Chapter XI: 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

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.