Leucite

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

Read more about Leucite:  Leucite Rocks

Other articles related to "leucite, leucites":

Borolanite
... From the analogy of certain leucite syenites which are known in Arkansas, it is very probable that these spots represent original leucites which have been changed into ... They resemble leucite in their shape, but have not yet been proved to have its crystalline outlines ... The pseudo-leucites, as they have been called, measure one-quarter to three-quarters of an inch across ...
Crown (dentistry) - Types and Materials - Restorations Without Metal - Leucite Reinforced
... Popularly known as the "Empress Crown," the leucite reinforced system is superficially similar to a gold crown technique in that a hollow investment pattern is made ... A specially designed pressure-injected leucite-reinforced ceramic is then pressed into the mold by using a pressable-porcelain-oven, as though the final all-ceramic ... the Umeå University in Sweden, led by Göran Sjögren, sought to study the effectiveness of leucite-reinforced crowns ...
Incongruent Melting
... For example, potassium feldspar (KAlSi3O8) decomposes to leucite (KAlSi2O6) when it melts ... Most of the feldspar does melt, a portion of it decomposes to leucite and some quartz (SiO2) is left over, since the chemical formulas of potassium feldspar ...
Lamproite - Nomenclature
... Historic Modern Wyomingite diopside-leucite-phlogopite lamproite Orendite diopside-sanidine-phiogopite lamproite Madupite diopside madupitic lamproite ...
Leucite Rocks
... Rocks containing leucite are scarce, many countries such as England being entirely without them ... necessary that the silica percentage of the rock should be low, since leucite is incompatible with free quartz and reacts with it to form potassium feldspar ... Because it weathers rapidly, leucite is most common in lavas of recent and Tertiary age, which have a fair amount of potassium, or at any rate have potassium equal to or greater than sodium if sodium is ...