Fig. 62.—Monoclinic Axes and
Hemi-pyramid.		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 {101}, {201}, {102} ... {hol}, 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 {111} ... {hkl}, 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 β.

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 (Na2SO4·10H2O), 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.

Hemimorphic Class

(Sphenoidal).

In this class the only element of symmetry is a single dyad axis, which is polar in character, being dissimilar at the two ends.

The form {010} perpendicular to the axis of symmetry consists of a single plane or pedion; the parallel face is dissimilar in character and belongs to the pedion {010}. The pinacoids {100}, {001}, {hol} and {hol} parallel to the axis of symmetry are geometrically the same in this class as in the holosymmetric class. The remaining forms consist each of only two planes on the same side of the axial plane XOZ and equally inclined to the dyad axis (e.g. in fig. 62 the two planes XYZ and XYZ); such a wedge-shaped form is sometimes called a sphenoid.

Fig. 64.—Enantiomorphous Crystals of Tartaric Acid.

Fig. 64 shows two crystals of tartaric acid, a a right-handed crystal of dextro-tartaric acid, and b a left-handed crystal of laevo-tartaric acid. The two crystals are enantiomorphous, i.e. although they have the same interfacial angles they are not superposable, one being the mirror image of the other. Other examples are potassium dextro-tartrate, cane-sugar, milk-sugar, quercite, lithium sulphate (Li2SO4·H2O); amongst minerals the only example is the hydrocarbon fichtelite (C5H8).