Minerals In Thin Section


Formula	Al₂SiO₅

Optic class & sign	Biaxial negative

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

Optical plane	(010)

Relief	Moderate positive

Refractive indices	n_x = 1.629 -1.640

	n_y = 1.634 -1.644

	n_z = 1.638 -1.650

	n increases with increasing Fe³⁺ and Mn³⁺

Birefringence (max.)	0.009 - 0.012

	In the andalusite-viridine series Δn decreases from the Al endmember (max Δn) to zero at about 6.5 mol% Mn + Fe, then increasing again where optic sign is positive (see under viridine).

Optic Angle	2V_x = 71 - 88°

	2V_z

Sign of elongation	Length-fast, l (-)

Interference figure	Acute bisectrix figures from sections ⊥ c with melatopes outside the field of view, due to high 2V

Colour / pleochroism	Commonly colourless; may show pink, yellow or green colour, then pleochroic with X = pink or yellow, Y = Z = colourless, pale yellow or pale green; colour intensity increases towards viridine; chiastolite varieties may have a distinct coloured-pleochroic core zone

Zoning


Form	Habit	Elongate-prismatic to short-columnar; also irregular forms; square-shaped (pseudotetragonal) sections ⊥ c of euhedral crystals

	Surface	Euhedral crystals are common in graphitic rocks; subhedral to anhedral crystals occur more typically in non-graphitic rocks

Cleavage		2 sets of {110} prismatic cleavage at nearly 90°

Twinning		Rare on {101}

Extinction		Symmetrical to {110} prism faces and cleavage in basal sections; straight to prism faces and cleavage in sections parallel to c


Reaction textures	Replacement of andalusite by sillimanite or kyanite, resulting from prograde metamorphism or polymetamorphism. Retrograde transformation of sillimanite or kyanite to andalusite is much less common.

Alteration / decomposition	Sericite, kaolinite, pyrophyllite


Occurence	Ign	Peraluminous granites; rare in pegmatites

	Met	Low-pressure Al-rich pelitic rocks in contact- and (low-pressure) regional-metamorphic environments

	Sed

	Hyd	May occur in quartz veins

	Other


Distinctive properties	Relief, low Δn, extinction characteristics, l (-), pseudo-tetragonal form if euhedral, association with other Al-silicates The chiastolite variety shows a unique and characteristic inclusion pattern mainly along the diagonals of the pseudo-tetragonal prism, forming a feathery cross in sections orthogonal to c. Accumulation of graphite on prism faces can be observed in carbonaceous rocks.

Additional comments	In schists, andalusite tends to form inclusion-rich porphyroblasts.

Andalusite