ALTERATIONS OF ORTHORHOMBIC PYROXENES. 269 



greater. The calculated increase in volume of the talc and magnetite, as 

 compared with the pyroxene, is 14.68 per cent, provided the average 

 specific gravity of bronzite be used, and 21.73 per cent provided the 

 average specific gravity of hypersthene be used. Probably the real 

 increase in volume is the average of the above, or about 18.20 per cent. 

 In the case of a hypersthene in which the iron is to the magnesium as 

 1:1 the alteration to talc may be as follows, provided the iron separate as 

 magnetite : 



(3) 3JtfgFeSi 2 6 +H 2 Q+0=H 2 Mg 3 SiA 2 +FeA+2Si0 2 +k. 



The increase of volume of the talc, magnetite, and quartz as compared 

 with the hypersthene is 12.84 per cent. 



Serpentine is produced from enstatite by the following reaction : 



(4) 3MgSi0 3 +2H 2 0=H 4 Mg 3 Si 2 9 +Si0 2 +k. 



In case the Si0 2 is dissolved, the increase in volume is 14.25 per cent; 

 if it separates as quartz (rhombohedral ; sp. gr. 2.6535) the increase in 

 volume is 38.36 per cent. 



If a rhombic pyroxene be taken in which the magnesium is to the iron 

 as 3:1 — i. e., stands on the border line between hypersthene and bronzite — 

 serpentine might be produced by the following reaction, with the simul- 

 taneous separation of hematite and quartz : 



(5) 2Mg 3 FeSi 4 1 -.+-tH 2 0+0=2H 4 Mg 3 8i ;! 9 +Fe,03+-lSi0 2 +k. 



Using the specific gravity of hypersthene, in case only serpentine and 

 hematite separate as solids, the decrease in volume is 2 21 per cent, and if 

 the silica separates as quartz the increase of volume is 33.94 per cent. 



Supposing the calcium is to the iron as 1:1 and the excess of iron 

 separates as magnetite, the reaction is : 



(6) 3MgFeSi 2 O s +2H 2 0+0=H 4 Mg 3 SiA+FeA+4Si0 2 +k. 



The increase in volume of the serpentine, magnetite, and quartz as 

 compared with the hypersthene is 20.24 per cent. 



Other reactions may be written which represent the alterations of 

 bronzites and hypersthenes, in which the proportions of magnesium and 

 iron are different. Also reactions may be written in which the oxide of 

 iron forms as magnetite or limonite. Where magnetite forms, the increase 

 in volume would be less than for hematite, and where limonite forms the 

 increase in volume would be considerably greater. 



