Page 118 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS PYROXENE GROUP
51 I RELIEF Moderate to high.
!--'"" ALTERATION opx minerals alter to serpentine as follows:
/ ~"-...
61 I
/v
3MgSiO, + 2H2 0--. Mg,(Si 2 0 5 )(0H) 4 + Si0 2
71 I
/ \
"' I "" 1\
<!)
~ XI I The serpentine mineral is sometimes called bastite. Orthopyroxenes
Oil may occasionally alter to amphibole, cummingtonite first being formed:
<!) I 2V" \
~ 91 I v
<l) 2V'~
Oil 1\ 8(Mg,Fe)Si0, + H 2 0--. (Mg,Fe)7Si 8 022(0H) 2 + (Fe,Mg)O
~ Rl I I
;::,. I I \
71 A rim of amphibole is formed around the opx crystal and iron ores are
released in the reaction, often seen in basic igneous plutonic rocks.
1/ \ +
6( *BIREFRINGENCE Low with interference colours from low first order greys (En) to yellow
1/ and reds (iron-rich members).
51 I INTERFERENCE Large biaxial figures are seen on sections out at right angles to the (100)
FIG URE
plane +ve or -ve (see data above).
1.8( )
*EXTINCTION All opx have straight extinction on prism edge or main prismatic
/ ANGLE cleavages.
1.78 TWINNING Absent from opx.
/
1.76 / *OTHERS One set of exsolution lamellae is usually present, parallel to prismatic
c /
.:::: / / face (100). Another set may be present at a high angle to this. The
] 1.74 v / explanation has been given under the heading 'Exsolution lamellae' .
~ n'l / DISTING UISHING Orthopyroxenes are distinguished from other clinopyroxenes by their
0 1.72 ,/ FEATU RES parallel extinction. Opx is length slow whereas andalusite is length fast;
>< / / ~ sillimanite, although length slow, has a very small 2V and higher inter-
~ 1. 7( I
/ ~ OCCURRENCE Orthopyroxenes occur in basic igneous rocks of all types. Mg-rich opx
c v ference colours.
1.68
/ / occurs in ultrabasic igneous rocks such as pyroxenites, harzburgites,
v Mg spinel. Orthopyroxenes occur in some regional metamorphic rocks,
1.66 lherzolites and picrites, in association with Mg olivine, Mg augite and
64
1. o 10 20 30 40 50 60 70 80 90 100 particularly charnockites and granulites, and may occur at h!_gh grades
during the thermal metamorphism of argillaceous rocks in hornfelses of
Mg 2 (Si0 ,),
the innermost zones of thermal aureoles.
e~statite I bronzite I persthene ~~~;~;sthenel eulite I rthoferrosilite
y
o
h
Figure 2.27 Variation of 2V angle and indices of refraction in the enstatite-
orthoferrosilite (orthopyroxene) series.
COLO UR Mg-rich opx are colourless; Fe-rich compositions show pale colours
from pale green to pale brown.
PLEOCHROISM Coloured opx show faint pleochroism with a pink to brown, f3 yellow to
2
brown, andy green, due to either Fe +, Ti or AI in the structure.
HABIT Early formed opx in i_gneous rocks appear as short prismatic crystals.
*CLEAVAGE Two good prismatic p 10} cleavages meet at nearly 90° on a basal
section. { 010} and { 100} are poor cleavages or partings.
106 107