148 THE ROYAL SOCIETY OF CANADA 



that calcite crystals with small percentages of MgCOg and dolomite 

 crystals with slightly too large percentages of CaCOj represent "solid 

 solutions" and not true mixed crystals: and that dolomitic limestones 

 are either mixtures of dolomite and calcite or regular intergrowths of 

 both minerals. Linck and his pupils have found a true isomorphous 

 series only in the form of the third (Vaterite) modification of CaCO, 

 with the corresponding MgCO^ modification and with intermediate 

 mixtures. These occur in spherulities which have no counterpart in 

 nature, and which seem on rise of temperature to go over into calcite, 

 dolomite and magnesite.* On the other hand, Pfafff obtained homo- 

 genous rhombohedra which gave on analyses percentages varying from 

 6-93 to 12 '77 CaCOg as impurities in a magnesite, and in another case 

 a fine powder consisting of 66-76% MgCO, and 26-48% CaCOg. It is 

 of interest to note also in this connection that the proportion in which 

 FeCOj occurs in dolomite (in the mineral usually known as ankerite) 

 is variable, and that it occurs replacing the MgCOj of the dolomite. 

 In any discussion on this subject, however, particular weight must 

 be laid on the conclusion arrived at by Gossner J as a result of his in- 

 vestigations, viz. that only those members of an isomorphous series 

 are widely miscible with each other, which possess molecular volumes 

 of closely comparable magnitudes. 



The most probable interpretation, in the opinion of the writer, 

 of the evidence that has accumulated is that which would be repre- 

 sented diagrammatically by Fig. 6, if the carbonates could be subjected 

 to thermal analysis. CaCO, (A) takes up a limited amount of MgCO, 

 (B) in isomorphous mixture: MgCOg (B) a limited amount of CaCOg: and 

 CaCOg. MgCOg (AB) — a true compound — a limited amount of either. 

 In each case the limits of miscibility are in all likelihood narrow. Out- 

 side these limits aggregates are formed. For instance, a carbonate 

 of the composition x represents presumably aggregates of p and q — 

 that is, calcite and dolomite with in each case the limiting amount of 

 isomorphous admixture. The results of the study of two component 

 systems by the aid of the methods of thermal analysis have amply 

 demonstrated that miscibility — limited or complete — is the rule and 

 not the exception among isomorphous substances. 



The diagrams discussed on the assumption that the three minerals 

 are completely immiscible would be somewhat modified if miscibility 

 is possible.* If the miscibility is limited in extent, the main features 

 of the discussion remains unchanged. The order of precipitation 



*Doelter's Handbuch der Mineralchemie I, 125. 

 tl. c. 



IZeitschr. f. Kryst. u.s.w., 43, 1907, 130. 

 *See Bancroft's Phase Rule, p. 203. 



