Optical Properties of Ejpidote. 



27 



Carefully selected crystals were taken for the analysis, which 

 was made in duplicate according to the usual methods. The 

 ferrous iron was determined as described by Pratt.* The 

 results are given below, together with the specific gravity 

 determinations, which were made upon a chemical balance : 



Specific gravity.. 



3-341 



3-379 



Average. 



3-389 



Average 



Ratio. 



..3-3 



SiO, 38-20 



37-78 



37-99 





•633 



6-00 



A1 2 3 ... 29-00 



30-06 



29-53 



•286 ) 

 •035 \ 



•321 



3-04 



Fe 2 O s _._ 5-63 



5-71 



5-67 



FeO 0-54 



0-52 



0-53 



•007 ) 







MnO ... 0-21 



0-21 



021 



•003 \ 



•436 



4-13 



CaO 23-82 



23-87 



23-85 



•426 ) 







H,0.-.- 2-05 



2-03 



2-04 

 99-82 





•113 



1-07 



Total... 99-45 



100-18 





The above ratios approximate closely to 6, 3, 4, 1, which, 

 regarding the water as coming from hydroxy I, gives the 

 usually accepted formula H 2 Ca 4 Al 6 Si 6 26 or Ca 2 (A10H) Al 2 Si 3 12 , 

 in which the calcium has been partly replaced by ferrous, and 

 aluminium by ferric, iron. The analysis is interesting as show- 

 ing a remarkably low iron percentage for epidote. 



The crystals, as is common with epidotes, show a prismatic 

 development parallel to the b axis and some of the largest 

 ones measured over 30 mm in length and 15 mm in diameter. 

 They are usually simple, but some are twinned parallel to 100. 



Although the faces of the crystals were brilliant, f yet the 

 reflections of the signal on the goniometer were usually poor 

 as the planes were commonly vicinal, which rendered the 

 measurements somewhat uncertain. Some of the faces in the 

 ortho-diagonal zone were deeply striated, so that there was a 

 continuous series of reflections of the signal. Upon a suite of 

 crystals which was measured, the following forms were identi- 

 fied : 



a, 100 



u, 210 



r, 101 



n, 111 



c, 001 



e, 101 



k, 012 



q, 221 



m, 110 



i, 102 



o, 011 



y, 211 



All of these were found upon 

 a single crystal, as shown in the 

 figure. 



On the simpler crystals the 

 forms a, c, e, r and n were 

 prominent and developed about 

 like fig. 5, page 517, 6th edition 

 of Dana's Mineralogy. 



This Journal, xlviii, p. 149, ] 894. 



