A. F. Rogers — Collophane. 275 



(C0 3 ,F 2 ,S0 4 ,0). Dahllite, which is crystalline, has the 

 formula 3 Ca 3 (P0 4 ) 2 .CaC0 3 . The fact that most amor- 

 phous minerals approach in chemical composition their 

 crystalline equivalents was brought out by Cornu. 17 This 

 principle he called the law of ^homoisochemite", and the 

 minerals themselves, ' * pseudostochiolite. ' ' 



The water evidently is also variable. Water is varia- 

 ble in all amorphous minerals, on account of their col- 

 loidal origin. "When the colloid is first formed the water 

 may be absorbed but during the hardening of the gel the 

 water probably becomes diffused through the dispersed 

 material and thus in time a solid solution is formed. 



Collopliane, then, maybe regarded as a solid solution of 

 calcium carbonate, calcium fluoride, calcium sulphate, and 

 water in tricalcium phosphate. 



Physical Properties of Collopliane. 



Collophane is usually massive, but sometimes has an 

 oolitic or concretionary structure. As is the case with 

 other amorphous minerals, colloform 18 crusts may be 

 present in cavities. 



The specific gravity of collophane varies from 2.6 to 

 2.9 ; the variation is due to variation in porosity as well as 

 to difference in chemical composition. 



The hardness varies from 3 to 5. The index of refrac- 

 tion varies about 1.57 to 1.63, but is usually 1.59 to 1.61. 

 The determination of the index of refraction is one of the 

 best means of identifying the mineral, for, with the excep- 

 tion of the very rare silicate, eudialyte, it is practically 

 the only amorphous or weakly doubly-refracting mineral 

 within these limits for its index of refraction. Like other 

 amorphous minerals of colloidal origin collophane often 

 shows double refraction due to strains set up in the hard- 

 ening of the gel. In colloform and oolitic forms, the 

 double refraction causes the mineral to take a spherulitic 

 appearance. These pseudo-spherulites, however, lack 

 the fibrous structure of true spherulites. 



17 Zeit. f . Chem. u. Ind. d. Kolloide, 4, 15, 89, 1909. 



18 This term was coined by the writer to designate the rounded, more or 

 less spherical surfaces assumed by amorphous and microcrystalline substances 

 in open spaces. It is a general term to cover mammillary, botryoidal, etc. 



