132 Pen field and Foroes — Chrysolite- Fay alite Group, etc. 

 Specific gravity 4*047, 4030, Average 4*038. 





I. 



IT. 



Average. 



Ratio. 



Si0 9 .... 



33-60 



33-94 



33-77 



•562 



FeO 



4-7-19 



47-32 



47-26 



•656^ 



MnO___. 



4-76 



4-32 



4-54 



•064 J- 



MgO____ 



14-02 



13-74 



13-88 



•347 | 



H 2 



0-48 



0-48 



0-48 



•026 J 



100-05 99-80 99-93 



Mixter. 

 Sp. gr. 3-91 



•562 1-00 33*59 



44*37 



1-093 1-95 4-35 



16-68 



K 2 0-39 



H 2 0-26 



99-64 



The ratio of the Si0 2 : EO = 1 : 1-95 or nearly 1 : 2, giving 

 the formula (FeMgMn) 2 Si0 4 , and, therefore, the mineral 

 holds an intermediate position between fayalite and the iron- 

 rich chrysolites. The cleavages are poorer than in fayalite, but 

 parallel to the same faces 001 and 010. Plates and prisms 

 were prepared as in fayalite, the optical orientation and the 

 character of the double refraction being the same. 



The following results were obtained : 



For yellow light Na, a = 1*7684, = 1-7915, y = 1-8031, 

 r - a -031:7. 2H, Li = 76° 59'; 2H, Na = 76° 00'; 2H, 

 Tl = 75° 45'; hence with n Na for a-mono-bromnaphthalene = 

 1-6567 at 25° C, Y ay = 34° 42'. From the values of a, fi and y, 

 Y was found to be 54° 55'; hence Y a = 35° 5', which agrees 

 closely with Y a as given above. The pleochroism was 

 similar to that of fayalite but weaker, for rays parallel to a and 

 c very pale yellowish green, and parallel to b pale yellow. 



Chrysolite. 



For the optical constants of chrysolite we are indebted to 

 Des Cloizeaux* who gives a = 1-661, 0= 1*678, y = 1*697, for 

 yellow Na, 2V y = 87° 46', the double refraction being posi- 

 tive and the dispersion p < o. The amount of ferrous iron, 

 however, is not given. He also determined the constants for 

 fosterite, the white chrysolite from Vesuvius, containing pre- 

 sumably, according to the published analyses, about two per 

 cent of ferrous oxide with the following results : /9 r = 1*657, 

 y = 1*659, fi bl = 1*670, 2V r = 86° 1', 2Y y = 86° 10', 2V W 

 = 86° 32'. 



Zimanyif has recently given the following determinations on 

 chrysolite from the East Indies, but also without any determina- 

 tions of ferrous oxide ; for yellow, Na, a = 1*6535, /3 = 1*6703, 

 y = 1-6894, 2V a = 87° 15'. 



* Memoirs De L'lnstitute De France, Tome xviii, p. 591. 



fZeitschrift fiir Krystallographie und Mineralogie, Band 22, p. 338, 1893. 



