.
[Illustration:
FIG. 84.—Magnesiumammonium phosphate.[154] ]
[Illustration:
FIG. 85.—Cæsium alum.[154] ]
Special Tests.
Distinction between haüynite (contains CaSO_{4}) and noselite (contains Na_{2}SO_{4}). Treat with HCl and on evaporation the characteristic crystals of gypsum will be seen if the mineral is haüynite. Dilute acid should be used and as low a temperature maintained as possible, otherwise crystals of anhydrite would form instead of gypsum.
Recognition of apatite by test for phosphorus. Treat with a drop of ammonium molybdate dissolved in HNO_{3}. After complete action remove the solution to a clean object glass, when after slight warming a large number of very small yellow crystals (rhombic dodecahedral in shape) will form. The test may be used to distinguish this mineral from nephelite, melilite and natrolite. In the presence of soluble silica evaporate to render it insoluble and treat again with HNO_{3} and the reagent.
Other micro-chemical tests are not mentioned for the reason that in elaborate chemical investigations of sections or isolated fragments recourse should be made to the most complete publications on the subject.
APPENDIX.
BRIEF SCHEME OF CLASSIFICATION INTO SYSTEMS BY OPTICAL DETERMINATIONS.
▄HOMOGENEOUS.▄ The whole substance shows the same optical character, except in the case of twin crystals when the different portions of the twin are affected differently.
▄Isotropic▄ All sections of the substance remain dark during a complete rotation between crossed nicols, and no interference figure is produced by convergent light.
Amorphous Absence of crystalline form or cleavage.
Isometric Presence of crystalline form or cleavage.
▄Anisotropic.▄ Sections generally show some interference color and extinguish four times, at 90° apart, during complete rotation.
_Uniaxial._ Determined by character of interference figures obtained by convergent light from sections which remain dark or nearly so during complete rotation.
All sections show parallel or symmetrical extinction.
Tetragonal Sections giving interference figures are four- or eight-sided, or show rectangular cleavage.
Hexagonal Sections giving interference figures are three-, six- or nine-sided, or show cleavage lines intersecting at 60°.
_Biaxial_ Determined by character of interference figures obtained by convergent light.
Orthorhombic Extinction is parallel or symmetrical in all sections parallel to _ă_, [_=b_] and _ć_. Color distribution is symmetrical to two lines and to the central point, see p. 48.
Monoclinic Extinction is only parallel or symmetrical in sections parallel to the ortho axis [_=b_]; all other sections show extinction angles. Color distribution is only symmetrical to one line or to the central point, see p. 48.
Triclinic Extinction angles in all sections, although in some minerals these angles may be very small. No symmetry in color distribution, see p. 48.
▄AGGREGATE.▄ Not homogeneous, but made up of an aggregation of individuals, all extinguishing at different times.
Double Refraction (maximum).[155]
0.287 Rutile
0.179 Dolomite
0.172 Calcite
0.141 Titanite
0.090 Titanite
0.072 Hornblende (basaltic)
0.062 Zircon
0.058 Biotite
0.050 Talc
0.050 Ægirite
0.041 Muscovite
0.037 Epidote
0.036 Chrysolite (Olivine)
0.036 Scapolite (Meionite)
0.034 Tourmaline
0.034 Phlogopite
0.030 Allanite (Orthite)
0.029 Diopside
0.027 Actinolite
0.027 Tremolite
0.024 Hornblende (common)
0.024 Diallage
0.024 Anthophyllite
0.023 Augite
0.021 Sillimanite (Fibrolite)
0.018 Glaucophane
0.017 Tourmaline (precious)
0.016 Cyanite (Disthene)
0.013 Hypersthene
0.013 Scapolite (Marialite)
0.013 Anorthite
0.012 Natrolite
0.011 Chlorite (Clinochlore)
0.011 Andalusite
0.011 Topaz
0.010 Staurolite
0.010 Gypsum
0.010 Enstatite
0.010 Serpentine
0.009 Corundum
0.009 Iolite (Cordierite)
0.009 Quartz
0.008 Topaz
0.008 Kaolin
0.008 Labradorite, Ab_{1}An_{1}
0.008 Oligoclase, Ab_{4}An_{1}
0.008 Albite
0.007 Orthoclase
0.007 Microcline
0.006 Vesuvianite
0.005 Zoisite
0.004 Nephelite (Elæolite)
0.003 Apatite
0.003 Melilite
0.002 Tridymite
0.002 Leucite
0.002 Allanite (Orthite)
0.001 Chlorite (Penninite)
0.001 Vesuvianite
Indices of Refraction (mean).[155]
2.712 Rutile
2.38 Perofskite
2.00 Spinel (Chrome)
1.963 Titanite
1.95 Zircon
1.920 Titanite
1.856 Garnet (Melanite)
1.792 Ægirite
1.78 Allanite (Orthite)
1.78 Garnet (Almandite)
1.766 Corundum
1.751 Epidote
1.75 Garnet (Pyrope)
1.741 Staurolite
1.723 Hypersthene
1.72 Spinel
1.720 Zoisite
1.720 Cyanite (Disthene)
1.719 Hornblende (basaltic)
1.715 Vesuvianite
1.711 Augite
1.699 Zoisite
1.697 Diopside
1.688 Diallage
1.675 Chrysolite (Olivine)
1.665 Enstatite
1.664 Sillimanite (Fibrolite)
1.674 Tourmaline
1.644 Anthophyllite
1.64 Hornblende (common)
1.637 Andalusite
1.635 Apatite
1.632 Glaucophane
1.632 Topaz
1.630 Melilite
1.633 Tourmaline (precious)
1.622 Dolomite
1.621 Actinolite
1.621 Tremolite
1.618 Biotite
1.608 Topaz
1.588 Chlorite (Clinochlore)
1.587 Muscovite
1.582 Anorthite
1.576 Chlorite (Penninite)
1.572 Talc
1.584 Scapolite (Meionite)
1.564 Phlogopite
1.56 Serpentine
1.559 Labradorite, Ab_{1}An_{1}
1.55 Kaolin
1.547 Quartz
1.551 Scapolite (Marialite)
1.601 Calcite
1.541 Oligoclase, Ab_{4}An_{1}
▄1.54▄ + ▄Canada balsam▄
1.539 Nephelite (Elæolite)
1.539 Iolite (Cordierite)
1.535 Albite
1.525 Gypsum
1.525 Microcline
1.523 Orthoclase
1.509 Leucite
1.503 Haüynite
1.488 Analcite
1.483 Natrolite
1.483 Sodalite
1.477 Tridymite
1.46 Opal
[Illustration:
Diagram showing relation between strength of Double Refraction, Interference Colors and Thickness of Section.[156] ]
ORDER OF CONSOLIDATION OF THE CONSTITUENT MINERALS IN PLUTONIC ROCKS.
“There is in plutonic rocks a normal order of consolidation for the several constituents, which holds good with a high degree of generality. It is in the main, as pointed out by Rosenbusch, a law of ‘decreasing basicity.’ The order is briefly as follows:
“1. Minor accessories (apatite, zircon, sphene, garnet, etc.) and iron ores.
“2. Ferro-magnesian minerals—olivine, rhombic pyroxenes, augite, ægirine, hornblende, biotite, muscovite.
“3. Felspathic minerals—plagioclase felspars (in order from anorthite to albite), orthoclase (and anorthoclase).
“4. Quartz, and finally microcline.
“In most rocks such minerals as are present follow the above order. The most important exceptions are the intergrowth of orthoclase and quartz and the crystallization of quartz in advance of orthoclase in some acid rocks, and the rather variable relations between groups 2 and 3 in some more basic rocks. The order laid down applies in general to parallel intergrowths of allied minerals; thus when augite is intergrown with ægirine or hornblende the former mineral forms the kernel of the complex crystal and the latter the outer shell; when a plagioclase crystal consists of successive layers of different compositions the layers become progressively more acid from the center to the margin.
“Certain constituents having variable relations are omitted from the foregoing list. Thus nepheline (elæolite) and sodalite belong to group 3, but may crystallize out either before or after the feldspar.”[157]
OPTICAL SCHEME.
INTRODUCTION.
The scheme is designed to furnish the student with a practical method of recognizing the common minerals in rock sections.
The arrangement followed has been to group minerals having general optical characters in common, at the same time giving their specific characters so as to make it possible to distinguish one from another. In each rectangle the minerals are arranged in order of their indices of refraction.
A tabulation of the minerals, with a list of optical characters appended, is of aid to the skilled investigator, but of very little assistance to the beginner.
The more common minerals, or those which are important petrographically, are printed in heavy-faced type; the minerals of less importance in small capitals.
ABBREVIATIONS AND CONVENTIONS USED.
A. = Amorphous.
I. = Isometric system.
T. = Tetragonal system.
O. = Orthorhombic system.
M. = Monoclinic system.
Tri. = Triclinic system.
H. = Hexagonal system.
M(H). = Monoclinic, with hexagonal form or characters, as in the case of biotite.
⟂ = At right angles to.
El. = Elongation.
Ex. = Extinction.
∥ Ex. = Parallel extinction, as when the crystals extinguish parallel to cleavage lines or crystal edges. Extinction which is symmetrical to intersecting cleavage lines is also included under this term.
The mean refractive indices are printed in heavy-faced type.
The term “grains” is used to describe not only minerals which occur in typically granular form, but also those which have coarser allotriomorphic form, as elæolite and sodalite in plutonic rocks, such as syenite, etc.
GENERAL RULES FOR USE OF SCHEME.
The division of the scheme into two vertical columns is based on the values of the mean refractive indices, as determined by the “relief” and appearance of the surface. When the refractive index is above 1.60, the relief is fairly well to distinctly marked and the surface rough to very rough, depending on the value of the index.
Most of the rock-forming minerals with indices below 1.60 show no relief and a smooth surface, except in the case of a few of the rarer minerals (mostly isometric), which have very low indices and hence rough surface.
Mistakes may easily be made in the case of minerals near the limit; but practice and the use of the Becke test should soon make possible the classification into the two groups suggested by the scheme, and the appended descriptions will help to check errors.
When an unknown mineral lies adjacent to one that is known, use the Becke test for obtaining the relative refractive index of the unknown mineral; focus sharply on the line of contact between the known and unknown minerals,[158] then raise the objective slightly and the “bright line” will appear on the side of the mineral having the higher index. The method of Schrœder van der Kolk may also be employed, see p. 22.
The horizontal divisions of the scheme depend on the relative strength of the double refraction based on the observed interference colors. These colors to be of use in classification must be correctly recognized. The lower and middle colors of the 1° order, from bluish-gray through white to yellow, are easily known. The bright red, blue, green, etc., colors of the 1°, 2° and 3° orders can also be differentiated without trouble from the very high order colors (4° and above), which are essentially white in tone with no decided color tint.
When confusion arises, the exact order of the color can be determined by a quartz wedge, as given on p. 35. Furthermore, a ¼ undulation mica plate serves to quickly distinguish between the 1° order white and the practically colorless, high order tint of calcite, titanite, etc.; as after the insertion of the test-plate the 1° order white suffers a marked change in color, while the very high order (practically white) tint shows no appreciable change.
In the determination of these interference colors care must be given to considerations of orientation and thickness. The section must give the maximum interference color of all the obtainable sections of the mineral in the rock section. Such sections will be parallel to the _ć_ axis in the uniaxial minerals and to the axial plane in the biaxial minerals; and will, therefore, in convergent light never show the emergence of an optic axis or bisectrix. Crystal “form,” cleavage, pleochroism, etc., may at times aid in the selection of these sections. The thickness of the section must also be considered, and it is well in all cases to pick out some known mineral in the section, as quartz, and note its maximum interference color. Knowing how this varies from the color given by the scheme for a section 0.03 mm.[159] in thickness, due allowance can be made for a like variation in the colors given by the other minerals. In the case of minerals with strong absorption the interference colors may not be noticeable on account of the absorption of parts of the light.
Under the subhead of pleochroism, the vibration direction of the ray of a definite color is given and not the direction of transmission of that ray.
The interference figures in convergent light increase in clearness and distinctness with the strength of the double refraction. In uniaxial crystals sections at right angles to the optic axis, _i. e._, sections which remain dark during a revolution between crossed nicols, show the best interference figures. In biaxial crystals the most characteristic interference figures are shown by sections at right angles to the acute bisectrix.
Crystal sections which are too small do not give very satisfactory interference figures with convergent light.[160]
Any scheme, however designed, makes a more or less arbitrary classification of the minerals, and when in doubt it is always safer to look for the mineral on both sides of the scheme line.
The rarer minerals are not included in this scheme, so when the determination of a mineral is uncertain or not positive recourse should be had to more elaborate tables.
INDEX.
MINERAL NAMES IN HEAVY-FACED TYPE.
Abbreviations, ix
Abnormal interference colors, 30
Absorption, 26 directions, 26 tints, 17
▄Acmite▄, 83
▄Actinolite▄, 84
Acute bisectrix, 5
▄Ægirine▄, 83
▄Ægirine-augite▄, 84
Aggregate structure, 38
▄Albite▄, 103
▄Allanite▄, 96
Allotriomorphic, 15
Aluminium, test for, by Borichy’s method, 137 test for, by Behren’s method, 139
Amorphous bodies, 2
Amorphous substances, test for, 26
▄Amphibole▄, 84
▄Analcite▄, 56
Analyzer, 10
▄Andalusite▄, 72
Anhedron, 15
Anisotropic character, test for, 27
Anisotropic crystals, 2
Anomalies, optical, 27
▄Anorthite▄, 103
▄Anorthoclase▄, 113
▄Anthophyllite▄, 88
▄Antigorite▄, 114
▄Apatite▄, 68
Apatite, recognition of, by test for phosphorus, 139
Apparatus for petrographical laboratory, 127
Appendix, 141
▄Arfvedsonite▄, 88
▄Augite▄, 80
Automorphic, 13
Axes of elasticity, 4
Axial angle, 5 determination of, 47
Axial plane, 5
▄Basaltic Hornblende▄, 85
▄Bastite▄, 77
Becke’s method for determining relative values of refractive indices, 19
Behren’s method. (Hydrofluoric and sulphuric acids), 137
Bertrand lens, 11
Biaxial crystals, 4, 5 vibration directions in, 4
Biaxial interference figures, 43
▄Biotite▄, 89
Bisectrix, acute, 5 obtuse, 5
Borichy’s method. (Hydrofluosilicic acid), 135
Broken or strained crystals, 15
▄Bronzite▄, 76
▄Calcite▄, 65
Calcium, test for, by Borichy’s method, 136 test for, by Behren’s method, 138
“Cap” nicol, 10
▄Carbonaceous Matter▄, 61
Carbonaceous particles, test for, 132
Carbonates, test for, 130
Cement, 122 for mounting, 126
Cementing, 121
Centering stage, 10
▄Chalcedony▄, 64
Characters observed by, convergent light, 40 crossed nicols, 26 reflected light, 13 polarized light, 25
Characters of opaque minerals, 13 transparent minerals, 13
Chemical and mechanical tests, 129
Chemical (micro) reactions, 134 separation, 134 tests on crystal in section, 129
▄Chiastolite▄, 72
▄Chlorite group▄, 92
▄Chromite▄, 52
▄Chrysolite▄, 77
▄Chrysotile▄, 114
Circular polarization, 3
Classification into systems by optical determinations, 141
▄Clay▄, 115
Cleaning and finishing sections, 126
Cleaning microscope, 11
Cleavage, 23
▄Clinozoisite▄, 96
Color, 17 diagram (interference), 143 fringes, 48
Condensing lens, 9
Consolidation of minerals in plutonic rocks, order of, 144
Conventions, ix
Convergent light, 40 characters observed by, 40
Convergent and parallel light, résumé of uses of, 49
▄Cordierite▄, 78
Corroded crystals, 15
▄Corundum▄, 62
Crossed nicols, characters observed by, 26
Crossed twinning, 37
Crystallites, 16
Crystallizations obtained by Behren’s method, 137 Borichy’s method, 135
Crystals bounded by planes, 13
Crystals without bounding planes, 15
Cutting, 120
Cutting and grinding machines, 117
▄Cyanite▄, 113
▄Damourite▄, 92
▄Delessite▄, 94
Diagram, showing relation between strength of double refraction, interference colors and thickness of section, 143
▄Diallage▄, 80
Diamond saws, 119
▄Dichroite▄, 78
▄Diopside▄, 80
▄Dipyre▄, 59
Directions of elasticity, ix
Dispersion, 5, 48
▄Disthene▄, 113
▄Dolomite▄, 67
Double image, 2
Double refraction, 2, 27 estimation of strength of, 30 strength of, measured by von Federow mica wedge, 35 table (maximum), 142
Effects produced by crystals on transmitted light, 2
▄Elæolite▄, 69
Elasticity, axes of, 4 directions of, ix
Electric motor for grinding machines, 117
Electro-magnetic separation, 134
Emery, grades of, for grinding, 124
▄Enstatite▄, 75
▄Epidote▄, 94
Etched figures, 130
Extinction, 31 angles, 5, 31 tests for, 32 wavy, 31
Extraordinary ray, 3
Eye-pieces, 11
Faster and slower rays, test for vibration directions of, 33
▄Fayalite▄, 78
▄Feldspar Group▄, 98
Feldspathoids, 69
▄Fibrolite▄, 73
Finishing and cleaning sections, 126
Focusing, 11
Form of crystals, 13
Fracture, 24
▄Garnet▄, 53
Gelatinizing silica, test for, 130
Glass slides, 125
▄Glaucophane▄, 88
▄Graphite▄, 61
Gravity (specific) separation, 133
Gridiron twinning, 37
Grinding, 123
Grinding and cutting machines, 117 apparatus for sections with parallel faces, 118 plates or laps, 121
▄Gypsum▄, 80
Gypsum test-plate, 33
Hardness of grains, 132
▄Haüyne▄, 56
▄Haüynite▄, 56
Haüynite and noselite, micro-chemical distinction between, 139
Heating sections to redness, 131
Heavy solutions, 133
▄Hematite▄, 61
Hexagonal crystals, 4
High relief, 17
▄Hornblende▄, 84
▄Hyalosiderite▄, 78
▄Hydrargillite▄, 115
Hydrofluosilicic acid test, 135
▄Hypersthene▄, 75
Idiomorphic, 13
▄Idocrase▄, 60
▄Ilmenite▄, 62
Inclusions, 24
Index of refraction, ix, 17 by Becke’s method, 19 by Schrœder van der Kolk’s method, 22 by Sorby’s method, 19
Indicators for specific gravity separation, 133
Indices of refraction (mean), table of, 142
Interference color, determination of order of, 35
Interference colors, 28
Interference colors (abnormal), 30
Interference figures, 40 biaxial, 43 uniaxial, 40
Investigation of microscopic and optical characters of minerals, 13
▄Iolite▄, 78
Iron, test for, by Borichy’s method, 137 test for, by Behren’s method, 139
Isolating crystals or mineral fragments for testing, 132
Isometric crystals, 2
Isotropic character, 26, 40 crystals, 2
▄Kaolin▄, 115
▄Labradorite▄, 103
Laps for grinding, 121
▄Leucite▄, 54
Leucoxene, 62
Light, ordinary, 1 plane polarized, 1
▄Limonite▄, 51
▄Lithia Mica▄, 92
Machines for cutting and grinding, 117
Magnesium, test for, by Borichy’s method, 137 test for, by Behren’s method, 138
▄Magnetic iron ore▄, 52
▄Magnetic pyrites▄, 51
▄Magnetite▄, 52
Measuring strength of double refraction by von Federow mica wedge, 35
Mechanical and chemical tests, 129
▄Melilite▄, 61
▄Menaccanite▄, 62
Methods of preparing sections, 117
▄Mica Group▄, 89
Mica plate (quarter undulation), 33
Mica wedge, von Federow, 35
Micro-chemical reactions, 134
▄Microcline▄, 102
Microlites, 17
Microscope (petrographical), 7
Microscopic and optical characters of minerals, 51
Minerals and thickness of section, determined by table and diagram, 36
Monoclinic crystals, 4 principal vibration directions in, 5
▄Monoclinic pyroxenes▄, 80
Mounting sections, 124 solutions, 125
▄Muscovite▄, 89
▄Natrolite▄, 79
Negative optical character, biaxial, 46 uniaxial, 42
▄Nepheline▄, 69
▄Nephelite▄, 69
Nicol prism, 7
▄Nosean▄, 56
▄Noselite▄, 56
Noselite and haüynite, micro-chemical distinction between, 139
Nose-piece for microscope, 10
Objectives, 10
Oblique extinction, 31
Obtuse bisectrix, 5
▄Oligoclase▄, 103
▄Olivine▄, 77
▄Opal▄, 51
Opaque minerals, characters of, 13
Optical, anomalies, 27 character, biaxial, 46 character, uniaxial, 42 classification into systems, 141 distinctions between orthorhombic, monoclinic and triclinic sections (perpendicular to bisectrices), 48, 141 distinctions between tetragonal and hexagonal sections (perpendicular to optic axis), 141 and microscopic characters of minerals, 51 principal section, 3, 4 scheme, 145
Optic axes, 4
Optic axis, 3
Optics for optical mineralogy, 1
Order of consolidation of minerals in plutonic rocks, 144
Order of interference color, determination of, 35
Ordinary light, 1 ray, 3
▄Orthite▄, 96
▄Orthoclase▄, 98
Orthorhombic crystals, 4 principal vibration directions in, 4
▄Orthorhombic pyroxenes▄, 75
Parallel and convergent light, résumé of uses of, 49
Parallel extinction, 31
Parallel face sections, grinding apparatus for, 119
▄Pargasite▄, 85
▄Perofskite▄, 57
▄Perovskite▄, 57
Petrographical apparatus, 127
Petrographical microscope, 7
Phenocryst, 14
▄Phlogopite▄, 89
▄Picotite▄, 53
▄Piedmontite▄, 95
▄Plagioclases▄, 103
Plane polarized light, 1
Plates, grinding, 121
Pleochroic halos, 59
Pleochroism, 25 test for, 25
▄Pleonaste▄, 53
Polarization colors, 28
Polarized light, characters observed by, 25 plane, 1
Polarizer, 7 test for vibration plane of, 9
Polishing sections, 124
Polysynthetic twinning, 37
Positive optical character, biaxial, 46 uniaxial, 42
Potassium, test for, by Borichy’s method, 136 test for, by Behren’s method, 138
Preparation of sections, 117
Principal section, optical, 3, 4
Principal vibration directions, 3, 4
Prism, nicol, 7
Pseudomorphic structure, 39
▄Pyrite▄, 51
▄Pyrites▄, 51
▄Pyroxenes, monoclinic▄, 80
▄Pyroxenes, orthorhombic▄, 75
▄Pyrrhotite▄, 51
Quarter-undulation mica plate, 33
▄Quartz▄, 63 wedge, 34
Reflected light, characters observed by, 13
Reflector, 7
Refraction, double, 2, 27
Refraction, index of, ix, 17
Refractive indices (mean), table of, 142
Relief, 17 test for, 17, 18
Resorption border, 15
Résumé of uses of parallel and convergent light, 49
Rotating stage of microscope, 10
▄Rutile▄, 57
▄Sagenite▄, 58
▄Sanidine▄, 102
▄Saussurite▄, 96, 113
Saws, 119
▄Scapolite Group▄, 59
Scheme of classification into systems by optical determinations, 141
Scheme, optical, 145
Schiller structure, 25
Schrœder van der Kolk’s method for determining refractive indices, 22
Sections, methods of preparing, 117 thickness of rock, 124
Selenite plate, 33
Separation, by chemical means, 134 by electro-magnet, 134 by specific gravity, 133
▄Sericite▄, 92
▄Serpentine▄, 114
Shagreened surface, 17
▄Shorl▄, 70
▄Sillimanite▄, 73
Simple twinning, 37
Single image, 2
Skeleton forms, 17
Slides, glass, 125
Slower and faster rays, test for vibration directions of, 33
▄Sodalite▄, 56
▄Sodalite Group▄, 56
Sodium, test for, by Borichy’s method, 136 test for, by Behren’s method, 138
Sorby’s method for determining index of refraction, 19
Special micro-chemical tests, 139
Specific gravity separation, indicators for, 133
Sphærulitic structure, 39
▄Sphene▄, 96
▄Spinel▄, 53
Stage of microscope, rotating, 10
▄Staurolite▄, 74
Strained or broken crystals, 15
Strength of double refraction, measure of, 30, 35
Structure, 37
Symmetrical extinction, 31
Systems, classification into, by optical determinations, 141
Table, double refraction, 142
Table, indices of refraction, 142
▄Talc▄, 94
Test for vibration plane of polarizer, 9
Tetragonal crystals, 4
Thickness of rock sections, 124
Thickness of section and minerals, determined by table and diagram, 36
▄Titanite▄, 96
Transmitted light, characters observed by, 13 effect on crystals, 2
Transparent minerals, investigation of characters of, 13
▄Tremolite▄, 84
Triclinic crystals, 4 principal vibration directions in, 5
▄Tridymite▄, 65
▄Topaz▄, 74
▄Tourmaline▄, 70
Twinning, 37
Uniaxial crystals, vibration directions in, 3
Uniaxial interference figures, 40
▄Uralite▄, 88
Uralitization, 88
Uses of parallel and convergent light, 49
van der Kolk’s, Schrœder, method for determining refractive indices, 22
▄Vesuvianite▄, 60
Vibration direction, 2 directions of faster and slower rays, test for, 33 directions, principal, 3, 4 plane of nicol, 9 plane of polarizer, test for, 9
von Federow mica wedge, 35
Wavy extinction, 16, 31
Wedge, mica, 35 quartz, 34
▄Wernerite▄, 59
Xenomorphic, 15
▄Zeolites▄, 79
▄Zircon▄, 58
▄Zoisite▄, 96 α and β, 96
Zonal structure, 38
ERRATA AND ADDENDA.
P. 33, line 23, _for_ beat _read_ be at.
P. 66, line 10, _after_ appearance _add_ (sometimes called “twinkling”).
Corrections for mean index of refraction _n′_: p. 60, line 1, 1.551 to 1.584; p. 62, line 29, 1.766; p. 66, line 3, 1.601; p. 67, line 24, 1.622; p. 71, line 10, 1.633 to 1.674.
Corrections for Indices of Refraction (mean) in Table on p. 142: Corundum 1.766; Tourmaline 1.674; Tourmaline (precious) 1.633; Dolomite 1.622; Scapolite (Meionite) 1.584; Scapolite (Marialite) 1.551; Calcite 1.601.
Corrections for mean indices of refraction in Scheme (insert folder): Scapolite 1.551 to 1.584; Calcite 1.601 and transfer to Dolomite rectangle; Dolomite 1.622; Corundum 1.766; Tourmaline 1.633 to 1.674.
-----
Footnote 1:
Often called a direction of maximum elasticity, c′ being a direction of minimum elasticity.
Footnote 2:
Rosenbusch’s _Microskopische Physiographie_, p. 156.
Footnote 3:
For a more complete discussion of optics, in connection with Optical Mineralogy, the student is referred to A. J. Moses’ _Characters of Crystals_, p. 85, et seq.; Moses’ & Parsons’ _Mineralogy, Crystallography and Blowpipe Analysis_, Chap. XVI., 4th Ed., 1909; Miers’ _Mineralogy_, 1902; L. Fletcher’s _Optical Indicatrix_, etc., 1892; Groth’s _Physikalische Krystallographie_, 3d Ed.; Rosenbusch’s _Mikroskopische Physiographie_, 4th Ed. and Iddings’ _Rock Minerals_, 1911.
Footnote 4:
The electromagnetic theory of light is now very generally held, but, whatever may be the recurrent change of state to which light is really due, the principles of wave motion furnish a satisfactory geometric description of optical phenomena.
Footnote 5:
These sections are supposed to have plane parallel faces, such being the case in ordinary practice, and to be examined with parallel perpendicularly incident light.
Footnote 6:
It is interesting to remember in this connection that in the isometric system there is also the greatest possible symmetry of “form.”
Footnote 7:
A. J. Moses, _Characters of Crystals_, pp. 85–97.
Footnote 8:
For this branch of optical physics, see A. J. Moses, _Characters of Crystals_, pp. 97–100.
Footnote 9:
This can be demonstrated by using a nicol and a plate of calcite which shows a double image. If the nicol is held between the calcite plate and the observer’s eye it can be so adjusted that only one image is seen. If now the nicol is revolved 90° the first image will disappear and the other image alone will be seen.
Footnote 10:
In some cases a peculiar form of double refraction does take place parallel to this direction, as in the circular polarization of quartz and cinnabar; but in very thin sections these results are not noticed and can be disregarded.
Footnote 11:
A. J. Moses, _Characters of Crystals_, pp. 98, 99.
Footnote 12:
The terms axes of elasticity are commonly used for these principal vibration directions in text-books on petrography.
Footnote 13:
Instead of ω and ε, for convenience in tables, etc., α and γ are used, denoting the indices of refraction of the rays traversing the crystal with greatest and least velocity respectively, without regard as to which is the _O_ or _E_ ray. A good reason for this convention is that the symbol (γ − α) is used to express in decimals the relative strength of the double refraction of a crystal, whether uniaxial or biaxial. γ is always greater than α.
Footnote 14:
For most cases in observations with white light the “optics axes” may be regarded as approximately fixed in position.
Footnote 15:
Often spoken of as: a, the axis of maximum; b, the axis of intermediate, and c, the axis of minimum elasticity.
In the more recent American text-books on Optical Mineralogy by Iddings, Winchell and Phillips a is denoted by _X_, b by _Y_ and c by _Z_.
Footnote 16:
For a short historical sketch of the use of the microscope in connection with Petrology, see G. H. William’s pamphlet, _Modern Petrography (Monographs of Education)_, Boston, 1886. For detail description and adjustments, see _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, pp. 11, 61.
Footnote 17:
For description of the ordinary microscope, eye-pieces, objectives, magnification, etc., see _Manipulation of the Microscope_, by Ed. Bausch.
Footnote 18:
Text Book of Mineralogy, by E. S. Dana, 1898 Ed., p. 176.
Footnote 19:
It is convenient to assume that the vibrations of the polarized light are taking place in this plane, called the “plane of vibration,” but all the phenomena caused by polarized light could be also explained on the assumption that the vibrations were taking place at right angles to this plane.
Footnote 20:
This condensing lens must be removed when very low power objectives are used.
Footnote 21:
Some microscopes are provided with adjusting screws bearing on the frame holding the objective, which can then be accurately centered to the axis of rotation of the stage.
Footnote 22:
In the Seibert microscope use objective No. 00 for the first general study of a rock section, No. II for general use and No. V for observations with convergent light. In the Fuess microscope use objective No. 4 for general use and No. 7 for convergent light tests. In the case of an English microscope a 1″ to ¾″ objective is used for general purposes and a ¼″ to ⅕″ for observations with convergent light.
Footnote 23:
See pp. 33 and 34.
Footnote 24:
In some microscopes the analyzer is in the form of a “cap” nicol, arranged to be fitted over the top of the eye-piece, and not introduced in the microscope tube as shown here. This form is not so convenient, as the “cap” nicol must be set by hand every time it is desired to make observations with crossed nicols. But at the same time it avoids any possible refocusing which may be necessary when the other type of analyzer is introduced in the tube.
Footnote 25:
When accurate adjustments are possible of the vibration planes of the nicols and the cross-wires parallel to these vibration planes, reference should be made to _Methods of Petrographic-Microscopic Research_, p. 61, 1911, Fred. E. Wright.
Footnote 26:
In the Seibert microscope eye-piece No. 0 is used for most purposes. Other eye-pieces, Nos. 1 and 2, with cross-wires, are used for different degrees of magnification, and one eye-piece, No. 3, without cross-wires, is provided to be used in connection with an eye-piece micrometer.
Footnote 27:
With the Seibert microscope, Fig. 2, the No. 0 eye-piece and the No. II objective will prove most satisfactory for the following tests. With the Fuess microscope use No. 4 objective. With an English microscope use an ordinary eye-piece and a 1″ or ¾″ objective.
Footnote 28:
Twins may be recognized just as in macroscopic specimens, and zonal structure noticed if the zones differ in color. When a colorless mineral is surrounded by other colorless minerals, of about the same index of refraction, its outline is often best brought out by observation between crossed nicols.
Footnote 29:
The term “Anhedron,” meaning without planes, has been suggested by L. V. Pirsson to describe in rocks the crystal fragments which have no plane faces, as, for example, the augites of augitic rocks. _Science_, Jan. 10, 1896, p. 49.
Footnote 30:
Partial resorption and recrystallization may produce a border of secondary minerals, surrounding the original crystal.
Footnote 31:
The surfaces of all minerals in sections are more or less rough, but this roughness is only made visible when there is a marked difference between the indices of refraction of the minerals, and the index of refraction of the balsam in which the minerals are embedded. The index of refraction of balsam is about 1.54, so it is only when the mineral has a higher or lower index of refraction that its surface appears rough. When internal structure is to be studied, the crystal should be surrounded by a fluid of nearly the same index of refraction as that of the crystal, and when the exterior of the crystal is to be studied, then a fluid should be used with a very different index of refraction.
Footnote 32:
The Seibert microscope has a very convenient and quick lowering adjustment, by means of the lever _d_, for making this test. For convenience the lower nicol and condensing lens are generally left in place below the stage of the microscope, as polarized light serves as well for these investigations as ordinary light. An additional advantage in this arrangement is that the condensing lens is always ready for the “relief” test and the lower nicol for the pleochroism test; but it must be remembered that the polarizer or lower nicol cuts out one half of the light, which comes to it from the reflector, and this loss is important when high power objectives are to be used. When very low power objectives are used, the condensing lens must be removed.
Footnote 33:
_Mem. de l’Acad._, Paris, 1767–68.
Footnote 34:
_Min. Mag._, Vol. I., p. 193; Vol. II., p. 1.
Footnote 35:
_Sitzungsberichte der k. k. Akad. der Wiss._, Wien, 1893, I Abt., p. 358. Translation by L. McI. Luquer in _School of Mines Quarterly_, Vol. XXIII., Jan., 1902, No. 2, p. 127. Review by Viola in _Min. Pet. Mitt._, Vol. 14, p. 554. _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 95.
Footnote 36:
Most of the petrographical microscopes carry over the polarizer a convex lens the effect of which is to widen the illuminating cone and hence make less visible this phenomenon.
Footnote 37:
In Seibert student microscope, No. II _a_, use next to smallest light-stop. Some of the Fuess microscopes are supplied with an iris-blende for limiting the cone of light.
Footnote 38:
In Seibert microscope use No. V, not sufficiently marked results being obtained with No. II.
Footnote 39:
See Viola’s diagram, _Minn. Pet. Mitt._, Vol. XIV., p. 556.
Footnote 40:
Seibert, No. II. Fuess, No. 4.
Footnote 41:
_Kurze Anleitung zur mikrosk._ _Krystallbestimmung_, Wiesbaden, 1898, and _Tabellen zur mikrosk._ _Bestimmung_, etc., Wiesbaden, 1900. _Methods of Petrographic-Microscopic Research,_ F. E. Wright, 1911, p. 93.
This method is particularly favorable for the accurate determination of the refractive indices of small isolated fragments or grains, by using liquids of known indices. In this case a bright line appears on the “near” edge and a dark line on the “far” edge if the grain has a higher index than the liquid. The reverse occurs when the index is lower than the liquid. When the index is the same (using white light) the “far” edge is blue and the “near” edge red and also the contours about disappear.
F. Krantz, of Bonn, furnishes a series of 21 liquids in small bottles, with indices from 1.447 to 1.83.
The indices of a few convenient liquids are: water, 1.34; alcohol, 1.36; glycerine, 1.41; olive oil, 1.47; nut oil, 1.50; clove oil, 1.54; aniseed oil, 1.58; almond oil, 1.60; cassia oil, 1.63; monobromnapthalene, 1.65; methylene iodide, 1.75.
Footnote 42:
Crystals that have two good cleavages often develop so that the direction of elongation is parallel to the intersection of the two cleavages, while in the case of crystals with one good cleavage the tendency seems to be towards a tabular habit parallel to the cleavage.
Footnote 43:
Harker’s _Petrology for Students_, p. 306.
Footnote 44:
The lower nicol is generally so adjusted that its plane of vibration is parallel to the north and south cross-wire in the eye-piece. This adjustment can be tested by means of a section of biotite, showing cleavage cracks. When the plane of vibration of the polarizer is parallel to the N. and S. cross-wire in the eye-piece, the biotite section becomes almost dark when its cleavage cracks are parallel to the same cross-wire. The upper nicol, or analyzer, must, of course, be removed during this test. This method is more convenient than taking the nicol out of its frame, in order to ascertain its plane of vibration (the direction of its shorter diagonal).
Footnote 45:
Although the “absorption directions” may not necessarily coincide with the principal vibration directions in Monoclinic and Triclinic crystals; still for convenience the absorption colors are usually given for the light rays vibrating parallel to these principal vibration directions.
Footnote 46:
In the Fuess and Seibert microscopes the analyzer or upper nicol is so fitted that it slides in and out of the tube of the microscope with its plane of vibration always at right angles to the plane of vibration of the polarizer or lower nicol.
Footnote 47:
Moses’ _Characters of Crystals_, p. 106. Moses and Parsons’ _Min. Cryst._ and _B. P. Analysis_, p. 163.
Footnote 48:
Iddings’ _Rock Minerals_, 1911, p. 172.
Footnote 49:
These sections always contain the principal vibration directions _a_ and _c_.
Footnote 50:
_Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 101.
Footnote 51:
A chart of interference colors can be obtained from Baudry et Cie, Paris, and is also published in _Les Minéraux des Roches_, by Lévy and Lacroix, _Rock Minerals_ (1911), by Iddings and in Rosenbusch’s _Mikroskopische Physiographie_.
Footnote 52:
Iddings’ _Rock Minerals_, 1911, pp. 141, 183.
Footnote 53:
_Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 132.
Footnote 54:
The ¼ undulation mica plate consists of a thin cleavage of mica on which is marked _c_, the vibration direction of the slower ray, which in mica is the line joining the “optic axes.” The thickness is such that the slower ray is ¼ wave-length behind the faster and the interference color is a bluish-gray. The gypsum plate is a thin cleavage of gypsum, on which is usually marked a, the vibration direction of the faster ray. The chosen thickness is such as to produce the red interference color of the 1° order.
Footnote 55:
The test-plates are generally introduced in the slot _k_, in a microscope of the Seibert type, or if a cap-nicol is used in a slot below this. In case no provision is made by the instrument maker for these test-plates, the regular analyzer is left out of the tube, and a simple nicol prism is used as an analyzer and is held by the observer over the eye-piece. Care must be taken to have the plane of vibration of this nicol at right angles to that of the polarizer, and to leave sufficient room for the introduction, by hand, of the test-plate between the eye-piece and the nicol. With care the plates can be introduced with sufficient accuracy to make the test practical.
Footnote 56:
A scale or chart of interference colors, or the interference color diagram, should be before the observer in order to avoid any mistakes as to whether the new color is higher or lower in the scale.
Footnote 57:
The quartz wedge is cut so that one of its faces is exactly parallel to the _ć_ axis (hence also parallel to the _c_ vibration direction) while the other face makes a very small angle with it. The direction _c_ is marked on the wedge.
Footnote 58:
Described under next test.
Footnote 59:
In applying this rule count the 1° order white as green and the 1° order gray as blue.
Footnote 60:
A. J. Moses, _Trans. N. Y. Acad. Sci._, Vol. XVI., p. 55, Jan., 1897.
Footnote 61:
E. von Federow, _Zeit. f. Kryst., etc._, Vol. XXXV., p. 340, 1895.
Footnote 62:
After the first rough determination of the phase difference by the mica wedge, the more exact phase difference can be obtained by the aid of a good color chart or diagram, see end of book.
Footnote 63:
It is not safe to use minerals near the edge of the section, as the thicknesses are apt to be unequal.
Footnote 64:
See at end of appendix.
Footnote 65:
In this way eliminate, so far as possible, the effect of the orientation of the mineral section.
Footnote 66:
The different mineral sections are all supposed to have the same thickness throughout the rock section.
Footnote 67:
Harker’s _Petrology for Students_, 1895, p. 14.
Footnote 68:
Iddings’ _Rock Minerals_, pp. 153, 173. Moses’ _Characters of Crystals_, p. 115.
Footnote 69:
In the Seibert microscope use No. V objective, in Fuess microscope No. 7 objective, and in English microscopes a ¼″ or ⅕″ objective.
Footnote 70:
Each convergent ray will have its vibration direction either in or at 90° to the plane through the ray and the optic axis. Hence all rays vibrating parallel to the vibration planes of both nicols will be completely cut out. As the section is rotated new rays successively come into these positions, so the same effect is maintained.
Footnote 71:
In the Seibert microscope there is a little slot _k_ for this purpose just above the objective.
Footnote 72:
The optical character may also be determined in parallel light by proving _ć_ = c(+), _ć_ = a(−). The optical character of the principal zone or the sign of the elongation is often given in tables. This optical character or sign is (+) when the principal zone axis or the direction of elongation is parallel to c and (−) when parallel to a.
Footnote 73:
Iddings’ _Rock Minerals_, 1911, p. 172.
Footnote 74:
This assumes the optic axes for different colors to emerge about at the same points. If there is marked “dispersion” the black bands and hyperbolas may be rainbow-hued, as with titanite.
Footnote 75:
The interference figure, perpendicular to the _obtuse bisectrix_, would be of the same type with a larger axial angle. Ordinarily this figure would not come within the limits of the field of view of the microscope. Confusion may arise, however, but in a section perpendicular to the acute bisectrix the cross dissolves more slowly into the hyperbolas than in the case of a section perpendicular to the obtuse bisectrix. At times it may be necessary to measure the axial angle to be sure. When, however, the mineral is known, the section perpendicular to the acute bisectrix can be recognized, because if the mineral is optically positive the trace of the axial plane is parallel to a and if negative parallel to c.
Footnote 76:
For construction of quartz wedge, see p. 34.
Footnote 77:
The wedge can be introduced in either of the several ways described for the introduction of the test-plates on p. 33.
Footnote 78:
For methods of measuring the axial angle, see _Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p. 147.
For convenience in many cases only 2_E_ is recorded, as then an indication is given as to whether the axial angle is visible with an ordinary microscope (arranged for observation with convergent light for interference figures). If 2_E_ is very large the axial angle can only be observed by covering the section with some transparent, strongly refracting fluid. For the Seibert microscope with objective V the limit for good results is about 2_E_ = 90°–100°.
Footnote 79:
For dispersion, etc., see A. J. Moses’ _Characters of Crystals_, p. 140.
Footnote 80:
The system of crystallization of leucite has been the subject of much discussion. Its habit is isometric. The consensus of opinion seems to be that leucite crystallizes in the isometric system, but that the isometric molecular arrangement, at least of the larger crystals, cannot exist for the temperature and pressure at the earth’s surface. Hence molecular displacement takes place, giving rise to a more or less complicated apparent twinning, and optical anomalies are noticed. The isotropic character returns if the section is heated to 500° C. Iddings’ _Rock Minerals_, p. 249, 1911.
Footnote 81:
C. W. Knight, _Canad. Rec. of Sci._, IX, No. 5. 265.
Footnote 82:
The interference colors of all minerals here recorded are those given by sections 0.03 _mm._ in thickness (very thin sections).
Footnote 83:
Shown by Mügge, Joly, etc., to be caused by radiations emanating from U, Th, R, etc. Iddings’ _Rock Minerals_, 1911, p. 189.
Footnote 84:
Iddings’ _Rock Minerals_, 1911, p. 392.
Footnote 85:
See page 31.
Footnote 86:
R. D. Irving, _Am. Jour. Sci._, June, 1883.
Footnote 87:
Granites of the central Alps, where the calcite crystals are intergrown with quartz.
Footnote 88:
The minerals Nephelite, leucite, sodalite (haüynite and noselite) and melilite are often grouped together under the name “_feldspathoides_”; on account of their relation in rocks being equivalent to that of the feldspars.
Footnote 89:
Oriented in conformity to the intergrowth with augite, etc., (010) and (100) are reversed.
Footnote 90:
For other alteration processes, see Iddings’ _Rock Minerals_, p. 380.
Footnote 91:
May be difficult to determine in the case of prism zone sections, showing large extinction angles.
Footnote 92:
See p. 88.
Footnote 93:
Iddings’ _Rock Minerals_, p. 456, 1911.
Footnote 94:
Depending on whether the axial plane is parallel or at right angles to the clino pinacoid (010) (the plane of symmetry), we have micas of the second (biotite) or first order (muscovite).
Footnote 95:
The distinction between zoisite α and β (essentially orthorhombic, but may be composite triclinic twins) and clinozoisite (monoclinic close to orthorhombic) depends on differences in position of plane of optic axes; axial figures shown by cleavage plates; dispersion; anomalous interference colors; etc. See Weinschenk’s _Die Gesteinbildenden Mineralien_, p. 83. 1901.
Footnote 96:
Test not easily made on account of the very high order interference colors, resulting from the strong double refraction.
Footnote 97:
See under quartz, p. 63.
Footnote 98:
Iddings’ _Rock Minerals_, p. 208, 1911.
Footnote 99:
On account of the weak double refraction the interference figures are not very sharp or well defined in thin sections. In most cases only the black hyperbolas are seen, without any colored curves.
Footnote 100:
By heating feldspar crystals the axial angle decreases to 0° and then increases in the plane of symmetry (at right angles to its former position). On cooling the axial angle returns to its former position if the temperature has not exceeded 500° C. If the temperature has been 600°–1000° C. for some time the axial angle will not return to its former position. This fact may give some clew as to the temperature at which the feldspar crystals formed.
Footnote 101:
This change to kaolin or clay in granite is called by Dolomieu “_La maladie du granit_.”
Footnote 102:
See p. 31.
Footnote 103:
Hatch’s _Introduction to the Study of Petrology_, p. 33.
Footnote 104:
J. W. Judd, _Geol. Mag._ [3], Vol. VI, p. 243, 1889.
Footnote 105:
The plagioclases have rather a complex composition; but may be regarded as forming a series from the composition NaAlSi_{3}O_{8}(Ab) to the composition CaAl_{2}Si_{2}O_{8}(An), consisting for the most part of isomorphous mixtures of these types, with some replacement by KAlSi_{3}O_{8}. The compositions of only a few of the common plagioclases are given above.
Footnote 106:
The lath-shaped feldspars, moulding the augite, give to diabases the so-called “ophitic” structure, Fig. 12. The peculiarity of this structure is that the feldspars crystallized before the augite, which is contrary to the usual order of formation.
Footnote 107:
For the positions of the optic axes, bisectrices, etc., relative to the cleavage plates of the different plagioclases, see Iddings’ _Rock Minerals_, p. 222, 1911.
Footnote 108:
_Die Gesteinsbildenden Mineralien_ (with tables), E. Weinschenk, Freiburg, 1901. _Étude sur la Détermination des Feldspaths dans les Plaques Minces_, Michel Lévy, Paris, 1894; and _The Determination of the Feldspars_, N. H. Winchell, _Am. Geol._, Vol. XXI, No. 1, 1898. Iddings’ _Rock Minerals_, Wiley & Sons, 1911. _Étude sur la Détermination des Feldspaths_ (troisième fascicule), Michel Lévy, Paris, 1904.
Footnote 109:
Glass models of the feldspars (size 20 × 10 cm.) by F. Krantz, Bonn. Diagrams, showing optical orientation in the plagioclases, in _Die Gesteinsbildenden Mineralien_, E. Weinschenk, p. 133, 1901.
Footnote 110:
For this method of investigation little cleavage flakes or plates can often be obtained from the crushed mineral, but, on account of Albite twinning, plates are more apt to be obtained parallel to the twinning plane than to the best basal cleavage. If a fragment with only one cleavage surface is obtained, it must be cemented to a glass by this surface and ground down to a thin section with parallel sides.
Footnote 111:
This test is only possible when suitable sections of the given feldspar in the rock section can be found. The method, however, can be used with great accuracy with the aid of some form of apparatus for properly orienting the section. See “Klein’s Apparatus for the Orientation of Thin Sections,” _Sitzungsber. Berlin. Akad._, 1895, 1151; (also in _N. Y. Acad. Sci._, Vol. XVI, p. 51, 1897); and Von Federov’s “Universal Table,” _Zeit. für Kryst, etc._, Vol. XXV., p. 351.
Footnote 112:
In the determination of feldspar microlites it is well to remember the following facts: “Microcline is rarely, or never, seen in the condition of microlites, while the associations of labradorite and albite are so different that there is little danger of confounding them. Labradorite is the commonest product of the consolidation of the basic eruptives, and albite almost invariably results from metamorphism, frequently from the contact of igneous rocks on the calcareous clastics.” N. H. Winchell, Determination of the Feldspars, _Am. Geol._, Vol. XXI, No. 1, p. 33, 1898.
Footnote 113:
These methods (both 2 and 3) are often not applicable on account of the tendency of the crystals in an effusive rock to parallel orientation, which may be so marked that the rock section does not show any favorable sections of the plagioclase.
Footnote 114:
These extinction angles, as well as those previously given, are those of only a few type feldspars of definite composition. As the composition varies through a long series, so the extinction angle changes, one being a function of the other.
For a complete list of compositions and related extinction angles, see Iddings’ _Rock Minerals_ and Lévy & Lacroix’s _Les Minéraux des Roches_.
Footnote 115:
Iddings’ _Rock Minerals_, p. 228, Wiley & Sons, 1911. _Étude sur la Détermination des Feldspaths_ (troisième fascicule), Michel Lévy, Paris, 1904.
Footnote 116:
In quartz ω is the refractive index of the ray with vibration direction ∥ a [that is the direction of vibration of the faster ray (the ordinary ray)]. Hence ω is direction ∥ a and ε ∥ c. In the feldspars; α ∥ a, γ ∥ c and β ∥ b.
Footnote 117:
_Am. Jour. Sci._, May, 1906. This method is specially useful in detecting presence of orthoclase, when plagioclase is the dominant feldspar.
Footnote 118:
These tests are only possible on pure and fresh material. The specific gravity increases with the Ca % (albite 2.62, anorthite 2.75).
Footnote 119:
In clear unstriated granules, which may be distinguished from quartz by biaxial interference figure in convergent light.
Footnote 120:
The tendency of labradorite in gabbros to twinning, after both _Albite_ and _Pericline_ laws, is to be noted.
Footnote 121:
Werveke’s (N. J. B., 1883, II, 97) theory is that a twin lamination may be caused by the forces producing mechanical deformations, as movement in the magma and mountain making pressure. Such lamellæ are characterized by the fact that their extent and course seem to depend on fracture lines in the crystal.
Footnote 122:
The derivation from pyroxene and amphibole appears to be doubtful, see Weinschenk’s _Gesteinsbildenden Mineralien_, 1901, p. 121.
Footnote 123:
Harker’s _Petrology for Students_, p. 63, 1895.
Footnote 124:
Sections can be obtained from Voight & Hochgesang, Göttingen; C. Marchand, rue Censier, 16ter, Paris; W. H. Tomlinson, Swarthmore, Pa., and G. D. Julien, 3 Webster Terrace, New Rochelle, N. Y.
Footnote 125:
Made by G. D. Julien, 3 Webster Terrace, New Rochelle, N. Y. Price: Power lathes (complete), $150.00 up; Foot-treadle lathes, $90.00 up.
Footnote 126:
A Crocker-Wheeler motor of ¼ to ½ H. P. will be large enough for an ordinary laboratory machine.
Footnote 127:
Made by Voigt and Hochgesang, Göttingen. Price, $15.00.
Footnote 128:
Saws charged with diamond dust can be obtained from Elisha T. Jenks, Middleboro, Mass. Price in 1910, $1.35 per diametrical inch. Foreign saws 6″ and 8″ diam. can be bought from G. D. Julien, New Rochelle, N. Y.
Footnote 129:
_School of Mines Quarterly_, Vol. XI, p. 32.
Footnote 130:
“Half a pound or less of ordinary shellac is melted in a flat-bottomed open vessel over a Bunsen burner. Then an equal quantity of Venice turpentine is carefully added under constant stirring. The mass should be allowed to boil for about ten minutes, during which the stirring is continued. Then small quantities are poured in separate heaps on an iron plate or other cold surface and rolled into sticks about seven inches long and half an inch thick.”
Footnote 131:
Square or standard 26 × 46 mm. glass slides are recommended instead of the oblong slides (1 × 3 inches), which are very apt to project beyond the edges of the stage and be struck by the fingers while rotating the stage.
Footnote 132:
Reference can be made to the publications on this subject by: Borichy, Behrens (Behren’s translation by Judd), Haushofer, Huysse, Klément, Streng, Rénard, etc.
Footnote 133:
In case an acid is to be used which would attack glass, the cover-glass can be replaced by a thin perforated disk of platinum.
Footnote 134:
The bases in solution can be determined by different methods of analysis, for which the student is referred to more elaborate works on this subject. In some cases it may be very advantageous to treat the section with acid, to remove certain soluble constituents, when other minerals not distinctly seen at first may be made more apparent.
Footnote 135:
The gas may be H_{2}S from a soluble sulphide, in which case the solution containing the bubbles will color filter paper moistened with lead water.
Footnote 136:
_Geol. and Nat. History Survey of Minn._, XIX., Ann. Rept., p. 42; also A. J. Moses, _Characters of Crystals_, p. 147.
Footnote 137:
The symmetry of the etched figures would, of course, be related to the system of crystallization.
Footnote 138:
A. J. Moses, _Characters of Crystals_, Chap. XVI.
Footnote 139:
When it is desired to preserve the section and at the same time to study the surface covered with a film of air, the edges alone of the cover-glass should be cemented.
Footnote 140:
_Geol. and Nat. Hist. Survey of Minn._, XIX, Ann. Rept., p. 50. Thin sections of 2–3 sq. mm. area are of a convenient size, and they should be subjected to a red heat for (1½)-3 minutes. Too long a continuance of heat may render the sections too dark or lessen their transparency or produce melting.
Footnote 141:
This test may vary, in many cases graphite not being consumed even after long heating.
Footnote 142:
The isolation of material for investigation is of more interest for the lithologist or chemist than for the student of optical mineralogy; therefore only a very brief outline of some of the methods employed will be given.
Footnote 143:
The grains passing through different meshes are investigated microscopically to ascertain which size grains are homogeneous; the rest of the sample should then be reduced to grains of this size.
Footnote 144:
Further reference can be made to _Iddings’ Rock Minerals_, pp. 25 and 95, 1911. A convenient form of apparatus is also described in _School of Mines Quarterly_, Vol. X., p. 284, 1889.
Footnote 145:
S. L. Penfield, _Am. Jour. Sci._, Vol. L., p. 446, 1895. In this article a convenient form of separating apparatus is also described.
Footnote 146:
A convenient list of minerals, arranged in the order in which they would be attracted by increasing the force of the electro-magnet, is given in Weinschenk’s _Tabellen_.
Footnote 147:
For micro-chemical work the reagents should be applied in very small drops, which spread out on glass to discs 2 mm. in diameter. For manipulating the reagents use platinum wires, 0.5 mm. in thickness.
Footnote 148:
_Elemente einer neuen chemisch-mikroskopischen mineral- und Gesteins-analyse_, Pragg, 1877.
Translation of above by Winchell in _Geol. and Nat. Hist. Survey of Minn._, Vol. XIX., Ann. Report, 1890.
Footnote 149:
The strength of the solution should be about 3½%; for if too weak many minerals do not give satisfactory results, and if too strong a very large number of fluosilicate crystals are formed together with the separation of much silica, thus making it impossible to carefully differentiate the crystals with a microscope.
Footnote 150:
As most of the rock-forming minerals that would be investigated are silicates, the hydrofluosilicic solutions can also be obtained by treatment with HFl.
Footnote 151:
After Lévy and Lacroix.
Footnote 152:
_Naturkunde_, Amsterdam, 2, Vol. XVII., 1881. _Chemical News_, Vol. LXIII., No. 1647, June 1891, et seq.
_Micro-chemical Analysis_, Behrens (Judd), Macmillan & Co., London, 1894.
Footnote 153:
After Lévy and Lacroix.
Footnote 154:
After Lévy and Lacroix.
Footnote 155:
Compiled from Weinschenk’s Tables, as revised in _Petrographic Methods_ by Weinschenk-Clark, 1912.
Footnote 156:
Pirsson and Robinson, _Am. Jour. Sci._, iv, Vol. X, Oct., 1900.
Footnote 157:
Harker, _Petrology for Students_, p. 28, 1895.
Footnote 158:
Have the convergent lens or condenser lowered and the analyzer out during this test.
Footnote 159:
The minerals are grouped according to the maximum interference colors given by sections of the thickness of 0.03 mm.
Footnote 160:
In the case of very small crystals, the centering must be accurate and a cover with a small hole in the center, should be placed over the top of the tube. In this way the eye is brought directly over the axis of the microscope and figures can be observed from very minute crystals.
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TRANSCRIBER’S NOTES
Page Changed from Changed to
80 occurs as an alternation product occurs as an alteration product of anhydrite. Gypsum is soluble of anhydrite. Gypsum is soluble in hydrochloric in hydrochloric
136 longer the higher the percentage longer the higher the percentage of calcium in the solution of sodium in the solution
● Typos fixed; non-standard spelling and dialect retained. ● The author used the old chemical symbol (Fl) for Fluorine. ● Corrected the Errata with the exception of the last paragraph "Corrections for mean indices of refraction in Scheme (insert folder)". ● Used numbers for footnotes, placing them all at the end of the last chapter. ● Enclosed italics font in _underscores_. ● Enclosed bold font in ▄lower half block▄. ● The caret (^) serves as a superscript indicator, applicable to individual characters (like 2^d) and even entire phrases (like 1^{st}). ● Subscripts are shown using an underscore (_) with curly braces { }, as in H_{2}O.