192 



The olivine, wiiicli extinguishes iindnlalorilj, bnt not to such a 

 degree as is (lie case in most peridotiles, looks very fresh and is 

 absohitelj' free from weathering. The apparent weathering in the 

 crust appears to be merely a brown colouring; serpentinizalion as 

 an effect of atmospheric intluence is absent. In another slide the 

 refractive indices were, by the immersion-method, fixed at 1.66 and 

 1.70, after the Canada balsam had been carefully extracted by the 

 use of xylol. The thickness of the slide was '/jj mM. (measured by 

 lietacliing the slide and fixing it with tallow vertically on the object 

 glass): the highest interference-colour observed was green 3"' order, 

 making y-u about 0.04, wliich agrees with the determination of the 

 indices. The observation perpendicular to an optical axis in con- 

 vergent light i-evealed on rotation of the table a slightly curving 

 beam, at which the optically po.silive sign and a large axial angle 

 could be establislied. Presumably one has to do here with a ferro- 

 magnesium olivine with about 10 % fo 127, '/• '^'^^'''O'iviue and 90 "/„ 

 to 87', J °, „ magnesium-olivine (See Doeltek Handbuch der Mineral- 

 cliemie II, I p. 16). 



The colourless mineral, however, caused most trouble in its 

 determination. Long as well as short laths occur; quadratic sections 

 are lacking; the biiefi'ingence is low, sometirjies next to zero; in 

 one and the same lath the double refraction is not always the 

 same, but vaiies, without attaining however, the so called ,,Pflock- 

 structure". All the laths show straight extinction; the elongation is 

 invariably positive; cleavage lines run lengthwise tlii'Oiigh the crystal, 

 especially in the middle and parallel to the outline. It was very 

 difBcult to obtain an interference figure. Therefore it was surmised 

 that the mineral might belong to the melilite group, but this surmise 

 proved to be untenable, as it was in no way supported by further 

 microchemical and optical testing. 



For this reason I applied to Dr. Lam for more material. This 

 additional supply enabled me to identify the mineral. The coverglass 

 was taken from all the slides, which were rinsed repealedly with 

 xylol, in order to remove any trace of Canada-balsam from the 

 margins of the slide before being examined by the immersion-method. 

 The refractive index appeared to be 1.58 



Being treated with hydrochloric acid and washed cautiously, 

 gelatination ensued ; when moistened with fuchsin and again washed 

 carefully, the olivine as well as the unknown mineral under con- 

 sideration appeared to be gelatinized. To make sure that the silex- 

 gel of the olivine had not spread over the unknown mineral as well 

 and might thereby be misleading, the whole procedure was repeated 



