the Monociinic Crystal System. 117 



morphism with reference to the vertical axis, as shown in figs. 

 5 to 8. It might, however, be the case that the forms of the 

 clinodiagonal zone were fully developed, when the crystal 

 would appear hemimorphic in the direction of the a axis. 



This selection of the planes in reality satisfies all the con- 

 ditions of hemihedrism since its planes are equally distributed 

 about both extremities of the axis of symmetry. The hemi- 

 hedrism is moreover inclined-faced, because its forms possess 

 no center of symmetry ;* or, in other words, their planes do 

 not have others parallel to them. In this case alone of the 

 three does the monociinic plane of symmetry remain. 



There is therefore possible in the monociinic system : 



1. Parallel-faced hemihedrism. Triclinic system. 



2. Hemimorphism. 



3. Inclined-faced hemihedrism. 



A similar suggestion has been offered by L. Wulff,t and in 

 his recent paper on the crystallization of cane-sugar the same 

 author has given figures very much like those here presented,^: 

 although I was not cognizant of his work until this article was 

 ready for publication. 



The theoretical possibility of such hemihedral and hemi- 

 morphic forms in the monociinic system has, however, for min- 

 eralogists but little interest apart from their actual occurrence 

 in nature. The first two are already well known and recog- 

 nized ; the third has heretofore lacked recognition on account 

 of the want of examples, and it is the especial object of this 

 paper to show that it is by no means uncommon on such an 

 important mineral as pyroxene, while it may possibly occur on 

 sphene. 



The earliest recorded, as well as one of the most remarkable 

 instances of this inclined-face hemihedrism in pyroxene which 

 has come to my notice, is to be found in a 5 - 



perfectly developed crystal of diopside, prob- 

 ably from the Mussa Alp in Piedmont, figured 

 and described by Hessenberg§ in 1856. His 

 figure is here reproduced (fig. 5), drawn so as 

 to show the clinopinacoid as the principal 

 face. Here, as in all other cases, the prismatic 

 zone is holohedral with the forms a, go Poo 

 (100); b, ccP^> (010); m, go P (110); /. 

 ccPs (130) ; and /, 00P5 (150). The hemi- 

 hedrism manifests itself as an apparent hemi- 

 morphism in the direction of the vertical 

 axis, at one end of which are the forms p, -f Poo (loi) ; d, + Ps 



* For an explanation of this term, see Sohncke, Zeitschr. fur Kryst., vol. xiv, 

 p. 134, 1888. \ Ibid., vol. xhi, p. 499, 1888. 



± Ibid., vol. xiv, p. 557, 1888. PL XI, figs. 3 a , 3 b and 3°. 

 § Mineralogische aSTotizen, No. 1. PI. II, fig. 18. 



