310 A TKEAT1SE ON METAMORPHISM. 



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

 as compared with the olivine is 37.13 per cent. Supposing the Mg and Fe 

 are present in equal proportions, the equation stands — 



(4) 3Mg 2 Fe 2 Si 2 8 +4H 2 0+2 0=2H 4 Mg 3 Si 2 9 +2Fe 3 4 +2Si0 2 +k. 



In this case, the olivine of which nearly corresponds to that of many rocks, 

 the increase in volume is 12.43 per cent. 



It would be easy to write other equations for different proportions of 

 magnesium and iron in the olivine, but this seems unnecessary. Also it 

 would be easy to write reactions by which other forms of iron compounds 

 than magnetite are produced, such as siderite, hematite, and limonite. If 

 this be done, and the volume reaction calculated, it will be found that the 

 increase in volume is still greater than when magnetite forms. 



Olivine is described by Becke as passing into anthophyllite (where 

 Mg : Fe : : 4 : 1, 3:1, etc., orthorhombic; sp. gr. 3.1-3.2). If the proportion 

 of the magnesium to the iron be taken as 3 : 1 in both the olivine and the 

 anthophyllite, the reaction may be written as follows: 



(5) Mg 3 FeSi 2 8 +2Si0 2 =Mg 3 FeSi 4 12 - k. 



The decrease in volume of the anthophyllite as compared with the original 

 olivine and quartz is 1.48 per cent. 



Various authors have also described the alteration of olivine into 

 actinolite. Supposing that the magnesium is to the iron as 3 : 1 in both the 

 olivine and the actinolite, and supposing the calcium to be derived from 

 carbonate and the silica from quartz, the reaction is as follows: 



( 6 ) 3Mg 3 FeSi 2 8 + 4CaC0 3 + 10SiO 2 =Mg 9 Fe 3 Ca 4 Si 16 48 +4CO,- k. 



The decrease in volume of the actinolite as compared with the olivine, 

 calcite, and quartz, is 13.34 per cent. 



In some instances the alteration into actinolite is described as taking 

 place in connection with feldspar as a reaction rim. In this case the calcuim 

 may be supposed to be derived from anorthite, as calcium silicate. The 

 aluminum may be supposed to pass into common spinel and hercynite 

 (isometric; sp. gr. 3.93), which are well known to be alteration products of 

 olivine. The reaction may be: 



(7) •4Mg 3 FeSi,0 8 +4CaA],SL0 8 =Mg 9 Fe 3 Ca 1 Sii 6 O iS +3MgAl 2 4 T-FeAl 2 4 -k. 



