The Several Forms of Calcium Carbonate. 479 



therefore the stable form at all temperatures from 0° (or 

 lower) up to 970°, at which temperature — and under the requi- 

 site pressure of carbon dioxide to prevent dissociation into free 

 lime and carbon dioxide — Boeke* observed the occurrence of a 

 reversible change into another form, called by him a-calcium 

 carbonate, which apparently differs very little crystallographi- 

 cally from calcite. 



Calcite is the general product from slow precipitations, for 

 special procedures must be adopted for the preparation of the 

 unstable forms, and even then calcite occasionally precipitates 

 instead of the desired less stable form. If either of the other 

 two pure forms is left in contact with an aqueous solution — or 

 indeed, with any liquid in which it dissolves — or if it is heated 

 alone, the ultimate product is calcite.f The hexahydrate 

 changes rapidly through solution in its water of crystallization 

 into fine rhombs of calcite even at room temperature. 



The properties which serve to characterize ordinary calcite;}: 

 are so well known that they need not be recounted here ; they 

 are collected with those of the other forms of CaC0 3 in Table 

 4. Attention is called to a specia] habit of calcite which 

 appeared in one experiment (see Table I, No. 34). The import- 

 ant question of its solubility will be discussed later. 



Aragonite (A-CaC0 3 ). 



At the pressure of one atmosphere aragonite is, to judge from 

 the available evidence, essentially unstable at all temperatures ; 

 it may possibly be stable at some temperature below 0° (upon 

 this point there is no direct experimental evidence one way or 

 the other) in which region it would be hard to secure decisive 

 evidence by reason of the extremely slow rate of reaction. 

 Moreover, it may have a stable region of existence at high 

 pressures, but with regard to this too there is no experimental 

 evidence. As a consequence of this instability, it is at the 

 present time impossible to specify any limiting condition which 

 determines absolutely the precipitation of calcium carbonate as 

 aragonite. A large amount of work§ has been done on the 

 conditions under which aragonite may form, yet no general cor- 

 relation of this evidence with specific factors is discernible. 

 As has long been known, aragonite is formed if the precipita- 

 tion is carried on at a temperature approaching the boiling 

 point of water, or in presence of a salt of a metal which yields 



*Boeke, Neues Jahrbuch Min., 1912, i, 91. 



fWith regard to the crystallographic character of,- this calcite, see p. 

 500 postea. 



% The properties of aggregates of calcite are treated under " vaterite." 



gin particular by Vater, whose results appear in a series of papers in Z. 

 Kryst., between 1893 and 1901. A summary of the literature may be found 

 in Doelter's " Handbuch der Mineralchemie," i, 346. 



