1911 Encyclopædia Britannica/Crocidolite

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CROCIDOLITE, a mineral described in 1815 by M. H. Klaproth under the name Blaueisenstein (blue ironstone), and in 1831 by J. F. Hausmann, who gave it its present name on account of its nap-like appearance (Gr. κροκύς, nap of cloth). It is a blue fibrous mineral belonging to the amphibole group and closely related to riebeckite; chemically it is an iron sodium silicate. Its resemblance to asbestos has gained for it the name Cape Asbestos, the chief occurrence being in Cape Colony. The mineral suffers alteration by removal of alkali and peroxidation of the ferrous iron, and further by deposition of silica between the fibres, or by their replacement by silica; a hard siliceous mineral is thus formed which when polished shows, in consequence of its fibrous structure, a beautiful chatoyance or silky lustre. This is the ornamental stone which is known when blue as “hawk’s-eye,” and when of rich golden brown colour as “tiger-eye.” The latter, which represents the final alteration of the crocidolite, has become very fashionable as “South African cat’s eye,” and is often termed “crocidolite,” though practically only a mixture of quartz with brown oxide of iron. The following are analyses by A. Renard and C. Klement of the unaltered crocidolite and of the blue and brown products of alteration:—

  Crocidolite. Hawk’s-eye. Tiger-eye.
 Silica 51.89  93.45  93.05 
 Ferric oxide 19.22  2.41 4.94
 Alumina · · 0.23 0.66
 Ferrous oxide   17.53  1.43 · ·
 Magnesia 2.43 0.22 0.26
 Lime 0.40 0.13 0.44
 Soda 7.71 · · · ·
 Potash 0.15 · · · ·
 Water 2.36 0.82 0.76
Total 101.69  98.69  100.11 

Another alteration product of the crocidolite, consisting of silica and ferric hydrate, has been called griqualandite. Crocidolite and the minerals resulting from its alteration occur in seams, associated with magnetite and other iron-ores, in the jasper-slates of the Asbestos Mountains in Griqualand West, Cape Colony. It is known also from a few other localities, but only in subordinate quantity. (See Cat’s-Eye.)