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PRECIOUS STONES.
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electrified when rubbed, or in some cases when pressed, and on heating.
The
specific gravity of Topaz lies between 3"40 and 3"65. The fracture is
subconchoidal ; the mineral has a hardness of 8 of Mohs' scale, and is
brittle. The streak is colourless. There is a highly perfect cleavage
normal to the vertical axis of the crystal. The crystalline form is
orthorhombic and the habit prismatic (Fig. 35), the prism usually
showing the faces of two forms at least, and often being vertically
striated, as is well shown in Fig. 36. Crystals often show an apparent
want of symmetry (Fig. 37). Topaz is usually found, when in situ, in
attached crystals in cavities in granite, in some dynamo-metamorphosed
rocks (which have probably been acted on by thermo-metamorphism as
well), and in some acid lavas known as liparites, though the last
occurrence seems rare. It will be seen that Topaz is thus more often
found in rocks rich in Quartz. It is also associated frequently with
Beryl, Tourmaline, and Felspar, especially where these have separated
from a granite magma in the later stages of consolidation. Other
frequent associates are Apatite, Fluor Spar, Cassiterite, and Mica.
Topaz is résistent to most forms of weathering and hence is often found
in rolled pebbles in the detritus from such rocks as have been
mentioned. In composition it is a fluo-silicate of aluminium,
in small quantity as an essential constituent, probably with the hydroxyl group (OH) replacing some of the fluorine.
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