274 A TREATISE ON METAMORPHISM. 



However, perhaps the most frequent and characteristic of the altera- 

 tions of the diopside-augite series is uralitization or change to amphibole 

 (monoclinic; sp. gr. 2.9-3.4). This process is particularly characteristic of 

 the ancient igneous rocks, and especially those which are under compara- 

 tively deep-seated conditions, although the alteration is by no means con- 

 fined to deep-seated rocks. It occurs on a great scale under the conditions 

 of the transformation of the igneous rocks into schists and gneisses. During 

 the process of uralitization epidote also very frequently forms. Not infre- 

 quently also magnetite (isometric; sp. gr. 5.16-5.18) and calcite separate. 

 In some cases the change is accompanied by the development of a feldspar, 

 such as albite (triclinic; sp. gr. 2.62-2.65). The kind of amphibole which 

 forms depends upon the variety of the pyroxene. From diopside, tremolite 

 (monoclinic; sp. gr. 2 9-3.1) is the ordinary product; from sahlite, actino- 

 lite (monoclinic; sp. gr. 3.0-3.2) is normally to be expected; from diallage 

 and omphacite (according to Zirkel," varieties of augite), smaragdite (a 

 variety of hornblende) is ordinarily produced; and from ordinary augite, 

 hornblende (monoclinic; sp. gr. 3.05-3.47) is usually developed. Finally, 

 it not infrequently occurs that augite changes directly into biotite (mono- 

 clinic; sp. gr. 2.90). 



The change of diopside to talc may be written as follows: 



(1) 3CaMgSiA+3C0 2 f H 2 0=H 2 Mg 3 Si 4 12 +3CaC0 3 +2Si0 2 +k. 



The increase in volume, supposing all compounds remain as solids, is 48.74 

 per cent. If only the talc remains, the decrease is 30.13 per cent. Sup- 

 posing the diopside were one in which a part of the calcium and magnesium 

 were replaceable by iron, so that the calcium and magnesium and iron are 

 present in equal proportion, thus approaching sahlite in composition, and 

 supposing the iron to pass into magnetite, the reaction is — 



(2) 3CaMgFeSi 3 9 +3C0 2 +H 2 0+0=H 2 Mg 3 Si 4 12 +3CaC0 3 +FeA+5Si0 2 +k. 



The increase in volume of the talc, calcite, magnetite, and quartz, as com- 

 pared with the diopside, is 27.88 per cent. 



The change of diopside to serpentine may be represented by the 

 following equation : 



(3) 3CaMgSi 2 O e +3C0 2 +2H 2 0=H 4 Mg 3 Si 2 O a +4Si0 2 +3CaC0 3 +k. 



a Kallmann, C. F., and Zirkel, F., Elementeder Mineralogie, Leipzig, 1898, p. 690. 



