Page 138 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
TOPAZ
•coLOUR Yellow, pale yellow.
coLOuR Colourless.
*PLEOCHROISM Always present and distinct in yellows with a colourless, {3 pale yellow,
HABIT Tabular crystals or pseudo-hexagonal plates, similar to micas.
andy rich golden yellow.
*cLEAVAGE Perfect { 001} basal.·
HABIT Staurolite occurs as squat prisms, usually containing inclusions, par-
RELIEF Low.
ticularly of quartz.
*BIREFRINGENCE High with interference colours of third order. Basal sections, similar to
CLEAVAGE {010} moderate.
those of muscovite, give very low first order greys.
RELIEF High.
INTERFERENCE Good Bx. figure with small 2V on basal section.
• ALTERATION Rare, but may alter to a green ferric chlorite. FIGURE
BIREFRINGENCE Low, but interference colours are masked by the yellow colour of ExTINCTION Straight.
mineral. •occuRRENCE Talc occurs by low grade thermal metamorphism of siliceous dolomites,
INTERFERENCE Since 2V is very large, a single optic axis is required. and by the hydrothermal alteration of ultrabasic rocks, where talc may
FIGURE
occur along faults and shear planes. The development of talc is often
EXTINCTION Straight on prism edge or cleavage. associated with serpentinisation, with the serpentine changing to talc
occuRRENCE Staurolite occurs only in regional metamorphic rocks which are rich in plus magnesite by addition of C02.
2
3
alumina and iron, with probably a high Fe +: Fe + ratio. In medium
grade pelitic rocks, staurolite develops from chloritoid and before
Topaz N esosil ica te
kyanite but kyanite and staurolite can coexist. With increasing grade
staurolite breaks down to give kyanite and garnet. Topaz AI,Si0 4 (0H,F)2 orthorhombic, 0.528: 1:0.955
C='Y
I
Talc Phyllosilicate I
I ',-
I /I
monoclinic I /I
0.577: 1 :2.068, {3 = 98°55'
010

------ b = f3

///
//
/ /
f3 //
/
a
n. 1.539-1.550 n. = 1.606-1.629
np = 1.589-1.594 np ·= 1.609-1.631
n, = 1.589-1.600 nr = 1.616-1.638
8 = 0.05
8 = 0.008-0.011
2V. = 0°-30° -ve 2V, = 48°-68° +ve
OAP is perpendicular to (010) OAP is parallel to (010) prism section is length slow
D = 2.58-2.83 H = 1 D = 3.49-3.57 H = 8
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