Page 121 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS PYROXENE GROUP
Clinopyroxenes * l!IREFRINGENCE Moderate, middle second order greens and yellows common.
Diopside (Di) - hedenbergite (Hed) CaMgSi,0 6 -CaFeSi,0 6 monoclinic INTERFERENCE Moderate 2V shown on a (100) section.
Di 1.091:1:0.589, {3 = 105°51' FIGURE
*EXTINCTION Large extinction angle seen in an (010) section. The angle 'Y (slow
Hed 1.091:1:0.584, f3 = 105°25'
ANGLE rayrcleavage is variable to over 45°. Extinction angles for all pyroxenes
are shown in Figure 2.22.
TWINNING Simple and multiple twins common on { 100} and { 001}.
OCCURRENCE Diopside occurs in a wide variety of metamorphic rocks, particularly
metamorphosed dolomitic limestones and calcium-rich sediments.
Diopside forms from the breakdown of the amphibole tremolite as the
temperature increases. Diopside is usually accompanied by forsteritic
olivine and calcite. Hedenbergite occurs in metamorphosed iron-rich
sediments, being found in eulysites and skarns. Diopside may also occur
in some basic extrusive igneous rocks, and hedenbergite may appear in
some acid igneous rock types such as fayalite granite, fayalite ferro-
gabbro, and some granophyres.

monoclinic
Pigeonite Ca(Mg,Fe)Si 2 0 6
f3 = 108°
rarely pigeonite has OAP
parallel to (0 I 0) with b = 13

1.664-1.726
1.672-1.730
1.694-1.751
0.030-0.025
2V, = 50°-62° +ve
OAP is parallel to (010)
D = 3.22-3.56 H = 51/z-61/z
coLouR Diopside is colourless and hedenbergite is brownish green.
PLEOCHROISM Hedenbergites are pleochroic in pale greens and browns but pleochro- I
ism is weak and not a diagnostic feature. I
13
HABIT Occurs as short subhedral prisms.
*cLEAVAGE { 110} good prismatic cleavages meeting on basal section at 87°. Several n. 1.682-1.722
partings { 100}, { 010} and { 001} are present. A diopside with the { 100} np 1.684-1.722
parting well developed is called diallage. n , 1.705-1.751
RELIEF Moderate to high. a = o.023-0.029
ALTERATION Similar to that of the orthopyroxenes. 2V, = 0-30° +ve
Diopsides will alter commonly to fibrous tremolite-actinolite, rarely to OAP is perpendicular to (010)
chlorite. D = 3.30-3.46 H = 6
108 109