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226 PRECIOUS STONES.
required to execute the work, and amongst the Chinese the rough material is greatly sought after and commands a high price.
341. Crocidolite.
Crocidolite
is another member of the Amphibole group and is of interest in that it
gives the characteristic appearance to the precious stones known as
Hawk's Eye and Tiger's Eye, though both of these are in a greater or
lesser degree alteration products of Crocidolite.
Crocidolite
occurs in narrow veins or bands having a transverse closely fibrous
structure due to the parallel arrangement of numerous minute acicular
crystalline growths. The colour of the unaltered mineral is a leek
green. The lustre of one of these fibrous pieces, even when broken, is
distinctly silky. On heating, a little water is driven off, and the
mineral then easily fuses. The specific gravity is 3"2 to 3-3,
but, as will be seen below, the specific gravity of the two varieties
mentioned may fall to nearly that of Quartz. The hardness of
Crocidolite is 4.
In composition it is a sodium ferri-ferrous metasilicate, NaFe (Si08)2, FeSi03.
Infiltration of this fibrous mineral with Quartz gives the variety
Hawk's Eye, of a blue green to indigo blue colour; the process of
infiltration is a gradual one, so all stages are found between what is
an undoubted Amphibole and a substance which may be regarded as a
variety of Quartz showing inclusions. Further by a gradual change the
Crocidolite may be decomposed, its iron passing into the state of the
hydrate (Limonite) and the silica remaining, with possibly addition of
silica from outside, as Quartz, and thus the golden brown stone
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