ALTERATIONS OF MINERALS OF GARNET GROUP. 305 



supposing- the Fe:Mg:: 2:1, about the proportion shown by analysis in the 

 case of the Lake Superior chlorite at the Spurr mine," is — 



(9) 4Fe 3 Al 2 Si 3 O 12 .2Mg 3 Al 2 Si 3 O 12 +15H 2 O=3H 10 Fe 4 Mg 2 Al 4 Si 4 O 25 +6SiO 2 +k. 



The increase in volume of the aphrosiderite and quartz as compared with 

 the garnet is 50.98 per cent. 



The alteration of almandite and pyrope to hypersthene and spinel, 

 supposing the Mg: Fe :: 1 : 1 in the hypersthene, is as follows: 



( 10 ) Fe 3 Al. 2 Si 3 12 . 2Mg 3 Al 2 Si 3 12 =3MgFeSi,0 6 + 3Mg A.l 2 4 +3Si0 2 + k. 



The increase in volume of the hypersthene, spinel, and quartz, as compared 

 with the garnet, is 12.66 per cent. If a hypersthene be produced which is 

 less rich in iron, the amount of pyrope molecule in the original garnet 

 must be increased. 



The alteration of grossularite and melanite to epidote, supposing an 

 average epidote be produced, in which the Al: Fe:: 2: 1 is probably — 



(11) 20a 3 Al 2 Si s O 12 .Ca 3 Fe 2 Si 3 O 12 +5C0 2 +H 2 O=2HCa 2 Al 2 FeSi 3 O 13 +5CaC0 3 +3SiO. 2 +k. 



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

 2.7135), and quartz, as compared with the garnet, is 40.88 per cent. 

 Similar equations can be written in which the pyrope molecule takes the 

 place of the grossularite molecule in large part. In this case magnesite, 

 instead of calcite, would be produced. Other reactions could be written 

 for the formation of epidote, in which the original molecule is a combination 

 of grossularite, pyrope, and melanite. The simplest case is as follows: 



(12) Ga 3 Al 2 Si 3 12 . Mg 3 Al 2 Si 3 12 . Ca5Fe 2 Si 3 12 + H 2 +5C0 2 = 



2HCa 2 Al 2 FeSi 3 13 +2CaC0 3 +3MgC0 3 +3Si0 2 +k. 



In this case the increase in volume of the epidote, calcite, magnesite, and 

 quartz, as compared with the garnet, is 39.53 per cent. 



The reaction for the passage of pyrope, almandite, and melanite into 

 hornblende may be written in many ways, depending upon the composition 

 of the particular hornblende produced. Taking the case of an average 

 hornblende, in which there are five of the actinolite molecules to two of the 



oPenfleld, S. L., and Sperry, F. L., On pseudomorphs of garnet from Lake Superior and 

 Salida, Colo.: Am. Jour. Sci., 3d ser., vol. 32, 1886, pp. 307-311. 

 MON XLVII — 04 20 



