The Several Forms of Calcium Carbonate. 483 



about 1 per cent) of the crystals was practically the same as 

 that of the well-formed crystals which separated at 60° C. 



Closer examination showed that these needles and associated 

 forms* were really aggregates of nearly parallel fibers ; we con- 

 clude therefore that the differences in apparent refractive in- 

 dex are to be ascribed to porosity. 



These experiments indicate that it is the sulphate, and not 

 the magnesia or other radicles present, which induces the 

 formation of aragonite in the sea, though it may well be that, 

 concurrently with this, other factors operate in the same 

 direction ; and it is entirely possible that to this admixture of 

 sulphate is due the apparent preservation of aragonite as such 

 for long periods in sea- water. 



Aragonite when dry can be kept for an apparently indefinite 

 period at ordinary temperature, as at 400° the inversion 

 requires several hours. In presence of water pure aragonite 

 transforms through solution very slowly ; a sample shaken up 

 with water at 25° only began to show an appreciable change 

 after 31 days. In another experiment aragonite enclosed with 

 water in a sealed tube was heated on a steam bath for two 

 weeks and then left at ordinary temperature for upwards of 

 two years ; when examined, it proved to be still mainly ara- 

 gonite although a few rhombs of calcite bad formed on the walls 

 of the tube. In accordance with this slow rate of transforma- 

 tion, we found that we could form crystals of aragonite on the 

 faces of a large calcite crystal, merely by placing the latter in 

 the beaker during the precipitation ; cases of this phenomenon 

 are known in nature. This cannot be done in presence of any 

 appreciable amount of really fine-grained calcite, in which case 

 the precipitate will consist largely or wholly of calcite, even 

 under conditions which otherwise would yield aragonite ; f 

 indeed we have attributed the failure of some of our experi- 

 ments to the presence of calcite dust in the laboratory atmos- 

 phere. The presence of calcite grains to serve as nuclei, from 

 whatever source derived, will hasten naturally the transforma- 

 tion of aragonite in water ; that such grains were present is the 

 only plausible explanation of certain cases in which the trans- 

 formation went rapidly. Similarly the inability of Footed to 

 conduct his determinations of relative solubility at temperatures 

 higher than about 50° indicates that some calcite was present 



* Aggregates with similar shape and structure, but different refraction, 

 appear also in precipitates of calcite. 



fCf. Vetter, Z. Kryst., xlviii, 45, 1911, who found that, whereas nuclei of 

 aragonite might fail to bring it down under conditions under which it would 

 not normally come down, nuclei of calcite always resulted in the precipitation 

 of calcite. 



JH. W. Foote, Z. physik. Chem., xxxiii, 740, 1900. 



