J!>2 nil hh rand and Wright — Plumbojarosiie. 



the three unit rhoinhohodral faces was 53° 41', while on a 

 second crystal a slightly lower average value, 53° 15', was 

 obtained. So far as can be judged from the measurements 

 made, the most i^robabl}' value is about 53° 40', a value which 

 may possibly be in error 20'-30'.* This value is a little 

 lower than that obtained by Fenfieldf on plumbojarosite from 

 Cook's Peak, New Mexico '(6':r = 54° 30'), and part of the dif- 

 ference may be due to diiference in chemical composition, and 

 part to uncertainty in angular measurements as a result of the 

 iiuperfect development of the crystal faces. In habit, the crys- 

 tals from Utah dilfer also slightly from those in New Mexico, 

 the latter crystallizing in thin flakes, while the former, although 

 tabular in shape, are much thicker and more nearly equant. 



In the aggregate the deep brownish red crystals glisten with 

 almost adamantine luster. They are brittle and easily crushdd, 

 and in powder form give an ochre-colored streak. Evidences 

 of fair cleavage after the unit rhombohedron (1011) were 

 observed. 



Under the microscope the grains are pale golden yellow to 

 brownish red in color and strongly pleochroic, c being dark 

 brownish red and a pale golden yellow. Absorption strong 

 c>a. 



The refraction indices are high and were determined by the 

 immersion method ; eis about 1"Y85 and («>• 1*825. The bire- 

 fringence is strong and greater than '040. In convergent polar- 

 ized light, the crystals are uniaxial, optically negative, with 

 strong birefringence. Occasionally a slight opening of the dark 

 interference cross was observed but was not of sufficient regu- 

 larity in its occurrence to be significant. 



Washington, D. 0. 

 July 1. 



* This angle c : r was also determined by Dr. O. C. Farrington (letter of 

 Jiily 6, 1910). His measurements averaged 53° 30', a value which agrees 

 fairly well with the above. 



t This Journal (4), xiv, 218-215, 1902. 



