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 this issue. 



GEOPHYSICS. — The hydr-othermal formation of silicates, a review. 



George W. Morey and Paul Niggli. J, Am. Chem. Soc, 35: 



10S6-1130. 1913. 

 This is a discussion of the theoretical principles underlying the be- 

 havior of water-silicate systems at temperatures ranging up to 500°, 

 followed by an annotated bibliography in which are assembled all of 

 the data relating to hydrothermal syntheses. These data unfortu- 

 nately afford practically no- reliable quantitative information; quali- 

 tatively, even, they leave much to be desired, for many minerals have 

 been obtained but once by a given investigator, in a manner which was 

 not reproducible and under conditions which were not specified. The 

 minerals which have been most commonly obtained are chiefly those 

 which are stable — or at any rate phanerostable — over a wide range of 

 •conditions; for example, quartz and the feldspars. In all cases the 

 'Crystals obtained are very small, so that accurate chemical analysis is 

 usually out of the question; their identification by optical methods may 

 even be doubtful. 



Hydrothermal syntheses, like the paragenetic relationships investi- 

 gated by Van't HofT, are determined by the solubility relations of all 

 the possible solid phases which may be formed from the components 

 present in the solution, even altho the concentration of these components 

 in the solution is vanishingly small. Many of the reactions are, without 

 doubt, practically restricted to the solid phase, altho they take place 

 thru the medium of the solution. These solubility relationships are 

 thus not simple ; but study of the question is further complicated by the 

 frequent appearance of metastable phases, which again is coordinated 

 with the rates of the various possible reactions. Now, as is well known, 

 rates of reaction are often affected very greatly by factors which in 

 other respects are of altogether minor importance; hence slight differ- 

 ences, e.g., in the composition, or even in the texture or fineness of 

 grain, of the initial solid phase — may exert considerable influence on the 



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