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.

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

“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]