208 



Polysilicates 



f{Z (sO ly- (Si308) 



(orthoclase) (albite) 



and orthosilicates of the less active metals, 

 calcium, magnesium and iron (pyroxenes, 

 olivine and anorthite) do. The relative 

 chemical activity of all of the elements 

 common to igneous rocks may be illustrated 

 in like manner, and the probabilities of 

 various pyrogenetic mineral compounds 

 forming from different rock magmas may 

 ,be made clear. 



One of the most important factors in the 

 •discussion of the chemistry of igneous rocks 

 is the role of hydrogen, whether as an active 

 H)ase-forming element, or as a catalytic 

 :agent, alone, as hydrogen (H), or com- 

 Ibined with oxygen, as hydroxyl (OH). Its 

 lexact behavior in each specific case is not 

 definitely known, but the principles appli- 

 cable to several distinguishable cases are 

 clearly established. 



Adopting the idea that an acid is a 

 hydrogen salt in which hydrogen plays the 

 role of a positive, base-forming element, 

 an acid salt may be considered as one in 

 which all of the hydrogen has not been 

 replaced by other base-forming metals. 

 Such an acid salt may be looked upon as a 

 substitution derivative from a hydrogen 

 salt (acid), or from a normal salt by the 

 introduction of hydrogen in place of other 

 positive metals. An example of such a 

 compound among pyrogenetic minerals is 

 to be found in museovite (K, H)Al(Si04). 

 This might be derived from H.SiO., 



KAl(SiOJ or AU(SiOj3. The formation 

 of such a compound involves the presence 

 of active hydrogen to play the role of metal. 

 Muscovite is a common pyrogenetic mineral 

 in some igneous rocks rich in silicon, with 

 much uncombined silica, and also in others 

 comparatively low in silica, accompanying 

 orthosilicate and nephelite. It forms by 

 the side of polysilicates— orthoclase and al- 

 bite— and even with the disilicate, petalite. 

 The formation of museovite must be as- 

 cribed to the action of hydrogen upon sili- 

 con, either directly in the first instance, or, 

 if previously formed silicates of aluminium 

 and potassium be assumed to be the source 

 of the compound in question, then its action 

 in replacing part of the potassium must be 

 that known as hydrolysis, whereby the 

 hydrogen ions from water replace metals 

 in the salt through a process of double 

 decomposition. 



It is known that the chemical activity of 

 hydrogen even toward a gas, like oxygen, 

 is greatly increased by rise of temperature ; 

 hydrogen being rather inert at ordinary 

 temperatures. The relative activity of hy- 

 drogen and potassium toward silicon is indi- 

 cated by the fact that the highest hydrogen 

 silicate definitely known is the orthosilicate, 

 H^SiOi (orthosilieic acid), whereas potas- 

 sium commonly occurs in a polysilicate, 

 KAlSigOs (orthoclase). It has been found 

 impossible to produce mica in open eru- 



