Page 129 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
SCAPOLITE
*EXTINCTION Both aegirine and aegirine-augite have small extinction angles in an n. = 1.616-1.640
ANGLE
(010) prismatic section. The extinction angles a (fasttc axis (or np = 1.628-1.650
a 'prismatic cleavage) = oo to < 20°. n, = 1.631-1.650
*oCCURRENCE Aegirine and aegirine- augite occur as late crystallisation products of a = o.013-0.014
alkali magmas, appearing in syenites and nepheline-syenites with alkali 2V. = 38°-60° - ve
amphiboles (see earlier section in this chapter). They may occur in alkali OAP is approximately parallel to (010)
granites, often with riebeckite, and may occur in some Na-rich schists D = 2.87-3.09 H = 4lf2-5
with glaucophane and riebeckite.
coLOuR Colourless.
HABIT Usually columnar or fibrous with rectangular cross sections.
Spodumene LiA1Si 2 0 6 monoclinic
*CLEAVAGE { 100} perfect and { 001} and { 102} good. A typical section shows two or
1.144: 1:0.632, f3 = 110°30'
n. = 1.648-1.663 three cleavages.
np = 1.655-1.669 RELIEF Moderate.
n , = 1.662-1.679 BIREFRINGENCE Low, with maximum first order yellow.
a = o.014-0.027 *INTERFERENCE Biaxial negative with a moderate 2V, seen almost on the basal face (i.e.
FIGURE
2V, = 58°-68° +ve at right angles to the length of the crystals).
OAP is parallel to (010) EXTINCTION Since the cleavages are different from those of pyroxene, the extinction
ANGLE
D = 3.03-3.22 H = 6112-7 angle is not so relevant a feature, but the crystals are almost length fast,
and a' c axis (crystal length) is 30°-40°.
Properties of spodumene are similar to those of diopside- colourless in DISTINGUISHING Identification of wollastonite is difficult. However, it should be noted
thin section and so on. But spodumene is a rare mineral, occurring in FEATURES
that although wollastonite is virtually identical to diopside, it is optically
lithium-rich acid igneous rocks such as granite pegmatites, where it is negative whereas diopside is optically positive.
associated with quartz, albite, lepidolite (lithium-rich mica), beryl and OCCURRENCE Wollastonite is a mineral formed in metamorphosed impure (cal-
tourmaline.
careous) limestones, usually as a result of the reaction
Pseudo-pyroxenes
CaCO, + Si0 2 (impurity)~ CaSiO, + C0 2
Wollastonite CaSiO, triclinic
1.082 : 1: 0.965 at fairly high temperatures (about 1000 oq, which may be reduced if
a= 90°0', f3 = 95°16', y = 103°22' volatiles are present. Wollastonite has been recorded from some
Although used as an end member of the pyroxene group of minerals alkaline igneous rocks.
Para wollastonite (the monoclinic form of wollastonite) has a similar
(Fig. 2.20) wollastonite does not possess a pyroxene structure, but is
chemically similar and is described with them. paragenesis.
c
I
I <X
I I Scapolite Scapo/ite Tektosilicate
I I
I I (Ca,Na).[ (Al,Si),AI,Si 6 0 24 ] (Cl,CO,) tetragonal cia = 0.44
I /}-
I
/ 1 n 0 = 1.540--1.600 } Indices increase with increasing substitution of Ca
n. = 1.535-1.565 for Na
a = o.oo4-0.037
Uniaxial -ve (length fast)
--- b = f3 D=2.50-2.78 H=5-6
coLOUR Colourless.
*HABIT Large spongy prismatic crystals are common in metamorphosed car-
bonate rocks. Granular and fibrous-looking aggregates are also
common, especially in garnet-bearing rocks.
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