[WALLACE] STUDY OF DOLOMITIZATION I47 



that m and n are simple integers (e. g. m=n=I). In such a case A B 

 may be taken as a true compound in the isomorphous series, with a 

 constant melting temperature like other compounds. It is not unreason- 

 able to suppose that the formation of a compound AB would take 

 place with contraction owing to the simple stoichiometric relations 

 between the two similar compounds A and B. The specific gravity 

 curve might here be represented as the dotted curve in the figure : and 

 Retger's reasoning with regard to the relationship of dolomite to calcite 

 and magnesite would not hold. It is impossible to apply the ordinary 

 methods of thermal analysis to the problem at issue owing to the in-, 

 stability of the compounds at higher temperatures : even under pressure 

 of an atmosphere of COg fusion is a matter of difficulty. We are of 

 necessity thrown back on independent considerations. 



The considerable difference in the molecular volumes of the three 

 salts has been frequently pointed out. This is due to the much closer 

 packing of the atoms hi the molecule of MgCOg than in the others, as 

 shown by the fact that the mineral of least molecular weight has the 

 greatest specific gravity: but the gradation in molecular volume is 

 constant in direction, and is not more considerable in magnitude than 

 in the case of the simple sulphates K2SO4, Rb2S04, CS2SO4, which 

 Tutton has shown to be a true isomorphous series. The changes in 

 rhombohedral angle and in topic parameters are such as would support 

 the theory of complete isomorphism. Against this must be placed the fact 

 that Ca salts occasionally stand rather apart crystallographically from 

 the salts of other elements that are closely related chemically: and the 

 further fact, already referred to, that the symmetry of dolomite is lower 

 than that of calcite and magnesite. The lowering of symmetry may be 

 accounted for by the lowering of symmetry of the actual structure of 

 the molecule.* The subject of continuous isomorphous mixtures has 

 not yet been gone into sufficiently fully from the crystallographical 

 standpoint; but it is not improbable that when work is directed to 

 this specific problem, it will be found that the mixed crystals of com- 

 paratively simple substances, while showing continuous gradation in 

 molecular volume, angles, and topic parameters, possess a lower sym- 

 metry than the simpler components owing to a less symmetrical 

 structure of the complex molecule or of the interlocking point 

 systems. 



Considering all the facts, we feel justified in taking calcite, dolomite 

 and magnesite to be an isomorphous series, though not a eutropic group in 

 the sense of the word understood by Linck. With regard to the possibility 

 of mixed crystals between the various compounds, experimental work 

 furnishes as yet somewhat contradictory evidence. Linck considers 

 *Becke: Tsch. Min.-Petr. Mitt. Bd. II, 1890, 257. 



