270 A TREATISE ON METAMORPHISM. 



If in the formation of bastite, a pyroxene be taken which stands 

 intermediate between bronzite and hypersthene — i. e., in which the magne- 

 sium and iron are as 3:1 — and if the same proportions of these constituents 

 be supposed to hold in the bastite, the reaction is as follows: 



(7) 3Mg 3 FeSi.,0 1; ,+8H 2 = H 16 Mg 9 Fe 8 Si 8 3l i+'lSi0. 2 +k. 



Using the specific gravity of hypersthene, if the silica be dissolved the 

 increase of volume is 22.77 per cent (if the specific gravity of bronzite be 

 employed, 15.65 per cent); if the silica separates as quartz, 46.87 per cent. 

 Similar reactions may be written which represent the formation of bastites 

 which are richer and poorer in iron, in which cases the volume changes are 

 slightly different. 



The passage of ferriferous rhombic pyroxene into anthophyllite may 

 be one of pure paramorphism, since in anthophyllite the proportions of 

 magnesium to iron have ranges paralleled by bronzite and hypersthene. 

 Therefore, the only necessary change is a molecular one, a mineral being 

 produced of lower symmetry and lower specific gravity as a result of the 

 alteration. If the specific gravity of hypersthene be used, the calculated 

 increase in volume due to the lower specific gravity of the resultant 

 mineral is 8.70 per cent. 



In the formation of actinolite from a rhombic pyroxene, it is necessary 

 that lime and silica be added. Supposing the magnesium is to the iron as 

 3:1 in both the rhombic pyroxene and actinolite, the equation is as follows: 



(8) 3Mg 3 FeSi. 1 O 12 +4CaCO 3 +4SiO 2 =Mg a Fe 3 Ca 4 Si 10 O, 18 +4CO, + k. 



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

 calcite, and quartz is 7.40 per cent if the specific gravity of hypersthene 

 be used, and if that of bronzite is 10.77 per cent. Similar equations may 

 be written in which the proportions of magnesium and iron are different. 



The changes of the rhombic pyroxenes to talc involve reactions of 

 carbonation and hydration, or of hydration and oxidation, or of all three 

 together. The changes of the rhombic pyroxenes to serpentine and bastite 

 involve hydration alone, or hydration and oxidation. All take place with 

 increase of volume and liberation of heat. 



Corresponding with these facts, as a matter of observation the devel- 

 opment of serpentine, bastite, and talc from the rhombic pyroxenes takes 

 place in the zone of katamorphism. The development of talc is especially 

 characteristic of the belt of weathering, and serpentine and bastite of the 



