272 Prof. Chapman on the Classification of the Silicates 



kinds of garnets (raelanite, uwarowitc, &c), although I consider 

 these quite as much entitled to the rank of distinct species, as 

 the various spinels and their allied aluminates, ferrites, chro- 

 mites, &c. ; the theoretical value of the term " species " will, 

 however, become obsolete as the science advances. In placing 

 isomorphous substances under one common type or sub-type, 

 we do not necessarily consider them to be one mineral, as some 

 opponents of the chemico-physical system have attempted to 

 establish. 



Idocrase Type : — Dimetric. 



Idocrase, 3RO, Si0 3 + R 2 3 , SiO 3 =rR; hausmannite, MnO, 

 Mn 2 3 ; anatase,Ti0 2 ; fergusonite, 6(YO, CIO), Ta 2 3 (?). 



Cassiterite Type : — Dimetric. 



Rutilc, TiO 2 ; cassiterite, SnO 2 ; zircon, Zr 2 3 , SiO 3 ; mala- 

 con, 2(Zr 2 3 , SiO 3 ) + H 2 ; oerstedite ; azorite, CaO, Ta 2 3 (?). 



Brookite Type : — Trimetric. 



Brookite,Ti0 2 ; polianite, MnO 2 ; pyrolusite, Mn O 2 ; gsethite, 

 Fe 2 3 + H 2 0; manganite,Mn 2 3 + H 2 0; diaspore,Al 2 3 + H 2 0; 

 columbite, 3(FeO, MnO), 2(Ta 2 O 3 , Pp 2 O 3 , Ni 3 ) ; yttro-tanta- 

 lite (?), 3YO, Ta 2 3 ; tantalite (kimitolite) ; samarskite; men- 

 gite; polymignite; polycrase; seschynite; ostranite; euxenite; 

 wcehlerite (?) ; eucolite (?). 



Pyrochlore Type : — Monometric. 



Perowskite, CaO, TiO 2 ; pyrochlore, 2RO, Ta 2 3 ; micro- 

 lite (?) ; pyrrhite, Zr 2 3 , Ta 2 3 . 



Cerite Type : — Hexagonal. 



Cerite, 3(CeO, RO), Si0 3 + 3H 2 0; thorite, 3ThO, SiO 3 

 + 3H 2 0; eudialite, 2(3RO, 2Si0 3 ) + Zr 2 3 , 2Si0 3 ; schorla- 

 mite, FeO, SiO 3 -f 2CaO, TiO 2 ? ; willemite, 3ZnO, SiO 3 ; diop- 

 tase, 3CuO, 2Si0 3 + 3H 2 (Appendix, chrysocolla) . 



Chrysocolla bears the same relation to dioptase as brown iron 

 ore to specular iron, and opal to quartz, amongst other examples. 

 Eudialite and dioptase are closely allied in a crystallographic 

 point of view. The two known rhombohedrons of eudialite give 

 for the relative lengths of the vertical axis, 2*110 and 05279; 

 whilst the dioptase forms yield for the same, 1*059 and 0*530. 

 If we consider the more obtuse forms as protaxial, we have, for 

 the notation of the above rhombohedrons, R and 4R in eudialite, 

 and R, 2R in dioptase. 



Electric Calamine Type: — Trimetric. 

 Electric calamine, 2(3ZnO, Si0 3 )+3H 2 0. 



