Leucite is a
rock-forming
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
of the
feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing ...
group, silica-undersaturated and composed of
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
and
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
tectosilicate KAlSi
2O
6.
Crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
s have the form of cubic icositetrahedra but, as first observed by Sir
David Brewster
Sir David Brewster KH PRSE FRS FSA Scot FSSA MICE (11 December 178110 February 1868) was a British scientist, inventor, author, and academic administrator. In science he is principally remembered for his experimental work in physical optics ...
in 1821, they are not optically isotropic, and are therefore pseudo-cubic.
Goniometric measurements made by
Gerhard vom Rath
Gerhard vom Rath (20 August 1830 – 23 April 1888), was a German mineralogist, born at Duisburg in Prussia.
Biography
Rath was educated at Cologne, at Bonn University, and finally at Berlin, where he graduated Ph.D. in 1853. In 1856 he became ...
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
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...
or
monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
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
The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by ...
is 5.5, and the
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its dens ...
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
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Feldspathoid
Potassium minerals
Aluminium minerals
Tetragonal minerals
Minerals in space group 88