Washington — Constitution of Some Salic Silicates. 555 



Art. XLIX. — The Constitution of Some Salic Silicates,' by 

 Henry S. Washington. 



Introduction . — In this paper is suggested an explanation of 

 the chemical constitution of the feldspars, lenads, scapolites, 

 and zeolites, which accounts' for the intimate relationships that 

 they show. In brief, the hypothesis is that they are alumino- 

 silicates, possess a chemical feature in common, the presence of 

 certain atomic groups, and that the formulas of the feldspars, 

 lenads,* and zeolites, are of identical type, while that of the 

 scapolites is different but closely analogous. 



The difficulties presented by the silicate minerals in the 

 study of their molecular constitution are so great, especially as 

 compared with organic compounds, that we are still only on 

 the threshold of knowledge adequate for discussion, and any 

 suggestions put forth at the present time must be regarded 

 as tentative and speculative. Still, certain facts are known, and 

 these, with recent applications of physical chemistry to miner- 

 alogy, give justification for dealing with the subject. 



The literature of the mineral groups mentioned is so exten- 

 sive that it cannot be gone into exhaustively here, and only a 

 few of the more recent papers will be mentioned. 



Relationships. — The minerals of the four groups under con- 

 sideration are alike chemically in being silicates of aluminum 

 and of potassium, sodium, or calcium, any two or all three of 

 the last being often present together. Some feldspars contain 

 barium in place of calcium, and this element and strontium are 

 present in a few. zeolites, while lithium is essential in eucryptite. 

 All the minerals containing these elements, however, are of 

 very rare occurrence. The function of the invariably present 

 aluminum is unknown, though it is generally assumed to he 

 present as a base. The zeolites and analcite are hydrated, 

 containing either water of crystallization, acid hydrogen, or 

 constitutional hydroxyl, in either case very loosely combined. 

 None of these minerals ever contains magnesium, ferrous or 

 ferric iron, manganese, titanium, boron, or fluorine, as essential 

 constituents, in this differing radically from the large and 

 important groups of the pyroxenes, amphiboles, micas, garnets, 

 vesuvianites, epidotes, tourmalines, and olivines. The scapo- 

 lites alone may contain small amounts of CI and S0 3 . These 

 are also present in the minerals of the sodalite group, which 

 show some affinities with the lenads, but which are now usually 



* The name lenad has recently (C. I. P. W., Quant. Class. Ign. Kocks, 

 Chicago, 1903, p. 132) been proposed to replace the term feldspathoid, 

 including leucite, nephelite and their congeners, and this will be used 

 throughout this paper. 



