Crystals - A. E. H. Tutton

A triclinic substance is represented in Fig. 32, Plate VIII., potassium ferricyanide, K₆F₂(CN)₁₂. The triply oblique nature of the symmetry is clearly exhibited by this salt, the absence of any right angles being very marked.

PLATE VII.
Fig. 30.—Monoclinic Crystals of Ammonium Magnesium Sulphate.

Fig. 31.—Monoclinic Crystals of Sodium Potassium Carbonate.
Monoclinic Crystals growing from Solution.

PLATE VIII.
Fig. 32.—Triclinic Crystals of Potassium Ferricyanide.

Fig. 33.—Tetrahedral Crystals of Sodium Sulphantimoniate, Cubic Class 28.
Crystals growing from Solution.

Fig. 33, also on Plate VIII., illustrates more particularly a class of one of the systems, the cubic, which is of lower than holohedral (full) systematic symmetry. This is the case also with hydrogen potassium tartrate and ammonium magnesium phosphate, but the forms shown of those salts on the slides represented in Figs. 28 and 29 are chiefly those which are also common to the holohedral classes of their respective systems, and the lower class symmetry is not emphasised. But here in Fig. 33, representing Schlippe’s salt, sodium sulphantimoniate, Na₃SbS₄.9H₂O, we have very clear development of the tetrahedron, belonging to the lowest of the five classes (class 28) of the cubic system. The crystals are almost all combinations of two complementary tetrahedra, one of which is developed so very much more than the other that the faces of the latter only appear as minute replacements at the corners of the predominating tetrahedron.