ALTERATIONS OF MINERALS OF GARNET GROUP. 303 



about the garnets. It seems probable that in such cases with the alman- 

 dite there is also present the pyrope molecule, and the reactions for the 

 formation of chlorite, spinel, and hypersthene, after almandite, as written 

 include the pyrope molecule. In the case of the Spurr mine chlorite, 

 secondary to garnet, the species has been determined to be aphrosiderite" 

 (massive; sp. gr. 2.90). 



Alterations of spessartite, melanite, and uvarovite, as pxire species, are 

 not described in the standard text-books. 



Common garnet most frequently alters into chlorite. Often also it 

 changes into epidote (monoclinic; sp. gr. 3.38) or into hornblende (mono- 

 clinic; sp. gr. 3.26). The mixture of almandite and pyrope altering' into 

 aphrosiderite, and into hypersthene and spinel, may be considered as alter- 

 ations of common garnet. Where epidote is produced it is probable that 

 the molecules from which it is derived are a mixture of grossularite and 

 melanite. Where hornblende is produced it is probable that the molecules 

 are a mixture of pyrope. almandite, and melanite. In the alterations of 

 the common garnets any of the iron oxides, magnetite (isometric; sp. gr. 

 5.174), hematite (rhombohedral ; sp. gr. 5.225), or limonite (amorphous; 

 sp. gr. 3.80), may be produced. 



The change from grossularite to meionite may be written as follows: 



(1) 3Ca 3 Al 2 Si 3 12 +5C0 2 =Ca 4 Al 6 Si 6 2 5+5CaC0 3 +3SiO,+k. 



The increase in volume of the meionite, calcite (rhombohedral; sp. gr. 

 2.7135), and quartz (rhombohedral; sp. gr. 2.6535) as compared with the 

 grossularite is 54.62 per cent. 



The change of grossularite to zoisite may be written as follows: 



(2) 3Ca 3 Al 2 Si 3 12 +5C0 2 +H 2 0=2HCa ; ,Al 3 Si30i 3 +5CaC0 3 +3Si0 2 +k. 



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

 with the grossularite is 40.49 per cent. 



The alteration of pyrope to talc may be written in two ways, depend- 

 ing upon whether the excess of magnesium over that required for the for- 

 ts Pumpelly, Raphael, On pseudomorphs of chlorite after garnet: Am. Jour. Sci., 3d ser., vol. 10, 

 1875, pp. 1-4. Penfield, S. L., and Sperry, F. L., Pseudomorphs of garnet from Lake Superior and 

 Salida, Colo.: Am. Jour. Sci., 3d ser., vol. 32, 1SS6, pp. 307-311. 



