Page 50 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS AbSiOs POL YMORPHS
*cLEAVAGE { 100} and { 010} very good. Parting present on { 001}. Sillimanite AlzSi0 5 orthorhombic
*RELIEF High : the high relief, which is easily seen if the section is held up to the 0.980 : 1:0.757
light, is a very distinctive feature.
ALTERATION As andalusite. Kyanite often occurs within large ' knots' of micaceous c=-y
minerals ; it also inverts to sillimanite with increasing temperature.
BIREFRINGENCE LOW.
*ExTINCTION Oblique on cleavages and prism edge; y ' prism edge is - 30°.
INTERFERENCE (100) sections give Bxa figures; but as with andalusite an isotropic
FIGURE section should be obtained and a single isogyre used to obtain sign and
size of 2V.
TWINNING Multiple twinning occurs on { 100}.
OTHER FEATURES The higher birefringence and excellent { 100} cleavage, intersected by
the { 001} parting on the prism face, help to distinguish kyanite from
andalusite and other index minerals.
•occuRRENCE See after sillimanite.

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/
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a=a

n. 1.654-1.661
n p 1.658-1.662
ny 1.678-1.683
a o.019-0.022
2Vy = 21°-30° + ve (a prism section is length slow)
OAP is parallel to (010)
D = 3.23-3.27 H = 61f2-1lh
OLOUR Colourless.
*HABIT It occurs as elongate prisms in two habits: either as small fibrous crystals
found in regionally metamorphosed schists and gneisses, or as small
prismatic crystals growing from andalusite in thermal aureoles.
•< 1 EAVAGE :010} perfect: thus a basal section of sillimanite, which is diamond
shaped, has cleavages parallel to the long axis.
~EUEF Moderate.
1\ 1 II,KAI"ION Rare.
'llttct 1 KIN<;ENCE Moderate.
1 \ ttNC"IION Straight on single cleavage trace.
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