[Illustration: FIGS. 1-6.--Crystals of Calcite.]

Crystals of calcite are extremely varied in form, but, as a rule, they may
be referred to four distinct habits, namely: rhombohedral, prismatic,
scalenohedral and tabular. The primitive rhombohedron, r {100} (fig. 1), is
comparatively rare except in combination with other forms. A flatter
rhombohedron, e {110}, is shown in fig. 2, and a more acute one, f {11-1},
in fig. 3. These three rhombohedra are related in such a manner that, when
in combination, the faces of r truncate the polar edges of f, and the faces
of e truncate the edges of r. The crystal of prismatic habit shown in fig.
4 is a combination of the prism m {2-1-1} and the rhombohedron e {110};
fig. 5 is a combination of the scalenohedron v {20-1} and the rhombohedron
r {100}; and the crystal of tabular habit represented in fig. 6 is a
combination of the basal pinacoid c {111}, prism m {2-1-1}, and
rhombohedron e {110}. In these figures only six distinct forms (r, e, f, m,
v, c) are represented, but more than 400 have been recorded for calcite,
whilst the combinations of them are almost endless.

Depending on the habits of the crystals, certain trivial names have been
used, such, for example, as dog-tooth-spar for the crystals of
scalenohedral habit, so common in the Derbyshire lead mines and limestone
caverns; nail-head-spar for crystals terminated by the obtuse rhombohedron
e, which are common in the lead mines of Alston Moor in Cumberland;
slate-spar (German _Schieferspath_) for crystals of tabular habit, and
sometimes as thin as paper: cannon-spar for crystals of prismatic habit
terminated by the basal pinacoid c.

Calcite is also remarkable for the variety and perfection of its twinned
crystals. Twinned crystals, though not of infrequent occurrence, are,
however, far less common than simple (untwinned) crystals. No less than
four well-defined twin-laws are to be distinguished:--

[Illustration: FIG. 7-10.--Twinned Crystals of Calcite.]

i. Twin-plane c (111).--Here there is rotation of one portion with respect
to the other through 180 deg. about the principal (trigonal) axis, which is
perpendicular to the plane c (111); or the same result may be obtained by
reflection across this plane. Fig. 7 shows a prismatic crystal (like fig.
4) twinned in this manner, and fig. 8 represents a twinned scalenohedron v
{20-1}.

ii. Twin-plane e (110).--The principal axes of the two portions are
inclined at an angle