Page 186 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES TUNGSTATE

Gypsum CaS04.2H,O monoclinic •occuRRENCE Gypsum is mainly found in sedimentary rocks, especially in evaporitic
0.6899: 1: 0.4214, .B = 99°18' sequences.
c Calcium sulphate can occur as either gypsum or anhydrite. Anhydrite
I
I may be formed by the dehydration of primary gypsum. In desert regions
I calcium sulphate is dissolved in percolating ground waters, which can be
drawn to the surface and deposit gypsum as 'desert roses' during very
dry spells.
Gypsum can form in fissures in shales and other argillaceous rocks by
the action of acid ground waters (sulphuric acid in solution), reacting
with calcium either from limestone nodules within the argillaceous rocks
or from intercalated limestone beds.

3.8 Phosphate

Apatite Ca,(P0 4 ) 3 (0H,F,Cl) hexagonal, cia 0.73
1.623-1.667
n 0
n. = 1.624-1.666
0 = 0.001-0.007
Uniaxial - ve (a prism section is length fast)
I
I I D = 3.1- 3.35 H = 5
I I
a
I coLOUR Colourless.
I *HABIT Small prismatic crystals with hexagonal cross section, often found with
I
C< ferromagnesian minerals in rocks, particularly amphiboles and micas.
no 1.519-1.521 cLEAVAGE Good basal { 0001} cleavage, imperfect prismatic cleavage { 1010}.
n p 1.523-1.526 RELIEF Moderate.
n, = 1.529-1.531 *BIREFRINGENCE Very low, maximum interference colours are grey.
0 = 0.01 occuRRENCE Important accessory mineral in igneous rocks, especially acidic plutonic
2V, = 58° +ve rocks, granite pegmatites and vein rocks, but common in diorites and
OAP is parallel to (010) gabbros also.
D = 2.30-2.37 H = 2 Apatite is common in metamorphic rocks, especially chlorite schists
and amphibole-bearing schists and gneisses.
coLouR Colourless.
Apatite occurs as a detrital mineral in sedimentary rocks. Sedimen-
HABIT Anhedral crystals occur usually in aggregate masses.
tary phosphatic deposits commonly contain a cryptocrystalline
CLEAVAGE {010} perfect, {100} and {011} good.
phosphatic mineral called 'collophane' , a term used if apatite cannot be
RELIEF Low, always less than CB.
positively identified.
ALTERATION With increase in temperature gypsum changes to anhydrite thus (about
200 °C):
3.9 Tungstate
f CaS04.2H,O ~ CaS0 4 + 2H,O
Wolframite (wolfram) (Fe,Mn)W04
BIREFRINGENCE Low, interference colours are first order whites.
INTERFERENCE Bxa figure is seen on a thin prismatic section but 2V is larger than field of The iron end member is called ferberite and the manganese end member
FIGURE
view; thus the sign is best determined by looking at an optic axis figure. huebnerite.
EXTINCTION Straight on the { 010} cleavage.
Crystals Wolframite is monoclinic, a : b : c = 0.8255: 1 : 0.8664, .B = 90°29'. It is
TWINNING Common on { 100}, repeated twinning usually seen. usually prismatic [ 001 J. Simple twinning is common and takes place on
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