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AUGITE, an important member of the pyroxene (q.v.) group
of rock-forming minerals. The name (from αὐγή, lustre) has
at various times been used in different senses; it is now applied
to aluminous pyroxenes of the monoclinic series which are
dark-greenish, brownish or black in colour. Like the other pyroxenes
it is characterized crystallographically by its distinct cleavages
parallel to the prism-faces (M), the angle between which is 87°.
A typical crystal is represented in fig. 1, whilst fig. 2 shows a
crystal twinned on
the orthopinacoid (r′).
Such crystals, of short
prismatic habit and
black in colour, are
common as phenocrysts
in many basalts,
and are hence known
as “basaltic augite”;
when the containing
rock weathers to a
clayey material the
augite is left as black
isolated crystals, and
such specimens, usually from Bohemia, are represented in all
mineral collections. Though typical of basaltic rocks, augite is also
an important constituent of many other kinds of igneous rocks, and
a rock composed almost wholly of augite is known as augitite.
It also occurs in metamorphic rocks; for example, in the
crystalline limestones of the Fassathal in Tirol, where the
variety known as fassaite is found as pistachio-green crystals
resembling epidote in appearance.
Chemically, augite resembles diopside in consisting mainly
of CaMgSi2O6, but it contains in addition alumina and ferric
iron as (Mg, Fe″) (Al, Fe″′)2 SiO6; the acmite (NaFe″′Si2O6,)
and jadeite (NaAlSi2O6) molecules are also sometimes present.
Variations in the amount of iron in mixtures of these isomorphous
molecules are accompanied by variations in the optical characters
of the augite.
(L. J. S.)
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