238 



THE GEOLOGIST. 



conversion into Clilorite and Steatite, the alcalies present in Tour- 

 maline are separated, and the Mg constantly increased. 



41. Garnet [1.0]. This mineral, so widely variable in its components, is 



yet— unlike Tourmaline— remarkable for the regularity of its chemical 

 relations, all the various replacements of its constituents being in 

 strict accordance with the doctrme of isomorphism. Its O.Q. is con- 

 stant, and likewise the oxygen-ratio of 11 to S = 1 : 1, answering to 

 the formula II^ Si^ + U Si, where R is Ca, Fe, Mg, Mn ; and 

 M, Al or Fe. The following examples will illustrate the common 

 variations. The common (alumina-lime) garnet contains principally 

 Al (20), Oa (32), with Fe (8). The noble {alvmina-iron) garnet 

 {Almandine\ principally J^e (40), Al (20). The black (magnesian) 

 variety Al (22), Mg (13), Fe (9), Mn (6), Ca (6). The manganese- 

 garnet contains ]\ln (33), Al (18), Fe (15). The iron-lime variety has 

 Fe (31), Ca (28), Mg (7). Two other varieties (Pyrope and Uwaro- 

 wite) contain respectively 9 per Cent, and 23 per cent, of the oxide 

 of Chromium. Thus every important base, both protoxide and 

 ■ sesqui-oxide, which each in their characteristic varieties reach to a 

 large per centage, in tm-n disappears, being replaced by their 

 isomorphs. 



Like all minerals containing considerable quantities of Ca and Fe, 

 the varieties of Garnet with these bases are very liable to decom- 

 position. The principal pseudomorphs of Garnet are Chlorite, Ser- 

 pentine, and Steatite. 



42. Idocrase, Vesuvian [1.0]. Like Garnet, to which Idocrase bears 



a great similarity, and with which it is frequently associated, the 

 O.Q. is 1.0 ; but the oxygen-ratio of the to K bases is probably 

 different, or nearer 3 : 2. The mineral consists essentially of a 

 silicate of Ca (33), Al (16), Fe (7), and 'M.g (5), replacing each other, 

 and with sometimes a httle Fe and Mn. 



43. CoRDiERiTE, lolite, Diehroit [.8] ; Finite; FaUunite ; and others. 



Cordierite is not frequently met with : its essential constituents are 

 a sihcate of Al (31), ^l^g (10), and Fe (8). The oxygen-ratio of the 

 and E bases is 2 : 3. This mmeral is principally important by 

 bemg regarded by Haidinger, Bischof, and others as the initial 

 member of a series of minerals, including Fahlunite and Finite, and 

 ending in Mica. The alteration to which Cordierite is subject has one 

 essential featm-e,— the elimination of the Mg, and the introduction of 

 water and alcalies. In one series of alterations the water only is 

 introduced ; but in the other (which includes Fahlunite and Finite) 

 the alcalies come in, and attain the maximum in pinite, which has 

 K (8). Rammelsberg considers the ratio of oxygen in the 'Si and Al 



