The Several Forms of Calcium Carbonate. 501 



ite, but inclined extinction is also observed. Such relations 

 indicate that transformation once started in a coherent mass of 

 aragonite proceeds through the mass and that the orientation of 

 the particles of aragonite encountered by the calcite as it 

 grows does not have a determining influence on the orientation 

 of the calcite. 



In contact with water pure aragonite persists as such for an 

 indefinite time at ordinary temperatures ; but, on the other 

 hand, in presence of calcite grains, it goes over into calcite at 

 a rate depending mainly on the temperature. We were unable 

 to detect any calcite amongst crystals of pure (artificial) 

 aragonite which had been in contact with water at 100° for 

 two weeks : the same material, after being in contact with 

 water at room temperature for two years, contained some cal- 

 cite, but the bulk of it still remained unchanged. This fact is 

 opposed to statements of some previous investigators ; the 

 transformation which they observed is due without doubt to a 

 lack of purity of their material. In order to ascertain the 

 behavior of pure aragonite in contact with water at higher 

 temperatures, a number of experiments were made, the results 

 of which are summarized below ; the mode of experiment was 

 to place 0*2 g. aragonite, together with the amount of water 

 noted in the first column, in a gold crucible contained within 

 a leak-proof steel bomb* (the volume of which was 70 cc ) which 

 was then heated in an electric furnace to the desired tempera- 

 ture. 



This table shows that the rate is not rapid — from a labora- 

 tory standpoint — unless the temperature is higher than 300° ; 

 and that the rate of transformation of pure aragonite to calcite 

 is greater in presence of water. The results at 340° are not 

 quite consistent, presumably by reason of the presence of a 

 little calcite; but since, as noted above, this lowest temperature 

 has no especial significance, it was not deemed worth while to 

 endeavor to secure thoroughly consistent results. The table 

 indicates moreover that aragonite yields calcite rhombs by trans- 

 formation when liquid water is present, but inverts to para- 

 morphs in water vapor as in air. 



The evidence which we have adduced demonstrates con- 

 clusively that puref aragonite is unstable with respect to calcite 

 at all temperatures above 0° under atmospheric pressure.^ 

 Whether it has a region of real stability at higher pressures or 

 at temperatures below 0° is an open question about which 

 little can be said. It may be mentioned, however, that the 



* For details see Morey, J. Am. Chem. Soc, xxxvi, 217, 1914. 



fWith respect to impure aragonite, see postea p. 509. 



\ The conclusions reached by Vaubel relative to the constitution of calcite 

 and aragonite (J. prakt. Chem., lxxxvi, 366, 1912), are merely fanciful and 

 have no foundation in fact. 



