500 Hewett and Shannofi — Orientite, a netv 



common habit are prismatic witli tlie length 11/2 to 4 times 

 the diameter. Tliese show only the basal pinacoid c (001) , 

 the brachypinacoid ^(010), and the unit prism m(llO). 

 The prism and brachypinacoid are about equally devel- 

 oped so that the crystals appear very much like hexagonal 

 prisms (fig. 5, B). The second habit has the same simple 

 combination but the crystals are distinctly tabular parallel 

 to the pinacoid h(OlO) with the length IVo to 2 times the 

 breadth (fig. 5, A). The larger crystals are often compo- 

 site being made up of smaller individuals in parallel posi- 

 tion. The faces of all crystals large enough for gonio- 

 metric measurement are imperfect, the prism faces being 

 striated horizontally and more or less bulged while the 

 base is usually somewhat concave. The only crystal 

 found which was not terminated by the base alone was 

 very minute and as shown in fig. 5 C was tabular parallel 

 to 6(010) and was terminated by forms which, by their 

 zonal relations, were identified as y{011), e(102) and 

 ^(112). This crystal could not be measured on the goni- 

 ometer and the only angular value obtained was measured 

 under the microscope. This was the angle e(102) : 

 e'( 102) =61-00' or, 



e (102) ct> = 90"00' p =30°00' 



The value given for the c axis, based, as it is, upon this 

 measurement alone, is obviously only an approximation. 

 The mean of 9 good measurements on crystals of the 

 simpler types gave as the angle for the prism : 



m = (110) <^ = o6°06' p = 90°00' 



or 7^^(110) : in'"(110) =67^48'. From the above angles 

 the following constants are derived: 



a = .6720 p^ = 1.1780 log. p^ = 10.07115 

 c = .7916 q^ = .7916 log. p^ = 9.89851 



These values give the following theoretical angles for the 

 forms observed : 



Forms^ Goldschmidt Symbols, and calculated angles 

 of orientite 



Letter Miller Gdt. symbol (^ p 



^ 010 00 0°00' 90°00' 



c 001 0°00' 0°00' 



771 



110 CO 56°06' 90°00' 



e 102 iO 90°00' 30°30' 



y oil 61 0°00' 38°22' 



^ 112. i 56°06' 35°22' 



