ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 277 



produced. It is assumed, further, that the alteration of a pyroxene results 

 in the production of the most closely allied amphibole. Of course this is 

 not always the fact, but it is believed to be usual. Following- this assump- 

 tion, the alteration of diopside is to tremolite, of sahlite is to actinolite, of 

 augite is to hornblende. 



The change from diopside to tremolite may be written as follows: 



(7) 2CaMgSi 2 6 +MgC0 3 =CaMg 3 Si 4 1 ,rCaC0 3 -i-k. 



Regardiug the magnesium carbonates as added in solution and the calcium 

 carbonate as subtracted in solution, the increase in volume is 5.68 per cent. 

 If the magnesium carbonate be considered as present as magnesite, and the 

 calcium carbonate be considered as present as calcite, the increase in volume 

 is 10.55 per cent. 



The change from sahlite to actinolite, supposing the magnesium and 

 iron to be present in the sahlite in equal proportions, is as follows : 



(8) 2Ca 2 MgFeSi 4 12 +FeC0 3 + MgC0 3 =Ca 2 Mg 3 Fe 3 Si s 21 +2CaC0 3 +k. 



Supposing the sahlite and actinolite only to be solids, the increase in vol- 

 ume is 7.28 per cent. If all the compounds are regarded as solids on both 

 sides of the equation, the increase in volume is 10.81 per cent. 



Supposing that in the augite compound there are two of the sahlite 

 molecules to one of the sesquioxide molecule, and supposing that the mao-- 

 nesium and iron are in equal proportions in both the augite and hornblende, 

 the general alteration may be written as follows: 



(9) 2[Ca 2 MgFeSi 4 ]2 . (MgFe) ( AlFe) 2 Si0 6 ] +FeC0 3 -j- MgC0 3 = 



Ca 2 Mg 3 Fe 3 Si s 24 . (MgFe) 2 (AlFe) 4 Si 2 12 +2CaC0 3 -!-k. 



But before the volume relations can be calculated it is necessary to 

 assume definite proportions between Mg and Fe, and Al and Fe, in the 

 second members of the augite and hornblende molecules. If the magne- 

 sium be taken to the iron as 2:1 and the aluminum to the iron as 2:1, 

 an average case, and only the augite and hornblende be considered as 

 solids, the increase in volume is 4.30 per cent. If all the compounds in 

 both equations are solids, the increase in volume is 6.14 per cent. 



An inspection of the above equations giving the alterations of the 

 diopside-augite series to amphibole shows that the chemical change in the 

 alteration of diopside and sahlite to tremolite and actinolite is relatively 

 greater than in the alteration of augite to hornblende. Moreover, if it be 



