Page 122 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS PYROXENE GROUP

*OPTICAL Very similar to diopside and augite except for 2V which is small ( < 30°). OAP is parallel to (010)
PROPERTIES A section cut parallel to face (101) will show a good Bxa figure. The D = 2.96-3.52 H = 5-6
section is difficult to recognise but the interference colours will be very
low (first order greys) and two cleavages will be present, meeting at an COLOUR Augite is colourless to pale brown. Titanaugite (Ti-augite) is pale
purple.
angle of less than 90°.
PLEOCHROISM In most varieties pleochroism is very weak but titanaugite is weakly
OCCURREN CE Pigeonite only occurs in rapidly chilled rocks. In most igneous rocks
pleochroic with a pale green, {3 pale brownish and y pale greenish
which have undergone slow cooling pigeonite is inverted to ortho-
purple.
pyroxene.
HABIT Variable from subhedral prismatic crystals in basic plutonic rocks to
euhedral crystals in basic extrusive rocks.
Augite (ferroaugite) Ca(Mg,Fe,Mn,Fe'•,AJ,Ti),(Si,Al),0 6 monoclinic
*CLEAVAGE Similar to diopside, with { 110} good and poor { 100} { 010} partings.
1.092 : 1:0.584, f3 = 105°50'
RELIEF Moderate to high.
c Augite ALTERATION Similar to diopside.
I BIREFRINGENCE Moderate, with maximum interference colours being low second order
I (also aegirine-augite)
I (blues, greens).
I
I * INTERFERENCE Good Bxa figure seen on the plane including the a and c axes, about the
I FIGURE
I position of the face (101).
*EXTINCTION Similar to diopside, yAel large (up to 45°+ ).
ANGLE
TWINNING Similar to diopside.
*OTHERS Hourglass zoning is occasionally seen on certain prismatic sections
especially in titanaugite.
DISTINGUISHING Augite is virtually indistinguishable from diopside. It may show a
FEATURES slighter smaller 2V. Augite is found in mafic and ultramafic plutonic
igneous rocks, whereas diopside occurs mostly in metamorphic rocks
and basic volcanics.
OCCURRENCE Augites occur mainly in igneous rocks and are essential mineral con-
---- - ----- b = 13
stituents of gabbros, dolerites and basalts. In plutonic gabbros augites
frequently occur with orthopyroxenes (as already described under
'Crystallisation trends'). Augitic cpx have been recognised in some very
high grade metamorphic rocks (granulites).

n. = 1.662-1.735 Refractive indices change depending upon the
+ ve I
np = 1.670-1.741 Mg: Fe ratio, and also on the amount and lattice
ny = 1.688-1.761 position of the minor constituents AI, Ti and
8 = 0.018-0.033 Fe'•. For example AI (Ti and Fe) in tetrahedral
2V 'Y = 25° -55° co-ordination (occupying Si sites) will increase
2V and lower Rls, whereas in the octahedral
sites (occupying Mg and Fe positions) they will
decrease 2Vand increase Ris. These factors may
affect compositions determined from optical
properties by - 5%
110 111