Lawson.] 



Malignite. 



345 



in the merest traces, and that we have to deal with a true potash 

 orthoclase. The powder of the orthoclase after digestion witli hot 

 hydrochloric acid remained unacted upon. Its specific gravity is 

 between 2.56 and 2.57. 



The pyroxene, which is abundant, is a deep green pleochroic 

 variety, with a pronounced elongation in the direction of the c axis. 

 It has the usual high refractive index, strong double refraction and 

 distinct prismatic cleavage of the augites. In clinopinacoidal sec- 

 tions the maximum observed value for the extinction angle C : c is 

 59 . The pleochroism is in shades of green and yellow, but can 

 not be adequately expressed by the simple formula usually 

 employed. In sections normal to the acute bisectrix the optically 

 positive character of the mineral is easily established by the use of 

 the quartz wedge. Such sections contain a and b, and the orienta- 

 tion of these axes is readily determined by the interference figure. 

 These sections show a feeble pleochroism tt=sap green, 6=whitish 

 or yellowish green. The absorption is cr b, but in many cases the 

 difference of absorption is so slight that no distinction can be made, 

 and = b. In sections parallel to 00 Poo , containing the axes a and 

 C, the pleochroism is also comparatively feeble, though generally 

 more pronounced than in the last case; a = sap green, C = yellowish 

 green to yellowish. The absorption is tt>c. In sections normal to 

 the obtuse bisectrix, containing b and C, the pleochroism is much 

 stronger. In these sections C = yellow to amber, b = whitish green. 

 The absorption is 6>c. From these observations the general 

 formula would be a=b>c, and the sections parallel to coPco contain- 

 ing a and r might from this formula be expected to exhibit a 

 pleochroism not less pronounced than any other section. This,, 

 however, is not the case. The very pronounced yellow and green 

 pleochroism in sections approximately transverse to c, and showing 

 the emergence of an optic axis, is a persistent feature of the min- 

 eral. If the axes of absorption are dispersed in the plane of 

 symmetry, as has been suggested by Laspeyres*, the yellow c ray 

 might be nearly normal to the optic axis, which lies near to c. But 

 inasmuch as the clear yellow to amber does not appear in clinopin- 

 acoidal sections, this supposition of the dispersion of the axes of 



*Z \ Kr}St, IV. 18S0, 454. 



