Diaspore

Diaspore
Diaspore from Slovakia
General
Category	Oxide mineral
Formula (repeating unit)	α-AlO(OH)
Strunz classification	4.FD.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbnm
Unit cell	a = 4.4007(6) Å b = 9.4253(13) Å c = 2.8452(3) Å; Z = 4
Identification
Color	White, pale gray, colorless, greenish gray, brown, pale yellow, pink, purple; may exhibit color change
Crystal habit	Platey, elongated to acicular crystals; also stalactitic, foliated, scaly, disseminated and massive
Twinning	Forms heart shaped twins on {021} or pseudohexagonal aggregates
Cleavage	{010} perfect, {110} distinct, {100} in traces
Fracture	Conchoidal
Tenacity	Very brittle
Mohs scale hardness	6.5 - 7
Luster	Adamantine, vitreous, pearly on cleavage faces
Diaphaneity	Transparent to translucent
Specific gravity	3.1 - 3.4
Optical properties	Biaxial (+)
Refractive index	nα = 1.682 - 1.706 nβ = 1.705 - 1.725 nγ = 1.730 - 1.752
Birefringence	δ = 0.048
Pleochroism	Strong
2V angle	Measured: 84° to 86°, Calculated: 80° to 84°
Dispersion	r < v, weak
Fusibility	Infusible
Solubility	Insoluble
Other characteristics	Decrepitates releasing water in closed tube on heating
References

Diaspore /ˈdaɪəspɔːr/, also known as diasporite, empholite, kayserite, or tanatarite, is an aluminium oxide hydroxide mineral, α-AlO(OH), crystallizing in the orthorhombic system and isomorphous with goethite. It occurs sometimes as flattened crystals, but usually as lamellar or scaly masses, the flattened surface being a direction of perfect cleavage on which the lustre is markedly pearly in character. It is colorless or greyish-white, yellowish, sometimes violet in color, and varies from translucent to transparent. It may be readily distinguished from other colorless transparent minerals with a perfect cleavage and pearly luster—like mica, talc, brucite, and gypsum— by its greater hardness of 6.5 - 7. The specific gravity is 3.4. When heated before the blowpipe it decrepitates violently, breaking up into white pearly scales.

The mineral occurs as an alteration product of corundum or emery and is found in granular limestone and other crystalline rocks. Well-developed crystals are found in the emery deposits of the Urals and at Chester, Massachusetts, and in kaolin at Schemnitz in Hungary. If obtainable in large quantity, it would be of economic importance as a source of aluminium.