MINEEALOGY; WITH A CLASSIFICATION OF SILICATES. S3 



4 : 9) are bisiliaxtes. While rhodonite and pyroxene are clinorhombic in crystallization, 

 the maguesian species, enstatite, with hypersthene and diaclasite, is orthorhorabic. 

 Anthophyllite appears to be an orthorhombic species having the composition of amphibole, 

 and kupfterite, a magnesian amphibole. Their very varied composition, and the great 

 number of bases which enter into the composition of some of the amphiboles and the 

 pyroxenes, are illustrations of the polybasic character of the silicates. 



With the pyroxenes some mineralogists have grouped spodumene, a^girite, arfvedson- 

 ite and acmite, the association being based on similarity of crystalline form, and supported 

 by a misconception of their chemical relations. All of these species find their position in 

 the next suborder, and the place of the last three is near to garnet and to epidote. We 

 have already noticed (§ 49) the presence of alumina in certain pyroxenes and certain 

 amphiboles, of which latter pargasite is the type, serving to connect the protosilicates 

 with the succeeding suborder of protopersilicates. 



§ G5. The relations of amphibole and pyroxene to each other and to wollastonite, as 

 shown in the unlike degrees of condensation made eA'ident by the different values of V, 

 were pointed out by the present writer in 1853, as examples of isomerism in polysilicates, 

 when the three were represented as belonging to as many homologous types (§ 18.) These 

 relations, so far as amphibole and pyroxene are concerned, were mentioned some years later 

 by Dana, in 1868, when he noticed that the pyroxenes have a specific gravity about one- 

 tenth greater than that of the corresponding^ amphiboles. ' The chemical difference 

 between these species and the corresponding spathoids is seen in the resistance of both 

 amphibole and pyroxene to acids, which decomfj^ose wollastonite. Ehodonite, a manga- 

 nesian species with the crystalline form of pyroxene, appears, from its volume, to be more 

 closely related to amphibole, and is partly decomposed by acids. Different and unlike 

 varieties of pyroxene agree closely with each other, with enstatite, and with chrysolite, in 

 the value of Y, as will be made evident by the accompanying table. No. IV. In this the 

 four pyroxenes compared were examined and analyzed by the writer. 



To this tribe of Protadamantoids we add titanite and guarinite, two titanosilicates 

 of unlike crystalline form, but of identical composition and specific gravity. The 

 solubility of titanite in acids has already been noticec^ in s^ ôt. Here also is the place of 

 danburite, a borosilicate remarkable for haAang a value of V, near to that of the pectolitoid 

 borosilicate, datolite. The amphiboles, rhodonite, chondrodite and monticellite, are the 

 adamantoids which approach nearest to the spathoids, from the denser species of which, 

 tephroite, he! vite and leucophanite, they are not far removed in volume. 



Tribe 4. — Pkotophylloids. 



S^ 66. The phylloid type in the protosilicates is represented by a small number of mag- 

 nesian minerals, of which the best known is talc, apparently including two species with 

 different atomic formulas, but iudistinguishaT)le save by chemical analysis. To these must 

 be added one or more of the species generally classed under the head of serpentine. 

 Among them is thermophyllite, having a recorded density of 2.56-261, while marmolite, 

 with a similar composition, should, if its density be really 2"41, constitute another Protophyl- 



' System of Minpralogy, 5tli TA., p. 240. 



