Page 125 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS PYROXENE GROUP

Jadeite NaAISi 20 . monoclinic •occuRRENCE Jadeite, is a rare pyroxene which can sometimes occur with albite in
1.103 : 1:0.613, .B = 107°16' regional metamorphic rocks, especially glaucophane schists, which have
formed under low heat flow, high P conditions. It is found m
Jadeite metamorphic rocks in association with lawsonite, glaucophane.
omphacite and spodumene
Omphacite is a cpx which has properties similar to jadeite and augite,
are similar
and is found exclusively in eclogites. In this rock omphacite occurs with
an Mg,Fe-garnet and the assemblage is considered to have formed at
high T and P and be stable at temperatures below 700 oc with pressures
of > 10 kb. The higher the P of formation the richer the omphacite is in
Na and AI.

na = 1.640-1.658
n p = 1.645-1.663
n r = 1.652-1.673
8 = 0.012-0.015
2Vr = 67°-70° +ve
OAP is parallel to (010)
D = 3.24-3.43 H = 6
COLOUR Colourless.
HABIT Usually occurs as granular aggregates of crystals, usually medium to
coarse grained.
CLEAVAGE Similar to diopside.
RELIEF Moderate.
ALTERATION Jadeite can alter to amphiboles or dissociate to a mixture of nepheline
and albite.
BIREFRINGENCE Moderate with second order colours.
EXTINCTION Similar to diopside, but smaller maximum angle withy (slowfcleavage =
ANGLE 33°-40° (max).
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