G. H. Williams — Monoclinic Hemihedrism. 



115 



Art. XIII. — On the possibility of Hemihedrism in the Mon- 

 oclinic Crystal System, with especial reference to the Hemi- 

 hedrism of Pyroxene ; by George H. Williams. 



In a recent " Note on some remarkable Crystals of Pyroxene 

 from Orange Co., N. Y.,* I described an unusual monoclinic 

 habit of this mineral which showed the planes of different 

 forms grouped about opposite extremities of the vertical axis. 

 These crystals were then called hemimorphic with reference to 

 the vertical axis, although it was recognized that such an 

 expression did not accord strictly with the accepted nomencla- 

 ture, because in the monoclinic system the vertical axis is not 

 an axis of symmetry. 



During the past summer it was suggested to me by Professor 

 Rosenbusch that these crystals might possibly be regarded as 

 hemihedral rather than hemimorphic, and such upon more 

 mature consideration appears to be the case. I therefore ven- 

 ture to refer once more to the Orange Co. crystals, both because 

 their apparently anomalous form receives a new interest when 

 viewed in this light, and also because they have been found, 

 upon further investigation, to offer by no means an isolated 

 instance of this peculiar development in pyroxene. 



As they are at present used in crystallography, the terms 

 hemihedrism and hemimorphism both express the independent 

 occurrence of one-half of the planes belonging to a crystalline 

 form. They differ, however, in the manner in which these 

 planes are grouped ; the former representing half the planes 

 as occurring equally about both extremities of an axis of sym- 

 metry, while the latter implies the presence of all the planes 

 of a form at one end of such an axis, and none of them at the 

 other. 



The most general holohedral form consistent with the sym- 

 metry of the monoclinic system is a hemipyramid (±P), com- 

 posed of four planes, one in each of the four similar octants. 



It is evidently possible to select one-half of these four planes 

 in three and only three different ways : 



* This Journal, III, xxxiv, pp. 275-276, Oct., 1887. 



