NO. 1867. CRYSTALLIZED VARISCITE FROM UTAH.—SCHALLER. 421 



mum velocity (unlike the green crystals), light brown; parallel to the 

 a axis, a light brown with a decided greenish-yellow tint. The bire- 

 fringence is about 0.006, the interference colors on the thicker crystals 

 reaching into first-order yellow. The areas of ''irregular aggre- 

 gates" (shown in fig. 1) are stronger colored and indefinite in character. 

 They somewhat resemble the kaolinization efl^ects seen in rock slides. 



RELATION TO ANALOGOUS MINERALS. 



A brief comparison of the optical properties of variscite, as given, 

 with those of the analogous minerals scorodite, strengite, and phos- 

 phosiderite, shows some close relationships as well as some marked 

 differences. 



The optical properties of variscite given by Lacroix ^ are not 

 entirely in accord with those given in this paper, but this is due to 

 the fact that Lacroix's material was fibrous and not in distinct 

 crystals. He gives the axial plane as parallel to h (010), wdtli nega- 

 tive acute bisectrix normal to a (100), whereas the positive acute 

 bisectrix is normal to c (001). The value of 2E is given as about 96° 

 and the mean index as 1.579, wdiich is a little liigher than the value 

 1.560 found on the Utah crystals, but is almost identical with the 

 maximum index found thereon, namely (j-) 1.578. The birefringence 

 is stated to be a little higher that that of quartz, but the birefringence 

 observed by Lacroix was probably not the maximum, which reached 

 a value of 0.032 in the crystals fi'om Utah. It is not known to what 

 extent, if any, the chromium and vanadium present in the green crys- 

 tals from Utah influence the optical values. 



The comparison of the chief optical properties of the different 

 analogous minerals can be most conveniently shown in tabular form. 

 The orientation of the variscite is so chosen that the axial plane is 

 h (010), but it is possible, as shown beyond, to so orient the crystals 

 that the axial plane is a (100), analogous to that of scorodite and 



strengite. 



Comparison of optical relations. 



In adopting the orientation for variscite as given in this paper, 

 namely, with the optic axial plane parallel to h (010), an interesting 

 condition obtains for the isomorphous mixtiu'es of variscite 

 (AIPO4.2H2O) and strengite (FeP04.2ll20), for in the latter mineral 

 the optic axial plane is parallel to a (100). Such an isomorphous 



1 Lacroix, A. Mineralogie de la France, vol. 4, 1910, p. 479. 



