Optical Properties of Epidote. 29 



obtained. In this crystal, a instead of being, as is usual, the 

 acute becomes the obtuse bisectrix and the optical character is 

 therefore positive. 



The dispersion was distinctly inclined. In the section at 

 right angles to c, the color to the right of each hyperbola was 

 violet and to the left red, but they were more diffused in one 

 than in the other. The dispersion p<u was shown by the fol- 

 lowing measurements : 2H a red, Li = 93° 10', 2H a yellow, N&= 

 93 c 25', 2H a green, Tl = 93° 51', but over the axis a the disper- 

 sion was /?>w, as in other epidotes. 



The pleochroism was quite striking, being in sections about 

 0'5 mm thick for rays vibrating parallel to 6 a lavender or pale 

 plum-color, but parallel to a and c almost colorless, the former 

 showing a slight tinge of lavender and the latter of green. 



By a comparison of the analysis of the Huntington with 

 those of other epidotes, it was found that the light-colored 

 one from Zillerthal in Tyrol, analyzed by Laspeyres,* was 

 almost identical in chemical composition, as shown by the fol- 

 lowing : 



Si0 2 A1 2 3 Fe 2 3 FeO MnO CaO H 3 Insol. Total 

 Huntington, 37-99 29*53 5-67 0*53 0'21 23*85 2*04 ... 90*82 

 Zillerthal, 38'46 28*59 5'76 0*53 ... 24*60 1*92 0*42 100*28 



These analyses are, moreover, the lowest in iron percentages 

 of any recorded epidotes. As no optical investigation of the 

 crystals from Zillerthal has been published, one was made, for 

 sake of comparison, upon a specimen in the Brush collection. 

 The material used was a fragmentary crystal about 10 mm in 

 diameter, decidedly zonal and of a pale red color on the 

 exterior, but pale green in the interior. The crystallographic 

 orientation was determined by measurement of the angle 

 100 /\ 001 and the cleavage parallel to 001. On a clinopinacoid 

 section the extinction, measured with the Bertrand ocular, was 

 found to be a^<? = 0° 30' in the acute angle ft for the interior 

 green portion of the crystal, but for the exterior, or reddish 

 part, it was about 0° 30' in the obtuse angle ft. 



Prisms were cut, as in the Huntington mineral, with their 

 edges in the inner green portion and the following indices of 

 refraction determined : For yellow, !N"a, a — 1*720, ft — 1*7245, 

 ?= 1*7344, y- a = 0*0144. From a plate parallel to 100 the 

 value of 2H , yellow ISTa, was found to be 96° 40'. (?*, Na .for 

 a-mono-bromnaphthalene being 1*6572) and from which 2Va, Na 

 was found to be 87° 46'. 



The pleochroism was very similar to that of the Huntington 

 epidote. For sections about 5 mm thick b plum color, a and c 

 alike and nearly colorless in the interior greenish part, while in 

 the exterior reddish envelope a was pale green and c pale rose. 



* Zeitschrif t fur Krystallographie und Mineralogie, iii, p. 562, 1879. 



