Minerals In Thin Section


Formula	Al₂SiO₅

Optic class & sign	Biaxial negative

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

Optical plane	Subparallel to a and inclined about 30° to both b and c

Relief	High

Refractive indices	n_x = 1.710 -1.718

	n_y = 1.719 -1.724

	n_z = 1.724 -1.734

	-

Birefringence (max.)	0.012 - 0.016

	-

Optic Angle	2V_x = 78 - 84°

	2V_z

Sign of elongation	Length-slow, l (+)

Interference figure	{100} sections yield acute bisectrix figures with melatopes well outside the field of view. Few isochromatic rings in optic axis figures

Colour / pleochroism	Commonly colourless, also pale blue and pleochroic with absorption Z > Y > X, whereby X = colourless, Y = light violet-blue, Z = light cobalt blue; colour distribution may be patchy

Zoning


Form	Habit	Bladed, columnar, also acicular

	Surface	Typically subhedral to euhedral

Cleavage		{100} perfect, {010} distinct; parting on {001}

Twinning		Simple or multiple twinning on {100} common, multiple twins on {001} less common

Extinction		Max extinction angle 30° for {100} sections; sections orthogonal to {100} show near-parallel extinction


Reaction textures	Replacement of andalusite by kyanite resulting from presssure increase during metamorphism, or from polymetamorphism

Alteration / decomposition	Sericite, kaolinite, pyrophyllite


Occurence	Ign	Rare in granite and pegmatites

	Met	Al-rich pelitic rocks from intermediate- or high-pressure environments, eclogites, high-pressure granulites, metabauxites

	Sed	Detrital in sands

	Hyd	May occur in quartz veins

	Other


Distinctive properties	High relief, birefringence, characteristic cleavage, twinning

Additional comments	Intermediate- to high-pressure Al₂SiO₅ polymorph

Kyanite