HOLOSYMMETRIC CLASS (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 and the vertical axis OZ and the clino-axis OX lie in the plane of symmetry.
All the forms are open, being either pin acoids 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 distin guished as the "clino-pinacoid" and has the indices { o r o } . 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" { ioo } whilst that parallel to the clino-axis is the "basal pinacoid" { ooi } ; pinacoids not parallel to the arbitrarily chosen clino- and vertical axes may have the indices { { 2o1}, { Io2} . . . or { roI},{ 2oI}, { io2 } . . . { 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 { oII }, { 021 10121 . . . { okl }, are called "clino-prisms"; those with edges parallel to the vertical axis, and with the indices { I Io }, { 210 } , { I 20 } . . . { Jiko }, are called simply "prisms." Prisms with edges parallel to neither of the axes OX and OY have the indices { III }, }22I}, 12111, {32I} . . . {hkl} or {III} . . . { and are usu ally 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 #.
Fig. 63 represents a crystal of augite bounded by the clino pinacoid (1), 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 am phiboles, pyroxenes and micas, epidote, monazite, realgar, borax, mirabilite melanterite (FeSO4. and many others; amongst artificial products are monoclinic sulphur, barium chloride potassium chlorate, potassium ferrocyanide ) , oxalic acid , sodium acetate and naphthalene.