The MonocUnic Pyroxenes of New York State. 129 



trant angle. Lustre vitreous to dull. Crystals often transparent 

 especially in the diopsides. Color mostly green, but varies from 

 colorless through various shades of green to black, also brown 

 and yellow. Streak white, green or gray. Fracture conchoidal. 



Cleavage parallel to m (110). Parting parallel to c (001) and 

 a (100). Hardness 5-6., Sp. Gr. 3.2-3.6. 



Etch figures on diopside usually show triangles the acute angles 

 of which point in the direction of the positive hemi-pyramid, or 

 a deltoid with rounded sides, whose acute angle jjoints towards 

 the negative and obtuse angle toward the positive pyramid. A 

 parallelogram is formed on the clino-pinacoid. 



The plane of the optic axes coincides with the plane of sym- 

 metry, and the acute bisectrix lies in front of the vertical axis. 

 Pyroxene is optically positive. c:C varies from 36°-54°. 



Tschermak* claimed that the variation of the extinction angle 

 is due to the chemical composition and that it is especially in- 

 fluenced by the percentage of iron m the non aluminous ones. 

 He noticed that a change of color was accompanied by a change 

 in the angle which the optic axis made with the normal to a. F. 

 J. Wiikf claimed that c : c increased with the percentage of FeO 

 in the Finnish pyroxenes. He, however, found that there were 

 pyroxenes which would not fit this rule, and came to the con- 

 clusion that the p3a-oxenes of the younger volcanic rocks fol- 

 lowed a different law. 



HerwigJ made a long series of similar determinations, but 

 they showed no deducible law. 



Doelter§ found in a series of augites that the FeO percent- 

 age (in those containing no FcgOg) had an effect on the optical 

 properties. JSTo regular law could be formulated if FcgOg were 

 present or if the sum of FeO and FegOg were used. A better 

 curve was obtained when the sum of FeO, re203 and AlgOg was 

 taken. A regular curve was only obtained when the sum of the 

 amounts of the component silicates are taken. 



Flink found that manganese had the same effect as iron on the 

 extinction angle. 



According to Des Cloiseaux|| an increase of temperature pro- 



*Miii. Mitth., 1871, p. 21. 



fZeitschr. fiir Kryst. u. Min., VIII, p. 78 and 208. 



X Ibid, XI, p. 67. 



iNeues Jahrb., 1885, I, p. 56. 



II Nouv. rech., 1867, p. 649. 



