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Ch. 12: Beryl - Garnet

Ch. 12: Beryl - Garnet Page of 311 Ch. 12: Beryl - Garnet Text size:minus plus Restore normal size   Mail page  Print this page
244
PRECIOUS STONES.
refracting, hence not showing dichroism. The refractive index for yellow light varies from 1'747 to 1'814; the dispersion is small in all except Demantoid. Almandine shows absorption bands in the spectrum ; some Garnets show asterism. Most Garnets fuse more or less easily, but Uvarovite is almost infusible. The specific gravity varies from 3º5 to 4"3 in the various kinds. The fracture is conchoidal to subconchoidal or uneven, and most speci­mens are brittle. The hardness is from 6-1/2 to 7-1/2, An indistinct cleavage parallel to the faces of the dodecahedron is sometimes present ; the streak is white.
The crystalline form is in all Garnets cubic, and the commonest forms are the dodecahedron (Fig. 23), and tris-octahedron (Fig. 24). The crystals, when in situ, are more often found embedded in a matrix, and they are then usually idiomorphic ; less often they are found attached at one point to the wall of a cavity. Garnets are usually of secondary origin, developed sometimes in metamorphosed calcareous rocks, more often in eruptive rocks (Figs. 25 and 26).
In chemical composition the Garnets are orthosilicates of the formwhere R" may represent
Ch. 12: Beryl - Garnet Page of 311 Ch. 12: Beryl - Garnet
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