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Potassium and aluminium tectosilicate mineral
Not to be confused with
Lucite
.
Leucite
(from the Greek word
leukos
meaning white) is a
rock
-forming
mineral
of the
feldspathoid
group, silica-undersaturated and composed of
potassium
and
aluminium
tectosilicate
KAlSi
2
O
6
.
[4]
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
λευκο?
, '(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
Rene Just Hauy
's name
amphigene
, but 'leucite' is the only name for this mineral species that is recognised as official by the International Mineralogical Association.
References
[
edit
]
Wikimedia Commons has media related to
Leucite
.