THE MINERAL EVANSITE 



225 



former, it will be seen that it is less than the average density as given by 

 Forbes — ^while according to the determination of this physical property 

 by Kovar, in one instance, namely, for the white variety, it is less (1.874) 

 and for the yellow it is nearly the same as the average density of the several 

 varieties obtained by Forbes as shown in the table above. If we take, how- 

 ever, an average of the higher values and also an average of the lower values 

 for densit}-, the figures obtained (1.93) and (1.86) give approximately the 

 range in specific gravity, due in the denser mineral to about 2 to 6 per cent 

 of ferric oxide, and in the latter to its absence or, if present, to just a trace 

 or to not over 1 per cent. On the other hand a further variation may 

 be charged to the presence or absence of lime. When, therefore, the speci- 

 fic gravity is greater than the average (1.86) for the purer mineral, in 

 general, it is due obviously either to the presence of foreign matter in the 

 form of iron oxide or calcium oxide, generally the former. This relationship 

 of specific gravity or density has been brought out by Schaller* in the 

 following table: 



Relation in Evansite of density to iron content. 



Since evansite is one of a large group of hydrated alumina phosphates, 

 it is difficult to distinguish it chemicallj^ ^vithout an ultimate analysis. It 

 possesses so many other properties that obtain in other members of its 

 group that physically it has very few peculiar and distinct diagnostic fea- 

 tures. Probably the mineral that it most closely resembles is coeruloloc- 

 tite, but it may be distinguished from the latter by inferior hardness and 

 specific gravity. The specific gravity of coeruloloctite ranges from 2.55 

 to 2.59, while that of evansite never exceeds a density of 2. In fact the 

 chief physical diagnostic feature of evansite is its specific gravity, which, 

 on the whole, is considerably less than that of most of the other minerals 

 of the same group. Consequently if a mineral be obtained which has the 

 following pyrognostic characteristics, namely, decrepitates and yields water 

 when heated in a closed tube; is infusible and leaves a milk-white powder; 

 colors the blow-pipe flame green when moistened wnth sulphuric acid; gives 



U. S. G. S. Bull. 490, p. ! 



