Page 74 - A Practical Introduction to Optical Mineralogy

EPIDOTE GROUP
SILICATE MINERALS
Epidote group Sorosilicates
all the crystal shows alteration, and no good crystal face edges occur. In
thermal aureoles, cordierite occurs in inner zones, and the fresh cordier- a-zoisite has ac as the optic axial plane.
ite crystals which develop often show good hexagonal crystal form. ,B-zoisite has ab as the optic axial plane.
Cordierite may occur in some igneous rocks, where it shows subhedral to All monoclinic epidotes have ac as the optic axial plane.
euhedral crystal form.
CLEAVAGE { 010} good, with a poor { 001} basal cleavage sometimes developing. Minerals in the epidote group belong to both the orthorhombic and the
monoclinic systems. Conventionally the mineral belonging to the higher
*RELIEF Low, similar to quartz; usually higher than 1.54. symmetry system (orthorhombic) is described first, and therefore the
'' ALTERATION Cordierite shows alteration at edges and along cracks to pinite (a mix- descriptions begin with zoisite and go on to the two important mono-
ture of fine muscovite and chlorite or serpentine), which usually appears
clinic varieties, clinozoisite and epidote. Two other varieties- piemon-
as a pale yellowish green mineral in thin section. A yellow pleochroi tite (or piedmontite), a manganese-bearing epidote, and allanite (or
halo may sometimes appear in a cordierite crystal, surrounding an
orthite), a cerium-bearing type- are not described in detail here as they
inclusion of zircon or monazite, similar to those found in biotite crystals.
are relatively rare, allanite occasionally being found as an accessory
Such haloes are caused by the elements of the radioactive series U-Ra
mineral in some syenites and granites.
and Th-Ac.
BIREFRINGENCE Low, similar to quartz or feldspar.
Zolsite Ca 2Al,(OH)Si,O, orthorhombic
INTERFERENCE 2V is very large, and so the best figure would be obtained by examining a
FIGURE near isotropic section giving an optic axis figure. Such a figure i~ 0.622: 1:0.347
approximately found in the position of face (011) or (011). a- zoisite c = ~ ~- zoisite
c=a I
*TWINNING Extremely common in all crystals except those in regional metamorphic
I
rocks, where, in any case, alteration masks any twinning. Fresh, clear I
I
crystals in thermal aureoles, and some 'partial melt' igneous rocks, show
IIIII IIIII
two kinds of twinning- cyclic and lamellar. Cyclic twinning on { 110} or
{ 130} produces a pseudo-trigonal or pseudo-hexagonal pattern. In
some instances twinning on these planes produces lamellar twinning
similar to twinning seen in plagioclase feldspars.
Cordierite is a mineral found in pelitic rocks which have been subjected
to metamorphism at low pressure. Cordierite occurs in the inner (high
temperature) zone of thermal aureoles, and in regional metamorphic
conditions of high heat flow and low pressure, such as Buchan-type
metamorphism, where the sequence of index minerals produced under b = ~
regional conditions is, progressively: biotite-andalusite-cordierite-sil-
limanite. In these rocks, cordierite occurs in high grade gneisses, either
under abnormal PT conditions, or where a thermal metamorphi ·
episode follows regional metamorphism, and pre-existing minerals such
as kyanite and biotite become unstable, reacting to give cordierite and / /
tl "Y a=-y /
muscovite.
*OCCURRENCE Cordierite may occur in some igneous rocks, especially cordierite
norites. Originally they were considered to represent the crystallised
.
products of basic magma contaminated by the assimilation of argil- 1.696 } The Rls may vary depending on the amount of trace
1.
laceous material. However, it has recently been suggested that such 1. 696 elements (FeJ+ etc.) in the structure
rocks represent partial melt products in which high temperature liquids 702
have formed from the pelitic (or argillaceous) rocks owing to extremely 0.006
high temperatures being developed from nearby emplaced basic intru- 2V, = 0-60° +ve
AP is either parallel to (010) in a-zoisite, or parallel to (001) in
sions, these crystallising to give cordierite-norites. In these partial melt
rocks there is no magmatic component. Cordierite has been known to ,B-zoisite
occur in some granites and granite pegmatites as a primary mineral. 0 = 3.15-3.36 H = 6
62 6