Page 106 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
OLIVINE GROUP
monoclinic grade increases, eventually becoming unstable at highest temperatures
0.574: 1: 2.217' 13 = 95°30' (> 600 °C) when the following reaction occurs:

('( muscovite + quartz---+ K-feldspar + sillimanite
I
Muscovite occurs in acid igneous plutonic rocks where it is a late crystal-
lising component, and it occurs in some detrital (clastic) arenaceous
sedimentary rocks.

Olivine group Nesosilicates
orthorhombic
Forsterite (Fo) Mg 2 Si0 4
Fayalite (Fa) Fe2SiO. 0.467 : 1 :0.587
------b ='I 0.458: 1 : 0.579
c = i3
I
Forsterite Fayalite
I
// I
/ / I
/ /
/ /
/ /
/ /
13 //
/ c = i3
/
a
n. = 1.552-1.574
np = 1.582-1.610
n, = 1.587-1.616
8 = 0.036-0.049
2V. = 30°-47° -ve
OAP is perpendicular to (010)
D = 2.77-2.88 H = 21/2 +
coLOUR Colourless.
HABIT Thin platy crystals; occasionally in aggregates. When the aggregates
consist of very fine grained crystals the mineral is called sericite.
*CLEAVAGE { 001} perfect.
RELIEF Low to moderate (if iron enters the structure).
ALTERATION Absent.
*BIREFRINGENCE High, upper second order to third order. Basal sections show a low olivine group
birefringence of first order grey which has a characteristic speckled
appearance. n. = 1.635 (forsterite)-1.824 (fayalite)
*INTERFERENCE A basal section gives an excellent Bxa figure which just fits into the field np = 1.651 -1.864
FIGURE of view; i.e. both isogyres will just be seen, indicating a 2V size of 40° ±.
n, = 1.670 -1.875
EXTINCTION Straight on cleavage. 8 = 0.035 -0.051
TWINNING Similar to phlogopite but not observable. 2V, = 82°-134~, i.e. 2V, 82°-90° +ve, 2V. 90°-46° - ve
occuRRENCE Muscovite is found in low grade metamorphic pelitic rocks where it OAP is parallel to (001)
forms from pyrophyllite or illite. Muscovite remains in these rocks as the D = 3.22 (forsterite)- 4.39 (fayalite) H = 61/2
94
95