as
garnet and quartz. In a very few crystalline minerals cleavage-planes
can hardly be said to exist. Cleavage must be carefully distinguished
from the planes of union in twin crystals, and the division-planes in
the laminar minerals.
Fracture surfaces are
formed when a mineral breaks in a direction different from the
cleavage-planes. They are consequently most readily observed when the
cleavage is least perfect. The form of the fracture is named conchoidal when composed of concave and convex surfaces like shells, even when nearly free from inequalities. The character of the surface is smooth ; or splintery when covered by small wedge-shaped splinters adhering by the thicker end; or hackly when covered by small slightly-bent inequalities, as in iron and other malleable bodies; or earthy when it shows only fine dust.
Hardness and Tenacity.
The
hardness of minerals, or their power of resisting any attempt to
separate their parts, is also an important character. As it differs
considerably in the same species, according to the direction and the
surface on which the trial is made, its accurate determination is
difficult, and the utmost that can usually be obtained is a mere approximation found by comparing different minerals one with another. For this purpose Mohs has given the following scale:
1. Talc, of a white or greenish color.
2. Rock-salt, a pure eleavable variety, or semi-transparent uncrystallizta gypsum., the transparent and crystallized varieties being generally too soft,
3. Calcareous spar, a eleavable variety.
4. Fluor spar, in which the cleavage is distinct.
5. Apatite ,the aspiragus-stone, or phosphate of lime.
6. Adularia felspar, any eleavable variety. 7. Rock-crystal, a transparent variety.
8. 'Prismatic topaz, any simple variety.
9. Corundum, from India, which affords smooth cleavage surfaces.
10. The Diamond.
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