40 On the Homceomorphism of Mineral Species 



of 119° in these species, parallel to faces of which the composi- 

 tion takes place, is the true vertical prism, as in Aragonite. The 

 fact that 120° X 3 or 60° X 6 equals 360°, is evidently the fun- 

 damental reason for the occurrence of such twins ; and hence 

 in other species a like angle for the vertical prism, especially if 

 the prisms are alike in their other dimensions, would be likely 

 to produce the same result. 



Hence we conclude that the sulphates (RO, SO 3 ), although 

 affording in one direction a prism near 120° in angle, have not 

 this prism as the fundamental vertical prism, for stellate com- 

 position does not occur parallel to it ; the true vertical prism is 

 the one usually so assumed — that of 101° to 104°. 



Bournonite affords another illustration of this subject. Gr. 

 Rose has assumed its homceomorphism with Aragonite, on the 

 ground that it has a vertical prism of 115° 58'. But this spe- 

 cies, instead of forming twins parallel to the faces of this prism, 

 actually affords cruciform twins parallel to a prism of 93° 40', 

 the one usually taken as the fundamental prism. The prism of 

 115° 58' is i -| (oo P I), and there is no reason for regarding it 

 as other than a secondary prism. 



Chrysoberyl has been placed near Chrysolite by the author, 

 and also by M. Scacchi, of Naples. In a certain position the 

 resemblance in angle exists. But still the species are rather 

 widely remote, inasmuch as the twins, like those of Aragonite, 

 parallel to faces of the prism of 119° 46', show that this is the 

 fundamental prism. Ch^solite affords no such twins; the angle 

 of its vertical prism is 94° 3', and it belongs to a different zone. 

 Chrysoberyl is actually near Aragonite in angle ; it has a bra- 

 chydome of 108° 26', and Aragonite one of 109° 39'. 



Monoclinic prisms near 120° in angle, never present stellate 

 twins like trimetric prisms. Such twins in oblique forms ap- 

 pear to be impossible, since they require a regular symmetrical 

 character in the molecule above and below the middle section. 

 This remark appears to apply also to hemihedral forms of the 

 trimetric system, like those of Datholite. 



