Page 49 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS AhSiOs POL YMORPHS
coLOUR Colourless but may be weakly coloured in pinks. Kyanite AI 2Si0 5 triclinic
PLEOCHROISM Rare but some sections show a pink, f3 andy greenish yellow. 0.902:1:0.710
*HABIT Commonly occurs as euhedral elongate prisms in metamorphic rocks a = 9005', f3 = 101°2', y = 105°44'
which have suffered medium grade thermal metamorphism (var.
chiastolite). Prisms have a square cross section (a basal section i
'I
square). --- I
*cLEAVAGE { 110} good appearing as traces parallel to the prism edge in prismatic
sections but intersecting at right angles in a basal section.
RELIEF Moderate.
ALTERATION Andalusite can invert or change to sillimanite with increasing
metamorphic grade. Under hydrothermal conditions or retrograde
metamorphism andalusite changes to sericite (a type of muscovite),
thus:
from feldspar senc1te
3AI,Si0 5 + 2H 20 + (3Si0 2 + K 2 0)----+ K 2 Al 4Si 6 AI 2 {0H) 4 0 20
*BIREFRINGENCE Low, first order (similar to quartz).
*EXTINCTION Straight on prism edge or on { 110} cleavages.
INTERFERENCE Basal section gives a Bx. figure but 2Vis too large to see in field of view.
FIGURE Look for an isotropic section approx. {101), and obtain an optic axis
figure which will be negative.
OTHER FEATURES Crystals in metamorphic rocks are usually poikiloblastic, and full of
quartz inclusions.
-------- b
*OCCURRENCE See after sillimanite.

11 , 1.7 12-1.71 8
11 1.721- 1.723
11
11 , 1.727- 1.734
fj 0.0 15- 0.0 16
2V, 82° - ve
OA Pis approx. perpendicular to (I 00) with thea axis approximately the
ncutc bisectrix
1
{) 3.58- 3.65 }-{ = 5 12-7
1 HIIII I K Usually colourless in thin section but may be pale blue.
t1 1ou llltHI M W •uk but seen in thick sections with a colourless, {3 andy blue.
11 1111 lJsun ll found as subhedral prisms in metamorphic rocks. The prisms
111 · hl11d • lik •, i. •. broad in one direction but thin in a direction at right
Ill\ I ., 10 llli~ .
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