Specific gravity 



I. 

 3022 



II. 

 3-047 



Mean. 

 3-035 



Analysis I. 



II. 



Average. 



Ratio. 



Si0 2 36-86 



36-70 



36-78 



•613 



FeO 4-61 



4-89 



4*75 



•066^ 



MnO 1-58 



1-67 



1-62 



•023 y 



MgO 21-44 



21-75 



21-60 



•540 J 



CaO 34-23 



34-39 



34-31 



•613 



H a O -97 



•93 



•95 







33-47 





5-01 





1-12 





20-61 





35-25 



Ign 



2-29 



PA 



2-03 



Al 2 O s 



•17 



Penfield and Forbes — Chrysolite- Fay alite Group, etc. 135 



Genth's analysis showed a loss by ignition of 2*29 per cent and 

 2*03 per cent of P 2 6 , it evidently was made on impure mate- 

 rial and consequently a new analysis seemed necessary. 



The material which we were able to select was of exceptional 

 purity and yielded the following results : 



Genth. 



•613 l'OO 

 •629 1-02 

 •613 l'OO 



Total 99-69 100-33 100*01 



99-95 



The ration of Si0 2 : (MgFeMn)O : CaO = l'OO : 1*02 : 1*00 is 

 very satisfactory, and gives the usually accepted formula 

 CaMgSi0 4 in which a little Mg is replaced by Fe and Mn. A 

 careful test was made for P 2 5 but none was found, and un- 

 doubtedly Genth's material, as assumed by him, was contami- 

 nated with apatite. From the crystal from which the material 

 for analysis was taken, a few colorless and transparent grains 

 were selected which when heated in the closed tube gave only 

 the merest trace of water, the pure mineral is therefore practi- 

 cally anhydrous, but in order to obtain sufficient material for 

 the analysis it was necessary to include some slightly brownish 

 grains, and these were permeated with cracks along which 

 decomposition had commenced, which accounts for the water. 



For the determination of the optical properties a single crys- 

 tal like the one figured by Pirsson was used. The indices of 

 refraction, determined by means of prisms, are as follows : for 

 yellow light, Na, «= 1-6505, £ = 1-6616, f= 1-6679, f — a = 

 0-0174 ; also /9, Li = 1*6594 and /?, Tl = 1-6653. The optical orien- 

 tation is a = c, b = a, and c = 6 as in chrysolite. The plane of 

 the optical axes is 001 and a is the acute bisectrix, consequently 

 the double refraction is negative. The dispersion is^ > u as fol- 

 lows : 2H,Li = 75° 55', 2R,Na = 75° 21', and 2H,T1 = 74° 52. 

 The measurements being made in a-mono-bromnaphthalene, 

 Y a ,Li = 37° 51', V„Na = 37° 31' and Y a ,Tl = 36° 28', while 

 from the values of a, /? and y as given above V a ,Na by calcula- 

 tion, was found to be 37° 9'. 



Laboratory of Mineralogy and Petrography, 

 Sheffield Scientific School, New Haven, October, 1895. 



