Minerals In Thin Section


Formula	Al₂SiO₅

Optic class & sign	Biaxial positive

Optical orientation	a = X, b = Y, c = Z

Optical plane	(010)

Relief	Moderate to high

Refractive indices	n_x = 1.653 -1.661

	n_y = 1.657 -1.662

	n_z = 1.672 -1.683



Birefringence (max.)	0.018 - 0.022



Optic Angle	2V_x

	2V_z = 21 - 30°

Sign of elongation	Length-slow, l (+)

Interference figure	Only sufficiently large crystals yield good interference figures on sections orthogonal or near-orthogonal to c.

Colour / pleochroism	Colourless

Zoning


Form	Habit	Long-prismatic crystals, or felt-like aggregates of intergrown fibres ("fibrolite"); sections orthogonal or near-orthogonal to c show characteristic square- to diamond-shaped outlines

	Surface	Typically euhedral to subhedral, except in high-grade reaction textures where irregular forms may occur

Cleavage		{010} perfect; more commonly observed in sections ⊥ c than sections (sub-)parallel to c

Twinning		None

Extinction		Straight to prism faces


Reaction textures	Replacement of andalusite by sillimanite is a typical prograde feature in low-pressure, high-temperature metamorphic rocks. Retrograde transformation of sillimanite to andalusite is less common.

Alteration / decomposition	Sericite, kaolinite, pyrophyllite


Occurence	Ign	Rare in granites or pegmatites

	Met	Al-rich pelitic rocks in contact- and regional-metamorphic environments; metamorphosed aluminous weathering products such a bauxites; metapelite-derived residual rocks from anatectic environments; sillimanite characterises low-pressure, high-temperature metamorphism under amphibolite- to granulite-facies conditions

	Sed	Detrital in sands

	Hyd	May occur in quartz veins

	Other


Distinctive properties	Form, relief, lower second-order interference colours, straight extinction, sign of elongation, mineral association

Additional comments	As fibrolite crystals are commonly much thinner than standard thin section thickness, second-order interference colours can only be observed where the fibrous aggregates show parallel crystal orientation such that cumulative retardation occurs across the crystal bundle

Sillimanite