_Potassium._ From hydrofluosilicic solutions,[150] isotropic, colorless crystals of K_{2}SiFl_{6}, in cubes, octahedra or combinations of these forms with the rhombic dodecahedron. Apparently orthorhombic crystals may form from acid solutions and at a low temperature, but if these crystals are dissolved in hot water and recrystallized they will assume the normal forms. Platinic chloride will produce under proper conditions sharp, yellow octahedra of K_{2}PtCl_{6}. [Illustration: FIG. 78.—Fluosilicate of potassium.[151] ] [Illustration: FIG. 79.—Fluosilicate of sodium.[151] ] _Sodium._ From hydrofluosilicic solutions, colorless, very weakly doubly refracting, hexagonal crystals of Na_{2}SiFl_{6}, which are generally longer the higher the percentage of sodium in the solution. This test is very certain even for small amounts. [Illustration: FIG. 80.—Fluosilicate of calcium.[151] ] [Illustration: FIG. 81.—Fluosilicate of magnesium.[151] ] _Calcium._ From hydrofluosilicic solutions, monoclinic crystals of CaSiFl_{6} + 2H_{2}O of various forms, generally spindle-shaped, with not very strong double refraction. The crystals have seldom straight-edged boundaries and are often grouped in rosettes. The addition of dilute H_{2}SO_{4} decomposes the crystals, recrystallization yielding long prismatic crystals of gypsum (distinction from strontium). Treatment with HFl and dilute H_{2}SO_{4} (in excess), producing on evaporation characteristic crystals of gypsum, furnishes a very delicate test for small percentages of calcium. _Magnesium._ From hydrofluosilicic solutions, rhombohedral crystals of MgSiFl_{6} + 6H_{2}O with plane faces and sharp edges. The crystals are colorless and strongly doubly refracting with positive optical character, Fig. 81. The formation of struvite crystals (NH_{4}MgPO_{4} + 6H_{2}O), of coffin-like forms, is very characteristic and takes place from very dilute solutions (rendered alkaline) on the addition of a grain of salt of phosphorus or a drop of sodium phosphate. _Iron._ From hydrofluosilicic solutions, crystals of FeSiFl_{6} + 6H_{2}O, which are isomorphous with those of magnesium salts, with the same optical characters. They may be differentiated by moistening with potassium ferrocyanide or ammonium sulphide, in the first case by turning blue, in the second case black. _Aluminium._ From hydrofluosilicic solutions, not satisfactory on account of the gelatinous formation. When the gelatinous formation is obtained by the action of hydrofluoric acid on an _aluminous silicate_, the staining test can be used to distinguish between fine grains of feldspar and quartz or iolite and quartz. Behrens’ Method.[152] (Hydrofluoric and Sulphuric Acids.) This method depends on common reactions that can be made rapidly, but has the disadvantage of being rather complicated and requiring delicate manipulation.