Page 136 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS STAUROLITE

Sphene Nesosilicate *BIREFRINGENCE Extreme, but colours tend to be masked by body colour and high relief
of mineral. (This mineral does not change appearance under crossed
Sphene CaTiSiO.(O,OH,F) monoclinic polars.)
0.755:1 :0.854, ,8 = 119°43'
INTERFE REN CE 2V is small; the small size of most crystals, coupled with the high
FIGURE dispersion of light, means that a good figure is not often obtained.
*TWINNING Single twins with { 100} twin plane common.
'I note th at the OAP section OCCU RREN CE It is a common accessory mineral in igneous and metamorphic rocks,
I particularly plutonic intermediate and acid rocks, such as diorite,
is lozenge shaped
granodiorites and granites and also alkali igneous rocks such as
nepheline-syenites. It is common in some skarns and in metamorphic
basic schists and gneisses. It is rare in sediments.

Staurolite Nesosilicate
Staurolite (Fe,Mg) 2 (Al,Fe'+) 9 0 6 Si 4 0, 6 (0 ,0H), orthorhombic
0.473 : 1:0.341
c=y
I
I
I
I
I
I

n. = 1.843-1.950
n p = 1.879-2.034
n, = 1.943-2.110
8 = 0.100-0.192
2V, = 17°-40° (usually 23°-34°) +ve
OAP is parallel to (010)
D = 3.45 H = 5
coLOuR Colourless, pale brown, dark brown.
PLEOCHROISM Well displayed in sphenes from alkali igneous plutonic rocks, with a
yellow or colourless, .B pinkish, yellow brown, y pink, yellow, orange
brown. 010
*HA BIT Anhedral to euhedral, often occurring as small lozenge or diamond
shaped crystals.
*CLEAVAGE {110} good.
* RELIEF Extremely high. n. = 1.739-1.747
ALTERATION Sphene alters to leucoxene, an aggregate of quartz and rutile to which n p = 1.745-1.753
ilmenite also alters, as follows: n, = 1.752-1.761
8 = 0.013-0.014
2CaTiSiO.(O,OH,F)--> 2Ti0 2 + 2Si0 2 + 2Ca(O,OH) 2V, = 82°-90° +ve
OAP is parallel to (100)
leucoxene D = 3.74-3.85 H = 71f2
124 125