Leucite

Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi₂O₆. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric measurements made by Gerhard vom Rath in 1873 led him to refer the crystals to the tetragonal system. Optical investigations have since proved the crystals to be still more complex in character, and to consist of several orthorhombic or monoclinic individuals, which are optically biaxial and repeatedly twinned, giving rise to twin-lamellae and to striations on the faces. When the crystals are raised to a temperature of about 500 °C they become optically isotropic and the twin-lamellae and striations disappear, although they reappear when the crystals are cooled again. This pseudo-cubic character of leucite is very similar to that of the mineral boracite.

Leucite
Leucite crystals in a rock from Italy
General
Category	tectosilicates
Formula (repeating unit)	KAlSi2O6
Strunz classification	9.GB.05
Crystal system	Tetragonal
Crystal class	Dipyramidal (4/m) (same H-M symbol)
Space group	I41/a
Unit cell	a = 13.056, c = 13.751 [Å]; Z = 16
Identification
Color	White to grey
Crystal habit	Commonly as euhedral, pseudocubic crystals; rarely granular, massive
Twinning	Common and repeated on {110} and {101}
Cleavage	Poor on {110}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	5.5 - 6
Luster	Vitreous
Diaphaneity	Transparent to translucent
Specific gravity	2.45-2.50
Optical properties	Uniaxial (+)
Refractive index	nω = 1.508 nε = 1.509
Birefringence	δ = 0.001
References	[1][2]

The crystals are white or ash-grey in colour, hence the name suggested by A. G. Werner in 1701, from λευκος, '(matt) white'. They are transparent and glassy when fresh, albeit with a noticeably subdued 'subvitreous' lustre due to the low refractive index, but readily alter to become waxy/greasy and then dull and opaque; they are brittle and break with a conchoidal fracture. The Mohs hardness is 5.5, and the specific gravity 2.47. Inclusions of other minerals, arranged in concentric zones, are frequently present in the crystals. On account of the color and form of the crystals the mineral was early known as 'white garnet'. French authors in older literature may employ René Just Haüy's name amphigène, but 'leucite' is the only name for this mineral species that is recognised as official by the International Mineralogical Association.