500 Johnston, Merwin* and Williamson — 



above ; and there is no doubt that of these Wells' figures are 

 the most trustworthy, followed by those of Kendall, which 

 agree with the former as closely as could be expected under 

 the circumstances. 



No attempt was made to determine the solubility of /z-CaC0 3 , 

 as, since all of our products contained some admixture of cal- 

 cite or aragonite, the results would have been illusory. But it 

 is quite certain that, under ordinary laboratory conditions, it is 

 less stable, and consequently more soluble, than either calcite 

 or aragonite. 



The Relative Stability of the Several Forms. 



Calcite. — At ordinary pressure calcite is the stable form into 

 which each of the others* ultimately transforms throughout 

 the range of temperature hitherto investigated, i. e., at temper- 

 atures higher than 0°, below which the rate of reaction is so 

 slow that it is difficult to determine the relation with any cer- 

 tainty. Calcite has at each temperature a definite dissociation 

 pressure of C0 5 , which is about 0*l mm at 500° and reaches one 

 atmosphere at 900° f and is a measure of the tendency of the 



reaction CaC0 3 > CaO 4- C0 2 to progress. If the progress 



of this reaction is inhibited by a sufficient pressure of CO„, 

 calcite goes over reversibly at 970° into a-CaC0 3 4 the 

 form of which is very similar to that of calcite ; the latter 

 melts at 1290°, the pressure of C0 2 required to inhibit dis- 

 sociation being then about 110 atm4 



Aragonite. — Pure dry aragonite persists for an indefinite 

 time at ordinary temperature ; but this persistence is only an 

 apparent stability conditioned by the extremely slow rate of 

 transformation under these circumstances. This rate increases 

 with temperature ; the transformation is complete at 470° in a 

 few minutes, at 425° in 90 minutes,§ whereas three hours heat- 

 ing at 400° induced no appreciable change in the aragonite. 

 There is of course no transformation point in this region, the 

 change being irreversible ; such figures as are given above 

 represent merely the temperatures at which the rate became 

 appreciable under the conditions of experiment. The calcite 

 which results when dry aragonite is heated retains the external 

 form of the aragonite, but is easily distinguished by the refrac- 

 tive index ; when a prism or aggregate of aragonite on inver- 

 sion yields a single paramorphic crystal of calcite, the latter 

 may extinguish parallel to the original extinction of the aragon- 



* It is possible that aragonite containing other material in solid solution 

 may not be unstable with respect to calcite ; see p. 509 postea. 



f See Johnston, J. Am. Chein. S03., xxxii, 938, 1910; also So3inan, Hos- 

 tetter and Merwin, J. Wash. Acad., v, 563, 1915. 



JH. E. Boeke, Neues Jahrbuch, 1912. i, 91. 



gSosnian, Hostetter and Merwin, J. Wash. Acad., v, 563, 1915. 



