Page 78 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS FELDSPAR GROUP
Epidote (pistacite) Ca 2Fe' • A1 2 (0H)Si,0, 2 monoclinic I II Kl ERENCE A section perpendicular to the c axis will give a biaxial negative figure
1.591 : 1: 1.812, .B = 115°24' HGURE with a large 2V; thus an optic axis figure is preferable when determining
the sign.
~ 1111 M II'.ATU RES Lamellar twinning may sometimes be present.
1111 IIKRENCE Epidote is an important mineral in low grade regional metamorphic
II
II rocks, where it marks the beginning of the epidote-amphibolite facies,
/I
a I I forming from the breakdown of chlorite. Epidote also forms from sauss-
I I I uritisation of plagioclase feldspar and from the breakdown of
1 1 I
1 I I amphiboles in basic igneous rocks, these changes being due to late stage
I I
I I I hydrothermal alteration. In highly amphibolitised basic igneous rocks,
V I
II I clusters of epidote crystals are commonly seen associated with plagio-
clase feldspar and often within amphibole minerals.
Feldspar group Tektosilicates
---- b = 13 Introduction
The feldspars are the most important minerals in rocks. They occur in
igneous, metamorphic and sedimentary rocks, and their range of com-
position has led to their use as a means of classifying igneous rocks.
Feldspars are absent only from certain ultramafic and ultra-alkaline
igneous rock types and carbonatites. Feldspars occur in almost all
I
I metamorphic rocks, being absent only from some low grade pelitic
I
a types, pure marbles and pure quartzites. In sedimentary rocks feldspars
are common constituents of many arenaceous rocks, but are less
common in shales (clay rocks) and mudstones. Feldspars are difficult to
n. = 1.715 } detect (although XRD investigation would reveal their presence)
n p = 1.725 Rls increase with increasing Fe'• content
ny = 1.734 because of the minute grain size ( < 0.002 mm) of these argillaceous
rocks.
a = 0.019-0.049 (variable with composition)
Although the most ubiquitous of minerals, feldspars have a restricted
2V. = variable 64°-90° -ve
range of composition. There are two main types of feldspars:
OAP is parallel to (010)
D = 3.38-3.49 H = 6
(a) Alkali feldspars, which range between the end members ortho-
coLOUR Colourless to pale yellowish green. clase KAISi,0 8 and albite NaAISi 3 0 8 •
*PLEOCHROISM Slightly pleochroic with a colourless to pale yellowish green, ,B greenish (b) Plagioclase feldspars, which range between the end members
and y yellowish. albite NaAISi 3 0 8 and anorthite CaAI 2 Si 2 0 8 •
HABIT Found in aggregates of elongate prismatic crystals with pseudo
hexagonal cross sections. From this it is obvious that albite is common to both feldspar types, and
CLEAVAGE {001} perfect (similar to clinozoisite). the most usual way of depicting the complete feldspar group is in a
*RELIEF High. ternary (or triangular) diagram with orthoclase, albite and anorthite
ALTERATION None. representing the composition of each apex (Fig. 2.7). This reveals that
*BIREFRINGENCE Moderate to high, showing low second order to upper third order the alkali feldspars can contain up to 10% anorthite molecule in their
colours. Some sections may show low anomalous interference colours structure, and similarly plagioclase feldspars can contain up to 10%
similar to clinozoisite. orthoclase molecule in their structure.
EXTINCTION Oblique to cleavage in pseudo-hexagonal sections (see figure), other· The optical properties and structure of the feldspars depend upon
wise straight on cleavage in prismatic section. their temperature of crystallisation and their cooling history. Thus for
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