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210 PRECIOUS STOXE.S.
habit
of the crystal is that of a stout prism terminated by one or more faces
in an oblique direction, the exact angles varying but little in the
different members of the group. All possess two cleavages ; in
Orthoclase these cleavages are at right angles to one another, while in
the other Felspars the angle does not differ widely from a right angle.
The hardness of all is equal to, or slightly exceeds, 6 of Mohs' scale.
The specific gravity varies between 2"5 and 2-9. All are
silicates of aluminium with either potassium, sodium or calcium.
Further, there is a certain amount of gradation seen from one member of
the group to another, and this is accompanied by a corresponding
gradual change in form and physical properties. Few of the Felspars
show transparency, most are translucent to opaque; most are of a light
colour or colourless. Several different members of the group are used
as precious stones under the same designation.
313. Orthoclase.
Orthoclase
occurs in several shades of colour, usually light in tint; rarely it is
colourless, as in the pm*e variety Adularia, which also shows a
moderate degree of transparency, other kinds being translucent to
opaque. Doubly refracting; specific gravity 2"53 to 2-59;
two cleavage planes at right angles to one another, both well marked.
Streak colourless. Its crystalline form belongs to the monosym-metric
system; the crystals are sometimes attached, often crystallised from a
semi-fluid magma and then frequently conrpletely developed. It is an
essential rock-forming constituent of several volcanic rocks. In
chemical composition it is an aluminium potassium silicate, K20, A1203, 6 Si02.
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