Calculated. 





Measured. 



Calculated 



64° 36' 50" 



c a r. 



001 a 101 = 63° 31' 



62° 42' 



54 59 54 



c a k, 



001 ^ 012 = 38 29 



39 10 



34 42 52 



c ~m 



001 ^ 110 = 75 4 



75 45 



58 27 45 



c ~n, 



001 ,,111 = 75 11 



75 11 



29 54 



c~q, 



001*221 = 89 32 



89 42 



34 21 









28 Forces — Epidote from Huntington, Mass., and the 



The following is a list of some of the measured angles com- 

 pared with the calculated ones derived from the axes of N. von 

 Kokscharow Jr.* 



a:b:c = 1'57874:1 : 1-80362 ; (3 = 64° 36' 50" 



Measured. 

 CaOC, 001 ^ 100 = 64° 31' 

 a*m, 100*110 = 54 34 

 c~e, 001 * 101 = 34 43 

 c*0 f 001 a 011 = 58 32 

 a*«, 100a]01 = 29 48 

 c*», 001a 102 = 34 00 



Each of the above is the mean of at least three separate 

 measurements, and their agreement is very satisfactory. 



The optical investigations were made upon a single large 

 crystal twinned about 100 and measured to make sure of the 

 orientation. A section parallel to the clinopinacoid showed a 

 slight zonal structure, and the central, or clearest portion, gave 

 the following extinction : a^c red, Li=l°51 / ; yellow, Na, 

 2° 9' ; green, Tl, 2° 12', and for white light with the Bertrand 

 ocular 2° 47', all inclined to the c axis in the obtuse angle /9. 

 The extinctions were measured on either side of the twinning 

 plane and the above are half of the recorded angles. The 

 value with the Bertrand ocular is considerably higher than the 

 others, due undoubtedly to the influence of the blue and violet 

 of white light. The IJntersulzbach epidote with about 14 per 

 cent of Fe 2 O a , gives, according to Klein, f cia<? for red 2° 56' 

 and for green, 2° 26', but on the opposite side of the c axis, 

 that is, in the acute angle /9. 



For the indices of refraction one prism was cut with a face 

 parallel to 010 and its edge, as near as possible, parallel to the 

 a axis, from which, by placing 010 perpendicular to the rays 

 coming from the collimator, a for yellow, Na = 1*714 and 

 y = 1*724 were determined. By minimum deviation a was 

 also found to be 1'714. By means of a second prism parallel 

 to the b axis, /9 was found to be 1'716. As the crystal under 

 examination had a zonal structure, the results have only a rela- 

 tive value, as they would be slightly different in another part 

 of the crystal. 



The low double refraction y— a = 0*010 is below that of any 

 recorded epidote. 



From plates cut at right angles to a and c the axial angles 

 were determined by measuring in cc-mono-bromnaphthalene 

 (n, Na, = 1*6577) on a large Fuess axial-angle apparatus. From 

 the plate at right angles to a, 2H - 94° 33' for yellow Na, and 

 from the second plate 2H a = 93° 25' were measured. From 

 these values 2Y - 90° 32', 2V a = 89° 28' and /? = 1*7144 were 



* Materialen zur Mineralogie Russlands, viii, p. 44. f Jahrb. f. Min., 1874, p. 1. 



