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(3) 4H 6 O. 5SiO 8 , or the equivalent fluorine compound, 4H 2 O. 2SiO 2 . 3SiF 4 . The first group is common to all species. The first two together form the magnesian micas without fluorine ; the first and third yield the muscovite and lepidolite series containing fluorine ; while the combination of all three groups gives the magnesia-fluorine and iron-lithia micas. Clarke (1889) regards all micas as substitution products of a normal orthosilicate, A1 4 (Si0 4 ) 3 , in which one, two, or three atoms of aluminium may be replaced by their equiva- lents. In micas containing fluorine the groups MgF and A1F 2 act as univalent. Where there is a defect of silica, the univalent A1O and MgOH are supposed to enter. If there be excess of silica, H 4 Si0 4 is partially replaced by H 4 Si 3 8 . Hence micas are isomorphous mixtures of salts varying in composition from A1 3 R" 3 (Si0 4 ) 3 to A1R% (Si0 4 ) 3 , and from A1 3 R' 3 (Si 3 8 ) 3 to A1 3 R" 9 (Si 3 8 ) 3 . CERAMIC CHEMISTRY. 5 Whatever be the real chemical classification of the micas, they are usually divided into two groups, the white micas, which are silicates of aluminium with alkalies and iron ; and the black micas, in which magnesia and iron are more conspicuous. The first group includes muscovite, lepidolite, and paragonite ; the second, biotite, phlogopite, and zinnwaldite. In granite, the most important mica is muscovite, the potash mica, which has the percentage composition : K 2 O 3.1-12.4,Na 2 O 0.0-4.1,FeO 0.0-1.2, Fe a O 3 0.5-8.8,MgO 0.4-3.1