;»;>•» Washington — Constitution of Some Salic Silicates. 



regarded as allied constitutionally to the garnets. The heavy 

 metals are never present in nature, though artificial feldspars 

 and lenads have been prepared which contain lead, zinc, silver, 

 or thallium replacing calcium and the alkali metals, and ferric 

 iron replacing aluminum. 



In the feldspars, lenads, and zeolites the ratio of K a O + 

 Xa.,()4-CaO to A1.,0 3 is constant, 1 : 1, while in the scapolites 

 the ratio does not differ much from this. The relation of 

 silica to these constituents varies from orthosilicate to trisili- 

 cate ratios, and the various minerals are commonly regarded as 

 either orthosilicates, metasilicates, trisilicates, or mixed inter- 

 mediate compounds. That is, in spite of their chemical 

 resemblances and other evidences of close relationship, pres- 

 ently to be noted, they are usually regarded as salts of at least 

 three distinct silicic acids. 



The intimate chemical relations between these minerals and 

 mineral groups are also evident from their alterations in nature 

 and from experiments which have been made in their artificial 

 reproduction. The feldspars are quite readily alterable, chang- 

 ing into scapolites and zeolites, as well as to muscovite, 

 zoisite, and so forth. Feldspars, nephelite, analcite, and other 

 zeolites are common natural alteration products of leucite, and 

 leucite and analcite are readily transformable artificially, the 

 reaction being reversible. Pseuclo-leucite will be mentioned 

 later. Nephelite readily alters, especially to analcite and 

 zeolites, and analcite has been formed artificially from some 

 feldspars, as andesine. The scapolites are often derived from 

 the sodicalcic feldspars and are easily alterable themselves, 

 forming a varied number of products, among which the zeo- 

 lites do not often occur. The zeolites are always secondary, 

 with the exception of analcite in some igneous rocks, and are 

 generally derived from feldspars and lenads, of which most 

 of them are commonly regarded as hydrated forms. Many 

 of them yield feldspars on fusion. Kaolin is the most usual 

 end product of the alteration of the feldspars, lenads, scapo- 

 lites, and also the zeolites. 



The erystallographic similarities and relations between the 

 minerals of these groups, as the feldspars and leucite, and the 

 feldspars, lenads, and zeolites, are commented on by several 

 authorities, as Dana and Hintze, and only need mention here. 

 It also seems scarcely necessary to dwell on the intimate 

 relationship shown by the feldspars and lenads in igneous 

 rocks, as their penological importance and mutual relations 

 are too well known to need more than mention. 



Behavior with Acids. — Of apparently minor importance, 

 but with bearing on their constitution, as will be seen later, 

 is the action of hydrochloric and other acids on the various 



