182 J. BarreU — BeJations of Subjacent Igneous 



The pressure and temperature relations wMch control 

 the mutual displacing power of CO2 and SiOs form a 

 problem somewhat analogous to, and yet different from, 

 that discussed for hydration and dehydration. It has 

 recently been discussed with the aid of a diagram by 

 Goldschmidt,-^ who bases his law upon the curve of vapor 

 tension of calcium carbonate. He does not, however, take 

 up the aspect more especially developed here — the in- 

 crease in the vapor pressure of carbon dioxide which 

 would be due to magmatic emanation coming in from 

 below and without free escape above. By raising the 

 vapor pressure, the formation of carbonates is theo- 

 retically favored. Nevertheless, in the case of such 

 hydrated rocks as serpentines and chlorite schists, the 

 carbonates are of secondary importance and consist of 

 intermixed calcite. This is true notwithstanding the 

 added fact that carbonates possess in general smaller 

 molecular volume than the corresponding silicates.-- It 

 would appear, therefore, that such carbonates as form 

 are largely eliminated because of the possibilities of the 

 slow escape of the gases and the solubility of the car- 

 bonates in water with abundant carbonic acid. It would 

 appear further, however, that where such hydrous sili- 

 cates as serpentine, talc, chlorite, sericite, kaolin, epidote, 

 and zeolites are formed, either carbon dioxide was only 

 sparingly present in the hydrating waters or else carbon 

 dioxide can not displace silica from any oxides except 

 lime and soda and in part potassa at those temperatures 

 where hydration goes on to a greater or less extent, and 

 especially hydration by magmatic waters. Konigsberger 

 mentions further that in water solution at 260'' C. silica 

 displaces carbonic acid from lime. This laboratory evi- 

 dence, as well as the mineral relations just cited, appears 

 to show that silica displaces carbon dioxide at tempera- 

 tures well below those determined by Goldschmidt by 



^^ V. M. Goldschmidt, Die Gesetze der Gesteinsmetamorphose mit Beispielen 

 aiis der Geologie des siidlichen Noi-wegens, 1912. 



^- [The reaction is also discussed by Johnston and Niggli in their paper 

 on the principles underlying metamorphic processes (Jour. Geology, 21, 481- 

 516, 588-624, 1913). In most of their discussion they assume that CO2 may 

 escape as produced. They find that temperature is much more important in 

 determining the direction of the reaction than pressure. However, '4t is 

 true that pressure is required in order to retain the volatile components. ' ' 

 They further emphasize the important distinction between uniform and non- 

 uniform pressure, and show on pages 614-615 that stress or non-uniform 

 pressure will cause reactions between carbonates and silica with the devel- 

 opment of CO2 to a much gieater extent than would be likely in the absence 

 of stress,] 



