Page 69 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS CHLORITE
*INTERFERENCE The colour of the minerals and their strong dispersion make interference Chlorite Phyllosilicate
FIGURE figures difficult to obtain, and these are usually indistinct with optic signs monoclinic
and size of 2V impossible to judge. c hh1 ltc (Mg,Al,Fe) 1z(Si,Al) 80zo(OH)I6
0.57: 1: 1.31, f3 = 97°
*EXTINCTION Oblique with both minerals having variable extinction angles; a ' cleav-
age varies from oo to 50° but this is also difficult to obtain.
TWINNING Simple or repeated on { 100}.
OCCURRENCE Both minerals occur as constituents in alkali plutonic rocks (soda-rich
rocks), such as nepheline- and quartz-syenites, where they occur in
association with aegirine or aegirine-augite and apatite. The minerals
are late crystallisation products.
- ----b = (3
Aenigmatite Aenigmatite (Na 2 Fe;+TiSi 6 0 20 ) is a mineral closely resembling the alkali
amphiboles. It has very high relief ( - 1.8) and a small positive 2V.
Aenigmatite is pleochroic with a red brown, f3 brown, andy dark brown.
It is similar to the dark brown amphiboles but has higher Rls. Aenigma-
tite often occurs as small phenocrysts in alkaline volcanic rocks such as
phonolites.

II ., = 1.57-1.66
Beryl Cyclosilicate 11 11 = 1.57-1.67
11 , = 1.57-1.67
hexagonal
cia 0.9956 6 = 0.0-0.01
2V = 20°-60° +ve or - ve
1.560-1.602
n 0 AP is parallel to (010)
n. 1.557-1.599 [) = 2.6-3.3 H = 2-3
a o.oo3-0.009
1 IIIIIIIK olourless or green.
Uniaxial +ve (a prism section is length slow)
It 11 11"111 M reen varieties have a pale green to colourless, f3 andy darker green.
D = 2.66-2.92 H = 71/2-8
11 ''" Tabular crystals with a pseudo-hexagonal shape.
coLouR Colourless, pale yellow or pale green. lll I' t11 Perfect { 001} basal cleavage.
PLEOCHROISM Weakly pleochroic in pale greens if section is thick. 111 1 11 1 w to moderate.
*HABIT Hexagonal prism with large basal face. 1 11" II IIN xidation of iron in chlorite may occur (the sign changes from +veto
CLEAVAGE Imperfect basal {0001}. ve).
RELIEF Low to moderate. 1 1 INIII Nl 1 Very weak, usually with anomalous deep Berlin blue colour.
ALTERATION Beryl easily undergoes hydrothermal alteration to clay minerals, as .,, Ml Nl 1 Biaxial Bxa figure on basal section with small 2V. Usually positive but
follows, the reaction releasing quartz and phenakite: '""'K 1 some varieties- chamosite in particular- are optically negative. Inter-
fe rence figures are rarely obtained.
liN I II liN Straight to cleavage but can be oblique with small angle y or a ' cl (fast or
2Be,Al 2 Si 6 0 18 + 4H 2 ~ Al.Si 40 10 (0H) 8 + 5Si0 2 + 2Be 2Si0 4
kaolin phenakite slow to cleavage) ; very small angle(< 5°) on (010) section.
, INN INII 1\s in micas: rare.
BIREFRINGENCE LOW first order greys. , 1'0 1 Ni l 'hlorite is a widely distributed primary mineral in low grade regional
TWINNING Rare 111 tamorphic rocks (greenschists), eventually changing to biotite with
*occuRRENCE Beryl occurs in vugs in granites and particularly in pegmatites, often increasing grade; muscovite is also involved in the reaction. The initial
associated with cassiterite. The precious stone variety, aquamarine, material is usually argillaceous sediments, but basic igneous rocks and
occurs in similar locations, but emerald is usually found in metamorphic tuffs will give chlorite during regional metamorphism. In some alkali-
biotite schists. ' i ·h rocks, chlorite will break down with increasing P and T and help to

56 ')7