306 A TREATISE ON METAMORPHISM. 



aluminous molecules, in which the MgO: FeO:: 2:1, and A1 2 3 : Fe 2 3 :: 3:1, 

 the reaction is as follows: 



(13) 3[2Mg 3 Al 2 Si 3 12 .Fe 3 Al 2 Si 3 12 .Ca 3 Fe 2 Si 3 12 ] +4C0 2 = 



5CaMg 2 FeSi 4 1 2-2[(Mg 1 Fe 2 )(Al 9 Fe 3 )Si 6 36 ]+4CaC0 3 +4Si0 2 +k. 



The increase in volume of the hornblende, calcite, and quartz, as compared 

 with the garnet, is 24.55 per cent, 



The alterations of the ferriferous garnets frequently produce iron 

 carbonate or iron oxides. • No reactions are written to illustrate these 

 changes; nor would it be easy to express these alterations by reactions 

 without knowing what becomes of the remainder of the garnet material. 



Of course, the alterations which are written above, instead of taking 

 place separately, may occur simultaneously. Thus the garnet may be a 

 complex one, which contains molecules of several of the simple garnets, 

 and there would be simultaneously produced a considerable number of 

 secondary minerals. Thus, chlorite and hornblende, chlorite and epidote, 

 or epidote and hornblende, might be simultaneously produced. For defi- 

 nite cases such as these, reactions might be written by combining the reac- 

 tions for the production of the individual minerals. 



An examination of the equations as written shows that in almost all 

 cases, simultaneously with the production of the minerals which are recorded 

 as secondary to garnet, quartz also appears, and in some cases calcium 

 carbonate also must separate, which may be deposited in the form of calcite. 

 Less frequently siderite and iron oxide form. It is well known that with 

 the minerals chlorite, epidote, hornblende, etc., secondary to garnet, quartz, 

 and calcite are often found, and that with serpentine, talc, spinel, hyper- 

 sthene, and enstatite, quartz is often found. However, the quartz and calcite 

 are usually not regarded as derived from the garnet and called minerals 

 secondary to them. But the equations clearly show that these minerals 

 should be regarded as secondary to garnet, just as certainly as epidote, 

 chlorite, etc. The almost universal presence of quartz with the minerals 

 mentioned, and the frequent presence of calcite, are thus completely 

 explained. The equations also seem to demand in the alteration to serpen- 

 tine and talc that gibbsite or diaspore shall be produced. However, some 

 of the alumina may unite with silica and water and form kaolin. The 

 equations suggest that a search be made for gibbsite, diaspore, and kaolin 



