510 Johnston, Merivhl, and Williamson — 



transformation of aragonite to calcite is accompanied by a 10 

 per cent increase of volume, so that rocks which initially con- 

 tained any considerable proportion of aragonite that subse- 

 quently transformed to calcite, should show evidences of this 

 fact. 



/u-CaCO s . — There have not been enough geological observa- 

 tions to enable one to discriminate between aragonite and 

 /x-CaC0 3 in recent deposits, so that the evidence does not war- 

 rant a definite conclusion as to the occurrence in nature of the 

 /-i-form : but our laboratory experiments lead us to believe that 

 it is formed in many places, but soon changes to calcite or 

 aragonite. That it has not been recognized is due to two facts, 

 namely : that, in presence of water, it persists presumably only 

 for a period measurable in days or weeks since the minute 

 crystals formed in the laboratory transform in a short time, 

 and that the color tests do not enable one to differentiate it 

 from aragonite. 



We have collated some of the characteristic properties of the 

 three forms of pure calcium carbonate, and what little is 

 known of the range of variation of the properties exhibited by 

 the natural minerals ; the results are presented in Table 4, 

 which shows at a glance the resemblances between them. 



Recapitnlation. 



Under ordinary conditions, calcium carbonate appears in 

 three crystalline anhydrous forms, viz., as calcite, aragonite, 

 and a form which we have designated u-CaC0 3 . The other 

 reputed forms, including "vaterite'' and "amorphous" 

 CaCO s , are not definite forms ; their divergent properties are 

 due mainly to differences in size of grain, and mode of aggrega- 

 tion. 



Of these three established forms, calcite is, at ordinary 

 pressure, the stable one at all temperatures from 0° (or lower) 

 up to 970°, at which temperature it inverts reversibly to 

 a-CaC0 3 ; under these conditions aragonite and the yu-form are 

 always unstable with respect to calcite, though there is an in- 

 dication that aragonite has a stable field of existence at about 

 — 100° or lower. Under all ordinary conditions, therefore, 

 pure aragonite tends to go over into calcite ; in how far it 

 actually does so depends upon the rate of this process under 

 the particular circumstances. There is therefore no definite 

 transition point; the interval required for the transformation 

 at 100° in presence of water and calcite is measured in days, 

 and for its inversion at or about 400° is measured in hours. 

 By reason of this instability, one cannot specify the factor or 

 factors which determine the precipitation of CaCO, as ara- 

 gonite ; indeed, its appearance would seem to be a matter of 



