Page 130 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS SERPENTINE
CLEAVAGE Good { 100} and { 110} prismatic cleavages. Serpentine Phyllosilicate
RELIEF Low. Serpentine Mg,Si 2 0,(0H). monoclinic
ALTERATION Scapolite may alter to a fine aggregate containing combinations of 0.57:1:1.31, f3 = 93°
various minerals including chlorite, sercite, epidote, calcite, plagioclase,
clays. Serpentine includes a variety of minerals, one fibrous (chrysotile) and
BIREFRINGENCE Low (Na-varieties) to moderate (Ca-varieties), with interference
two tabular (Iizardite and antigorite)
colours varying accordingly.
INTERFERENCE Aggregates of crystals are usually too small for sign determination. Antigorite Chrysotile
FIGURE c=o.
ZON ING Compositional zoning is common. c=o.
DISTINGUISHING Na-rich scapolite is similar in RI and birefringence to quartz,
FEATURES K-feldspar, plagioclase and cordierite, but it is uniaxial negative and
untwinned whereas quartz is uniaxial positive. K-feldspars and cordier-
ite are biaxial, and plagioclase is biaxial and invariably twinned.
Nepheline is also uniaxial negative but its Rls are lower (often < CB)
and it has a different occurrence.
occuRRENCE Scapolite may occur in some pegmatites, replacing plagioclase or quartz,
but its main occurrence is in metamorphic or metasomatic rocks. ----- b = 13
Scapolite may form as a primary mineral in calcareous rocks subjected
to regional metamorphism at amphibolite facies. It is associated with
calCite, sphene, diopside, plagioclase, epidote and garnet. In contact
metamorphism scapolite forms in carbonate rocks by introduction of / /
/
sodium and chlorine from the igneous intrusion. Grossular garnet, /
wollastonite, and fluorite are associated minerals. a = 'I

Chrysotile Lizardite Antigorite
n. 1.53-1.55 1.54-1.55 1.56-1.57
np 1.57
n, 1.55-1.56 1.55-1.56 1.56-1.57
8 0.013-0.017 0.006-0.008 0.004-0.007
2V. variable -ve ? 37°-61° -ve
OAP parallel to (010) ? parallel to (010)
D 2.55 2.55 2.6
H 2% 2% 2-31/2
coLOuR Colourless to pale green.
HABIT Chrysotile is fibrous elongated parallel to the a crystallographic axis, and
Iizardite and antigorite are both flat, tabular crystals.
cLEAVAGE Chrysotile has a fibrous cleavage, and lizardite a basal cleavage.
RELIEF Low.
*BIREFRINGENCE Low or very low, often with anomalous pale yellow colours shown (cf.
chlorite).
INTERFERENCE Antigorite shows a medium sized negative 2V on a basal section.
FIGURE
EXTINCTION Straight on fibres, cleavage or crystal edge. Chrysotile is length slow.
118 119