Minerals in rock sections

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, Ab₁An₁
0.008: Oligoclase, Ab₄An₁
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, Ab₁An₁
1.55: Kaolin
1.547: Quartz
1.551: Scapolite (Marialite)
1.601: Calcite
1.541: Oligoclase, Ab₄An₁
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

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]