508 Johnston, Merwih, and Williamson — 



more generally appreciated and acted upon, a critical review 

 of the behavior and properties of a mineral would be more 

 readily prepared and would help greatly to settle many ques- 

 tions still outstanding. 



Natural calcite* is in many occurrences very pure, and its 

 physical properties are well known; but little is known as to 

 the effect of impurities upon its properties. Foote and Brad- 

 leyf analyzed specimens of calcite which had deposited in open 

 spaces on dolomite, and found close to 1 per cent MgC0 3 as 

 a probable maximal limiting amount in solid solution under the 

 various conditions of formation of these specimens. Observa- 

 tions of the refractive index of the calcite in dolomitic lime- 

 stones lead us to the belief that under some conditions more 

 than 1 per cent MgC0 3 may be present ; moreover, there is a 

 greater likelihood of equilibrium in the limestones themselves 

 than in the deposits in open spaces, so that this is not in con- 

 flict with the observations of Foote and Bradley. In this con- 

 nection we remark that the stable solid phase in contact with 

 solutions containing magnesium is calcite only so long as the 

 concentration of magnesium does not exceed a certain limiting 

 value, beyond which it is dolomite;^; but whether this limiting 

 concentration of magnesium carbonate — which, moreover, will 

 vary with the temperature — is near that now present in the 

 sea is a question about which nothing has yet been definitely 

 ascertained. § 



Natural aragonite is usually not pure; all available analyses 

 indicate that it contains significant amounts of admixed sub- 

 stance, commonly the carbonate of lead, strontium or zinc, | pres- 

 ent presumably in solid solution. The aragonite found in the 

 sea may contain calcium sulphate; we have identified, sepa- 

 rated, and analyzed about a gram of the minute separate needles 

 of aragonite occurring in the muds of the shoal waters of the 



* A discussion of some of the agencies which, influence the deposition of 

 calcium carbonate is given in another paper now in course of publication in 

 J. Geology. 



t Foote and Bradley, this Journal, xxxvii, 339, 1914. The crystals anal- 

 yzed were clear, so that there is here little question that the magnesium was 

 present as carbonate, even though the C0 2 was not determined directly. 

 But it is to be noted that in fine-grained material the magnesium might in 

 some cases be present as hydroxide, as it is in fact in the outer layer of port- 

 land cement which has set in sea-water. 



\ Just as, in presence of sodium, with gaylussite (CaCO3.Na2COs.5H2O) 

 and pirssonite (CaCO3.Na2CO3.2H2O). See Wegscheider and Walter, Monat- 

 sheft, xxviii, 633, 1907. 



§The latest work along this line is that of Spangenberg (Z. Kryst., Hi, 

 529, 1913) who succeeded in producing undoubted dolomite at a temperature 

 of 180-200° and a pressure of C0 2 of about 50 atm. 



I G. M. Butler (Econ. Geol., viii, 8, 1913) describes a variety containing 

 nearly 10 per cent Zn. This fact accords with the idea that there is a form 

 of ZnC0 3 isomorphous with aragonite (see Griffiths and Dreyfus, Chem. 

 News, liv, 67, 1886). 



