304 A TREATISE ON METAMORPHISM. 



mation of talc is regarded as passing into magnesite (rhombohedral ; sp. 

 gr. 3.06) or into spinel. The first reaction is — 



(3) 4Mg 3 Al 2 Si 3 12 +15H 2 0+3C0 2 =3H 2 Mg 3 Si 4 12 +3MgC0 3 +8Al(OH) 3 +k. 



The increase in volume of the talc, magnesite (rhombohedral; sp. gr. 3.06), 

 and gibbsite (monoclinic; sp. gr. 2.35) as compared with the pyrope is 75.91 

 per cent. The second reaction is — 



(4) 4Mg 3 Al 2 Si 3 O v ,+6H 2 0=3H 2 Mg 3 Si 4 O v ,+3MgAl,0 4 +2Al(OH) 3 +k. 



The increase in volume of the talc, spinel, and gibbsite as compared with 

 the pyrope is 36.84 per cent, 



The change of pyrope into serpentine is — 



(5) Mg3A] 2 Si 3 12 +5H 2 0=H 4 Mg3Si 2 9 -f2Al(OH) 3 +SiO,+k. 



The increase in volume of the serpentine, gibbsite, and quartz as compared 

 with the pyrope is 81.61 per cent. 



If amesite (hexagonal plates; sp. gr. 2.71) is produced from pyrope 

 the equation is — 



(6) Mg 3 Al 2 Si 3 I2 +2H 2 0+C0 2 =H 4 Mg 2 ALSi0 9 +MgC0 3 +2Si0 2 +k. 



The increase in volume of the amesite, magnesite, and quartz as compared 

 with the pyrope is 62.26 per cent, 



In the alteration of pyrope to chlorite, supposing an intermediate 

 chlorite be taken, the reaction is — 



(7) 3Mg 3 Al 2 Si 3 12 +8H 2 0=H 16 Mg 9 Al 6 Si 5 36 +4SiO 2 +k. 



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

 pyrope is 56.02 per cent, Reactions could be written which represent 

 other varieties of chlorite. 



The change of pyrope to enstatite and spinel is — 



(8) Mg 3 Al,Si 3 12 =2Mg Si0 3 +MgAl 2 4 +Si0 2 +k. 



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

 with the pyrope is 13.51 per cent. 



The alteration of almandite and pyrope to chlorite (aphrosiderite), 



