The Several Forms of Calcium Carbonate. 503 



ratio of the solubility of aragonite to calcite is, according to 

 Wells' results, diminishing towards unity as the temperature 

 diminishes, and would, if the rate of change remain constant, 

 become unity in the neighborhood of —100°; consequently, 

 on this basis, there would be in this region a transition point 

 below which aragonite would be the stable phase under 

 atmospheric pressure.* 



/x-CaCO s . — Under ordinary conditions this substance is unsta- 

 ble with respect to both calcite and aragonite, and goes over 

 into one or other of these forms at a rate depending upon the 

 conditions — into calcite in 5 minutes at 410°. 



CaC0 3 .6H 2 is likewise unstable with respect to calcite at 

 temperatures above 0°, and is transformed at a rate depending 

 upon the conditions ; the relations below 0° are unknown. 



The Heat of Transformation of Aragonite to Calcite. 



Closely related to the question of the stability relations of 

 aragonite is that of its heat of transformation to calcite ; this 

 we shall now discuss briefly, as part of the evidence, being in 

 Russian, is not generally available. 



Favre and Silbermann state that aragonite on going over 

 to calcite gives out 20 calories per gram ;f .Le Chatelier,^ on 

 the other hand, claims an absorption of 3 cal.; but little reli- 

 ance can be placed upon either of these figures, representing 

 as they do a difference between two quantities a hundredfold 

 larger and determined with an accuracy not better than one 

 per cent of their values. Boeke§ endeavored to estimate it at 

 470° by a differential method capable of detecting a sudden 

 heat change (e. g. such as accompanies the melting of a salt) of 

 0'5 cal. per gram; but he was unable to detect any heat effect. 

 We have tried similar experiments in which the sensitiveness 

 was more than 10 times as great as Boeke employed, but with 

 the same result ; but this negative result does not necessarily 

 imply that the heat of transformation is of the order of 0*1 cal. 

 per gram or less, because the transition actually is not a sudden 

 one, but is spread out over a considerable temperature interval. 

 The most recent publication dealing with the question is that 



* An analogous case of a reversible transformation is benzilortho-carboxylic 

 acid, studied by Soch (J. Phys. Chem., ii, 364, 1898. Tbe white a form is 

 stable below about 65°, the yellow /3-form between 65° and 132° ; yet on 

 heating the substance to 180° and cooling very suddenly both white and 

 yellow crystals are formed. Indeed the proportion of a in the liquid increase 

 with rising temperature, which indicates that at some temperature between 

 65 and 132° the heat of transformation a->/? is zero. 



■j- Cited from Abegg's Handbuch II, ii, 155. 



X Le Chatelier, Comp. rend., cxvi, 390, 1893. 



§ Boeke, Z. anorg. Chem., 1, 246, 1906. 



