(Holohedral; Prismatic).
Here there is a single plane of symmetry perpendicular to which is a
dyad axis; there is also a centre of symmetry. The dyad axis coincides
with the ortho-axis OY, and the vertical axis OZ and the clino-axis OX
lie in the plane of symmetry.
[Illustration: FIG. 62.--Monoclinic Axes and Hemi-pyramid.]
[Illustration: FIG. 63.--Crystal of Augite.]
All the forms are open, being either pinacoids or prisms; the former
consisting of a pair of parallel faces, and the latter of four faces
intersecting in parallel edges and with a rhombic cross-section. The
pair of faces parallel to the plane of symmetry is distinguished as
the "clino-pinacoid" and has the indices {010}. The other pinacoids
are all perpendicular to the plane of symmetry (and parallel to the
ortho-axis); the one parallel to the vertical axis is called the
"ortho-pinacoid" {100}, whilst that parallel to the clino-axis is the
"basal pinacoid" {001}; pinacoids not parallel to the arbitrarily
chosen clino- and vertical axes may have the indices {101}, {201},
{102} ... {hol} or {1'01}, {2'01}, {1'02} ... {h'ol}, according to
whether they lie in the obtuse or the acute axial angle. Of the
prisms, those with edges (zone-axis) parallel to the clino-axis, and
having indices {011}, {021}, {012} ... {okl}, are called
"clino-prisms"; those with edges parallel to the vertical axis, and
with the indices {110}, {210}, {120} ... {hko}, are called simply
"prisms." Prisms with edges parallel to neither of the axes OX and OY
have the indices {111}, {221}, {211}, {321} ... {hkl} or {1'11} ...
{h'kl}, and are usually called "hemi-pyramids" (fig. 62); they are
distinguished as negative or positive according to whether they lie in
the obtuse or the acute axial angle [beta].
Fig. 63 represents a crystal of augite bounded by the clino-pinacoid
(l), the ortho-pinacoid (r), a prism (M), and a hemi-pyramid (s).
The substances which crystallize in this class are extremely numerous:
amongst minerals are gypsum, orthoclase, the amphiboles, pyroxenes and
micas, epidote, monazite, realgar, borax, mirabilite (Na2SO4.10 H2O),
melanterite (FeSO4.7H2O) and many others; amongst artificial products
are monoclinic sulphur, barium chloride (BaCl2.2H2O), potassium
chlorate, potassium ferrocyanide (K4Fe(CN)6.3H2O), oxalic acid
(C2O4H2.2H2O), sodium acetate (NaC2H3O2.3H2O) and naphthalene.