Page 77 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS EPIDOTE GROUP
n. = 1.710 } Rls are based on a clinozoisite with about 1 per cent
COLOUR Colourless; a pink variety (thulite) may occur in Mn-rich environments,
n ~ = 1.715 Fe 0 present. These will increase with increasing
HABIT Usually found in clusters of elongate prismatic crystals, with rectangului 2 3
n y = 1. 719 Fe 2 0, content or decrease if clinozoisite is iron free
cross sections.
o = 0.005-0.015 (variable with Fe2 0 3 content)
CLEAVAGE Perfect { 100} prismatic cleavage, poor { 001} cleavage sometimes
present. 2Vy = variable, usually 14°-90° +ve
*RELIEF High. OAP is parallel to (010)
ALTERATION Since zoisite forms under conditions of low P and T, it remains stable" D = 3.12-3.38 H = 6lf2
and is not subject to further reactions. Zoisite may form from tht•
COLOUR Colourless.
breakdown of calcium plagioclase from basic igneous rocks which have II A BIT Found in columnar aggregates of crystals, which are usually quite small.
suffered hydrothermal alteration (a process called saussuritisation). ClEAVAGE Perfect { 001} cleavage, appearing as a prismatic cleavage in sections,
*BIREFRINGENCE Very low (varies between 0.004 and 0.008). a-zoisite shows low first since mineral is elongate parallel to b axis.
order colours (greys, whites) but ,8-zoisite shows anomalous interfc1 • RELIEF High.
ence colours of a deep Berlin blue. I II• RATION None.
EXTINCTION Straight on prism edge or { 100} cleavage. 1~1 I KINGE NCE Very low with anomalous first order interference colours (deep blue,
*INTERFERENCE A basal section (001) gives a biaxial positive figure with a moderate 2 V. greenish yellow: no first order white).
FIGURE
I IINC fiON Oblique, variable extinction angles depending on mineral composition,
OTHER FEATURES Since ,8-zoisite differs from a-zoisite in containing up to 5% Fe 2 0 3 , t111
but most elongate prismatic sections have straight extinction on
intermediate variety between a- and ,B-zoisite may occur, which ~~
cleavage.
distinguished by possessing a very small 2V ( = 0°).
OCCURRENCE Zoisite is found in basic igneous rocks which have been hydrothermally IN II K11 1 RENCE A (100) section will give a biaxial positive figure, but since 2Vis large an
II G RE isotropic section should be selected and a single isogyre examined for
altered, where it develops from calcic plagioclase. It also occurs i;1
medium grade metamorphosed schists in association with sodic pi agio sign and size.
clase, amphibole, biotite and garnet. It may occur in some meta IIIIIIR RHN E Clinozoisite occurs primarily in regionally metamorphosed low grade
rocks forming from micaceous minerals. Its other occurrences are simi-
morphosed impure limestones.
lar to those of zoisite.
Clinozoisite Ca2Al,(OH)Si,0, 2 monoclini1
1.583: 1: 1.814, .B = 115°30'
c

-----b j{

I I
I I
I I
{/ 'Y
64 65