482 Johnston, Merwin, and Williamson — 



pbous nucleus is required to produce aragonite. Under these 

 circumstances it separates as minute prismatic forms, occasion- 

 ally as fine-textured aggregates of irregularly radiating fibres 

 which could be recognized only by refractive index determina- 

 tions on fragments of very small size broken off from these 

 aggregates. These crystals when heated lost the theoretical 

 percentage of weight and therefore are pure aragonite ; their 

 optical properties are given in Table IV. 



If the precipitation is carried out at a temperature of about 

 60°, the product is a mixture of pure aragonite mixed with 

 /x-CaC0 3 , which can be separated by a flotation method ; but 

 at temperatures lower than this, the product is a mixture of 

 /x-CaCOg and calcite, except in presence of sulphate in which 

 case the product consists in part of aragonite containing some 

 proportion of sulphate, even when the temperature is less than 

 60°. Thus when calcium sulphate is used instead of calcium 

 chloride, or when potassium sulphate is admixed with the 

 potassium carbonate solution, or when solid calcium sulphate 

 is present in the medium during precipitation, the prisms 

 obtained, if well-formed, have a refractive index lower by. 

 about O'Ol in each direction ; the crystals, though perfectly 

 clear,* contain sulphate, in two analyses 1*2 and 1*4 per cent 

 reckoned as CaS0 4 . Something very like this occurs in nature ; 

 for in a stalactite from a cave near Luray, Virginia, composed 

 partly of aragonite, there was a thin layer which had optical 

 properties nearly identical with those of the above artificial 

 product containing sulphate, but the total quantity was so 

 small that a satisfactory chemical analysis of it could not be 

 made. The inference from this is that aragonite is able to 

 take up some proportion of calcium sulphate in solid solution ; 

 on this basis one is enabled to account more readily for certain 

 observations, for the presence of sulphate in solid solution 

 might reduce the solubility of aragonite to such an extent as 

 to render it not unstable with respect to calcite in solutions 

 containing sulphate. We have made a large number of ex- 

 periments on this influence of sulphate ; and have obtained 

 undoubted aragonite at temperatures as low as 19°, from 

 solutions containing sulphate (see Table I, No. 23). This ara- 

 gonite was difficult to identify, for although it consisted of very 

 small needles which superficially resembled the usual precip- 

 itates of aragonite, its apparent refractive indices were very 

 low, ranging down to 1*51 and 1*65 respectively, and varied 

 with the conditions of precipitation. It was surmised that this 

 variation was caused by a variation in the proportion of 

 sulphate present, but analyses showed that the sulphate content 



* When these clear aragonite crystals were heated, the calcite formed was 

 clouded by inclusions. 



