MINERALOGY ; WITH A CLASSIFICATION OF SILTCATIÎS. 77 



Moufiua, iu ItsHy, consist of a hydrous soda-alumina silicate, having the composition of 

 aualcite. This observation the writer has confirmed, a large crystal from the same locality, 

 soft, white and earthy in texture, having yielded him on analysis ten per cent, of soda, 

 besides a little potash. 



§ 101. In the study of these persilicates it becomes necessary in each case to deter- 

 mine whether we have to do with really amorphous and argilloid species, or, as in the 

 case of crystalline kaolin and pholerite, with phylloids, which are apparently less hydrated 

 than the argilloids. The origin of the more and of the less aluminous species, like allophane 

 and schrotterite on the one hand, and like montmorillonite and cimolite on the other, I'emains 

 be discovered. These seem, for the most part, to be like halloysite, true colloids, and their 

 separation from aqueous solution is apparent from the occurrence noted by Daubrée of an 

 amorphous halloysite-like matter deposited by the thermal water of Plombières in France, 

 which is probably identical with the saponite of Nickles, a highly hydrated silicate, more 

 siliceous than halloysite, from the same thermal spring. 



These aluminous silicates, like other colloids, such as opal and beauxite, are probably 

 capable of assuming a soluble modification, and have all been deposited from solutions. 

 That such a dissolution and deposition takes place on a large scale is apparent from the 

 existence of the so-called indianaite. This name has been given to a material found in the 

 coal-measures in the State of Indiana, where it has been mined and emj^loyed to a consid- 

 erable extent in the manufacture of potter's ware. It occurs in irregular beds, often 

 several feet in thickness, beneath a stratum of sandstone, and is associated with and over- 

 lies limonite. It is often translucent in aspect, with a conchoidal fracture, and has all the 

 aspect of a colloid. In composition it is somewhat more basic than halloysite, the 

 atomic ratios of ahimina and silica being about 6 : t, and may perhaps be regarded as 

 this species with an admixture of allophane, translucent masses of which, iu a pure 

 state, are found imbedded therein. Indianaite contains about 2.30 hundredths of water, 

 which after a long exposure to a temperature of 100° centigrade, is reduced to 14".5.' The 

 species, wolchonskoite and chloropal, which we have placed in the table near to keramite 

 and montmorillonite, show that chromic and ferric silicates present similar conditions to 

 the silicates of alumina. 



§ 102. We have thus far, iu the third part of this essay, briefly explained the applica- 

 tion of our system of mineral classification to the natural silicates, and have endeavored to 

 show how the three suborders into which these may, on chemical and genetic grounds, 

 be separated, are each divided into five tribes by physical differences repeated more or 

 less completely in each suborder. The spathoid type is naturally separated into two parts, 

 the one highly hydrated, constituting the hydrospathoid tribes, represented iu the first and 

 second of these suborders by what we have called the pectolitoids and zeolitoids, both 

 readily decomposed by acids, even in the case of the more highly siliceous species. The 

 other part, anhydrous, or with a smaller portion of (•ombin(>d water, seldom over one or 

 two hundredths, includes the protospathoid and protoperspathoid tribes, in the latter of 

 which the more siliceous species resist the action of acids. The hydros^iathoids and spa- 

 thoids in the first two suborders (with a few exceptions which have been noted) 

 agree in having a larger volume than the species of the succeeding tribes. The 



' Reports Geological Survey of Indiana, 1874, p. 15, and 1878, p. 154. 



