ALTERATIONS OF MINERALS OF GARNET GROUP. 307 



where the serpentines and talcs are secondary to garnet. Of course, in 

 many cases the silica, calcium carbonate, and possibly the excess of 

 aluminum hydrate, may be dissolved and transported elsewhere, and thus 

 their absence would be no proof that the compounds were not really pro- 

 duced by the alteration of the garnet. 



The alterations of the various kinds of garnet into different combina- 

 tions of the following minerals, serpentine, talc, chlorite, epidote, and zoisite, 

 magnesite, and gibbsite (equations 2, 3, 4, 5, 6, 7, 9, 11, 12), are all 

 alterations of hydration, and the majority of them of carbonation and 

 desilication. These reactions are notable in the amount of increase in 

 volume, ranging from 36 to 80 per cent. ■ This increase in volume is a 

 natural consequence of the high specific gravity of the garnet. The altera- 

 tions of grossularite to meionite, calcite, and quartz (equation 1), and of 

 pyrope, almandite, and melanite to hornblende, calcite, and quartz (equa- 

 tion 13), are alterations of carbonation and desilication. There can be no 

 better illustrations of reactions characteristic of the zone of katamorphism. 

 It will be seen (pp. 683-685) that the development of garnet is a process 

 of the zone of anamorphism where the jjressure is great and the tempera- 

 ture probably high. Naturally the extensive destruction of garnet is a 

 process of the upper physical-chemical zone. 



The alterations of pyrope to enstatite, spinel, and quartz (equation 8), 

 and of almandite and pyrope together to hypersthene and spinel (equation 

 10), are common reactions. They do not involve hydration. They do, 

 however, involve desilication. The increase in volume for these changes 

 is comparatively small, 12 or 13 per cent. One would expect that these 

 reactions would take place either in the lower part of the belt of cementa- 

 tion or possibly in the upper part of the zone of anamorphism. 



