60 Ford diul Bradley — On Hydrozincite. 



mined by the immersion of the crystals in high refracting oils. 

 The results were, a = 1*650 and y = 1*740 ; both, +'005. In 

 spite of the high birefringence, which equals about '09, the 

 interference colors shown are usually gray or yellow of the first 

 order, although occasionally second order colors were seen. 

 This is due of course to the extreme thinness of the crystal 

 plates. 



In studying the optical nature of minerals similar to hydro- 

 zincite it was found that there is an almost complete agree- 

 ment between the characters given above with those of auri- 

 chalcite as determined by Buttgenbach.* His observations, 

 which have been confirmed by the present authors, were made 

 on the fine material found recently at the Kelly mine, Mag- 

 dalena, New Mexico. lie describes this material as follows! : 

 " The aurichalcite is in small elongated plates with a pearly 

 luster. Between crossed nicols the very thin plates give 

 various colors which do not go higher than the yellow of the 

 second order. The extinction is parallel to the elongation of 

 the crystals with this direction coinciding with the c vibration 

 direction. The plates, examined in converging light, are 

 parallel to the plane of the optical axes. By the movement, 

 however, of the dark curves which leave the center of the 

 field when, after having formed the black cross, the table of 

 the microscope is turned a little, it is easily determined that 

 the direction of the acute bisectrix coincides with the direc- 

 tion a. This proves that the sign of the mineral is negative. 

 The indices of refraction of aurichalcite are given by Lacroixf 

 as approximately between 1*67 and 1'155.^ 



From the above it is readily seen that as far as the crystal 

 and optical characters are concerned the two minerals are prac- 

 tically identical. The refractive indices of aurichalcite are a 

 little higher, but some variation here would have been ex- 

 pected because of the presence of copper in that mineral. 



It would naturally be expected, therefore, that the formula 

 of hydrozincite should be analogous to that of aurichalcite. 

 The latter mineral was analyzed by Penfield^: on excellent 

 material and with results that left no doubt but that its formula 

 should be 2(Zn,Cu)C0 3 .3(Zn,Cu)(OH) 2 . Hydrozincite, there- 

 fore, if the agreement between the physical properties as given 

 above is significant, should have the same formula, but without 

 the presence, of copper, or 2ZnC0 3 .3Zn(OH) 2 . If that is true 

 the theroretical composition of hydrozincite would be that 

 given below. 



- 



* Ann. Soc. geol. de Belgique, xl, B119, 1913. 



t Min. de la France, iii, 739. 



% This Journal (3), xli, 106, 1891. 



