MICA. |----------------| Potash. Magnesia. Silica 46.36 42.65 Alumina 36.80 12.96 Peroxide of iron 4.53 --- Protoxide of iron --- 7.11 Oxide of manganese 0.02 1.06 Magnesia --- 25.75 Potash 9.22 6.03 Hydrofluoric acid 0.70 0.62 Water 1.84 3.17 ---- ---- 99.47 99.35 Mica undergoes decomposition with extreme slowness, as is at once illustrated by the fact that its shining scales may frequently be met with entirely unchanged in the soil. Its persistence is dependent on the small quantity of alkaline constituents which it contains; and for this reason it is observed that the magnesian micas undergo decomposition less rapidly than those containing the larger quantity of potash. Eventually, however, both varieties become converted into clay, their magnesia and potash passing gradually into soluble forms. Hornblende and augite are two widely distributed minerals, which are so similar in composition and properties that they may be considered together. Of the former two varieties, basaltic and common have been distinguished, and their composition is given below:-- Hornblende. |----------------| Common. Basaltic. Augite. Silica 41.50 42.24 50.12 Alumina 15.75 13.92 4.20 Protoxide of iron 7.75 14.59 11.60 Oxide of manganese 0.25 0.33 -- Lime 14.09 12.24 20.55 Magnesia 19.40 13.74 13.70 Water 0.50 -- -- ---- ---- ---- 99.24 97.05 99.67 In these minerals alkalies are entirely absent, and their decomposition is due to the presence of protoxide of iron, which readily absorbs oxygen from the air, when the magnesia is separated and a ferruginous clay left. The minerals just referred to, constitute the great bulk of the mountain masses, but they are associated with many others which take part in the formation of the soil. Of these the most important are the zeolites which do not occur in large masses but are disseminated through the other rocks in small quantity. They form a large class of minerals of which Thomsonite and natrolite may be selected as examples-- Thomsonite. Natrolite. Silica 38.73 48.68 Alumina 30.84 26.36 Lime 13.43 -- Potash 0.54 0.23 Soda 3.85 16.00 Water 13.09 9.55 ---- ---- 100.48 100.83