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CHAPTER XIII.
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376. OLIVINE----SPHENE.
Some of
the minerals known to the ancients as Beryllus, Smaragdus, and Topazios
may have been Olivine ; but their Chrysolithus was our Topaz, although
Olivine is now known sometimes as Chrysolite. When undecomposed,
Olivine is some shade of green, usually olive-green or greyish-green,
but on exposure to weathering it turns brown or red through the
formation of ferric salts. It is transparent in the gem varieties, and
has a vitreous lustre. It shows strong double refraction, the refractive indices
in yellow light being, for the greatest 1.697, and for the least 1.661.
Dispersion is weak, and it is only feebly dichroic. When heated it
turns white, but does not fuse easily. The specific gravity is 3'33 to
3.44. The fracture is conchoidal, and the mineral is brittle. It has
two cleavages, neither very perfect, normal to the two horizontal axes
of the crystal. The hardness is 6-1/2 to 7 ; the streak is usually
white. It is orthorhombic in crystalline form, and commonly occurs in
stout rhombic prisms (Fig. 28). Sometimes twinned forms are seen ; more
often it occurs as embedded grains, thus often allotriomorphic. It is
practically always found, when in situ, in basic eruptive rocks as an original
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