Classification of Natural Silicates. 131 



imations to the more complex ratios really existing. These, 

 from the frequent impurities of natural silicates, can seldom be 

 fixed with exactness, although with sufficient precision to give 

 very nearly the values of P and V, which latter serves to deter- 

 mine the place of the species in the natural system of classification. 

 Water being an element universally distributed in nature, its 

 presence or absence in a silicate becomes of subordinate impor- 

 tance in determining alike the genesis and the natural affinities 

 of species. Hence the water-ratios are omitted in the tables of 

 classification, wherein the various natural silicates are from the 

 chemical side, considered with regard to the atomic ratios of 

 the fixed bases to each other and to silica. 



There are genetic reasons (which were explained at length) 

 for separating silicates of sesquioxyd-bases, like alumina, from 

 protoxyd-silicates. The former of these constitute the Persili- 

 cates, and the latter the Protosilicates, those containing both prot- 

 oxyds and sesquioxyds being designated Protopersilicates. Ferric 

 oxyd and zirconia are classed with alumina, while titanic and 

 boric oxyds in silicates are counted with the silica in determi- 

 ning the atomic ratios. 



In the table of the Protosilicates, and in that of the Persili- 

 cates, both hydrous and anhydrous, the generally accepted atomic 

 ratios of the fixed bases to the silica are noted, but in the table of 

 the Protopersilicates regard is had to the more important ratios 

 of the sesquioxyd and fixed protoxyd bases to each other, inas- 

 much as the ratio of the silica to both of these is found to vary 

 greatly in closely related species, as may be seen in zeolites, 

 feldspars, scapolites and micas. In these tables the three groups 

 of silicates are arranged with primary reference to physical 

 characters. Thus for Protosilicates we have in parallel vertical 

 columns Pectolitoid, Spathoid, Adamantoid, Phylloid and Ophi- 

 toid, for each of which the range of values for V is given, 

 while in an adjacent column are inscribed the approximate atomic 

 ratios of fixed protoxyds to silica. Among pectolitoids are 

 included with pectolite, apophyllite and datolite, hydrorhodonite, 

 dioptase, pyrosmalite, calamine, cerite and thorite. The spathoids 

 embrace tephroite, willemite, gadolinite, helvite, leucophanite, 

 tscheffkinite and wollastonite ; the adamantoids, chondrodite, 

 chrysolite, phenacite, bertrandite, hornblende, pyroxene, titanite, 





