_Potassium._—Fragments containing potash give isotropic clear cubes, or octahedra of low refracting power, or combinations of these forms with each other and with rhombic dodecahedra. These crystals have the composition K₂SiF₆. Their forms are shown[166] in Figs. 46 and 47. In case much sodium be present, the first crystals obtained may be strongly double refractive rhombohedra, but on dissolving in water and allowing to recrystallize, the normal forms will be obtained. If the crystals be dissolved in hydrochloric or sulfuric acids, and treated with platinum chlorid, the characteristic yellow octahedral crystals of K₂PtCl₆ will be obtained. Ammonium and cesium compounds also give this reaction. _Lithium._—When fragments containing lithium are treated with the solvent mentioned, monoclinic crystals are produced on drying. These crystals dissolved in sulfuric acid and freed from calcium sulfate by treatment with potassium carbonate give aggregates of lithium carbonate resembling a snowflake. At a high temperature lithium solutions treated with sodium phosphate give spindle-shaped crystals of lithium phosphate. The double lithium aluminum silicofluorid is shown in Fig. 48. The ease with which traces of lithium may be detected by the spectroscope renders unnecessary any further description of its microchemical reactions. _Calcium._—Nearly all mineral particles, save quartz grains, contain calcium. When these particles are dissolved by treatment with hydrofluosilicic acid, they form on drying hydrated monoclinic crystals of calcium silicofluorid (CaSiF₆ + 2H₂O). These crystals assume many forms, some of which are shown in Figs. 49 and 50. These crystals are easily decomposed by sulfuric acid, the well-known long prismatic crystals of gypsum taking their place. On treatment of silt particles containing lime with hydrofluoric and sulfuric acids, only a part of the lime passes into solution if the content thereof be large. Where but little lime is present and the sulfuric acid is in large excess, all the lime passes into solution and the characteristic gypsum crystals appear as in Fig. 51. _Magnesium._—Rhombohedral crystals of magnesium silicofluorid separate from the solution of particles containing magnesium in hydrofluosilicic acid. They have the composition MgSiF₆6H₂O and their common forms are shown in Fig. 52. Quite characteristic also are the crystals of struvite (NH₄MgPO₄ + 6H₂O), which are produced in a very dilute solution of the magnesium compound first obtained by carefully adding ammonium hydroxid and chlorid until a faint alkaline reaction is produced, and then placing a drop of dilute sodium phosphate at the edge of the solution. The crystals should be allowed to form slowly in the cold. Their form is shown in Fig. 54. [Illustration: FIGURE 38. PHOTOMICROGRAPHS OF SILT PARTICLES. ]