The
following brief notes as to the physical or mechanical properties of
minerals not already discussed or described must suffice .
Form.—The forms
of crystals are all referable to one or other of these six
crystallographic systems, (i) The cubic or monometric; (2) the
pyramidal, dimetric, or tetragonal ; (3) the rhombohedral or hexagonal
; (4) the prismatic, trimetric, or orthorhombic ; (5) the oblique or
monoclinic ; (6) the triclinic or anorthic.
Structure.—The
mode of mechanical aggregation or intimate texture of minerals may
often be learnt by disruption of the mass, or by splitting or cleaving
it. Structure is often crystalline, laminar, fibrous, or columnar.
Fractured, not cleaved, surfaces are less instructive—they may be
conchoidal, uneven, splintery, or hackly.
Transparency.—For
want of a more comprehensive term, the various degrees of resistance to
the transmission of light through minerals are included under this
title. The degrees are five :
Transparent—when
objects can be seen distinctly. Semi-transparent—when objects can be
seen dimly. Translucent—when light, not objects, can be seen.
Subtranslucent—when light is transmitted through thin
splinters. Opaque—when light is not transmitted.
Lustre.—This
character, although it needs some practice to discern it accurately, is
of importance as an element not merely of the,beauty but also in the
discrimination of precious stones. The terms employed to designate its
various qualities are these :
