Leucite

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Leucite
Leucite in rock w- nepheline Potassium aluminum silicate Albano Hills Italy 1927.jpg
Leucite crystals in a rock from Italy
General
Category tectosilicates
Formula
(repeating unit)
K[AlSi2O6]
Strunz classification 9.GB.05
Crystal system Tetragonal
Dipyramidal class
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
Crystal symmetry Tetragonal
H-M Symbol: (4/m)
Space group: I41/a
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]
Not to be confused with Lucite.

Leucite is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi2O6]. 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.

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.

References[edit]