ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 275 



Supposing- all the compounds separated as solids, the increase in volume is 

 56.32 per cent. If only the serpentine and quartz remain as solids, the 

 increase in volume is 0.44 per cent. 



The change from sahlite to ferriferous bastite, provided that in both 

 compounds the magnesium is to the iron as 3 : 1, is — 



(4) 3Ca i Mg 3 FeSi 8 2 4+12C0 2 +8H 2 0=H l6 Mg 9 Fe s Si 8 Os,+16Si0 2 +12CaC0 3 +k. 



If all the compounds separate as solids, the increase in volume is 56.41 per 

 cent; if the bastite and quartz remain as solids, 1.93 per cent. Supposing 

 the calcium, magnesium, and iron were in equal proportions in the sahlite, 

 and that in the bastite the magnesium were to the iron as 3 : 1, the equation 

 may be written — 



(5) 9CaMgFeSi,0 9 +9COj+8H ! 0+20=H w Mgi,Fej8i 8 SG +2Fe,0 4 +9CaCO,+19SiO,+k. 

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

 as compared with the sahlite, is 37.50 per cent. 



It is, of course, not impossible that serpentine shall develop as one of 

 the products from augite. In this case it doubtless forms from the sahlite 

 molecule of the augite compound, the sesquioxide compounds passing into 

 some other mineral. It hardly seems advisable to attempt to write equa- 

 tions representing such an alteration. 



In writing equations for the alterations of the aluminous pyroxenes into 

 chlorite and epidote it is necessary that certain assumptions shall be made 

 in reference to the relative proportions of the various elements. Moreover, 

 if equations are written which produce chlorite alone, a large amount of the 

 sesquioxide bases must be left over. If an equation be written for the forma- 

 tion of epidote, a large amount of magnesium is unaccounted for. Since it 

 is very common for the minerals chlorite and epidote to form simultaneously, 

 an equation is written on this supposition. In order to give defmiteness to 

 the compound, it is sujsposed that there are twice as many molecules of the 

 diopside part of the augite molecule as of the other part. Furthermore, it 

 is supposed for the diopside molecule that the magnesium is to the iron as 

 2:1; and for the other molecule that the aluminum is to the iron sesquioxide 

 as 3:1. An epidote is taken in which the aluminum is to the iron as 2:1. 

 A chlorite between clinochlore and prochlorite is taken, as such a chlorite 

 is at about the middle of the series. As may be seen by reference to the 

 analyses of augites and epidotes, the proportions taken represent about their 



