MINEEALOGY; WITH A CLASSIFICATION OF SILICATES. 45 



etc. These anhydrous spar-like species we designate as the tribe of the Protospathokls. 

 In the third place we note a group of species not unlike the second in general aspect and, 

 like them, generally anhydrous ; which are, however, harder and considerably denser, as 

 appears from the reduced value of Y. This grouj) is represented by chondrodite, chryso- 

 lite, phenacite, amphibole, pyroxene, dauburite, titanite, etc. Many of these species pre- 

 sent a hardness and transparency which caused them to be included by Mohs and his 

 school in i he order Gem, and this gem-like or adamantoid character si^ggests for them 

 the tribal name of Protadamantoids. As regards their relation to acids, it may be noted 

 that while the spathoid wollastonite is readily decomposed thereby, the corresponding 

 adamantoid bisilicates, amphibole and pyroxene are unattacked. The highly basic chryso- 

 lite pectises with acids, but the more condensed phenacite and bertrandite, with the same 

 atomic formula, resist their action. The case of titanite, a titanosilicate, is peculiar, for the 

 reason that the titanic oxyd, of which it contains so large a proportion, is soluble in hydro- 

 chloric acid. This decomposition of titanite was long since studied by the writer, who 

 showed that the titanic oxyd thus dissolved presents chemical reactions very different 

 from that got from menaccanite by the same solvent, or from titanite itself by the action 

 of hot sulphuric acid, and then described it as a peculiar modification of titanic acid. ' 



§ 51. TTe recognize in the fourth place among the protosilicates a group characterized 

 by a hardness less than that of the three preceding tribes, and by a very marked basal 

 cleavage, yielding more or less flexible laminae, as is well seen in talc and in the foliated 

 serpentines. This type, but sparingly shown in the first suborder, is largely developed in the 

 micas and the chlorites of the second, and from its foliated structure is naturally designated 

 as phylloid, for which reason the fourth tribe of the protosilicates may be called Frotopliijl- 

 loids. There still remains in this order a considerable group of soft hydrous silicates, chiefly 

 magnesian, represented by much that is called serpentine, by deweylite, cerolite, chryso- 

 colla, etc. To these the tribal designation of Ophitoids, suggested by the Greek name for 

 serpentine, is given. They are, for the most part, readily decomposed by acids without 

 pectisation, and are amorphous colloid substances. The crystalline silicates which 

 approach these protocolloids in physical and chemical characters — in part phylloids. and in 

 part perhaps spathoids, — will be considered in the further discussion of the ophitoids. 



§ 52. Passing now to the second suborder, that of the Protopersilicates, we re- 

 cognize, as in the protosilicates, five tribes, which repeat the general characters of those 

 just noticed. The first is that of the great family of the zeolites, with prehnite and some 

 related species, which together constitute a hydrospathoid tribe conveniently designated 

 as ZeoUtoids. The resem.blances, as regards hardness, density, aspect and mode of occur- 

 rence, between this tribe of hydrous silicates and the pectolitoids, are such that, notwith- 

 standing their wide difference in chemical composition, the calcareous pectolitoids have 

 generally been confounded with the zeolites. 



The spathoids of the second suborder, constituting the tribe of the Proioperspallioids, 

 include a large number of species represented by melilite, gehlenite, ilvaite, the whole 

 family of the feldspars, sodalite, nephelite, leucite, iolite and petalite, of which the more 

 basic species are, like the protospathoids, decomposed by acids. The Protoperadamanfoids 

 form a large and important tribe of hard and gem-like species, including pargasite, ido- 



' Amer. .Toui-. Science, 1852, xiv. 340. 



