THE ORTHORHOMBIC PYROXENES. 267 



As before, supposing- the potassium carbonate to be taken into solution, the 

 decrease in volume of the orthoclase and muscovite as compared with the 

 leucite is 12.43 per cent. 



In the passage of leucite into orthoclase and nephelite it is necessary 

 to suppose that a part of the potassium of the leucite is replaced by sodium. 

 Supposing the nephelite formed to be a pure soda-nephelite, the reaction 

 would be: 



(4) 4KAlSi 2 6 +Na 8 C0 3 =2KAlSi s 8 +2NaAlSi0 4 +K 2 C0 3 +k. 



Ignoring the carbonates, the decrease in volume of the orthoclase and 

 nephelite as compared with the leucite is 7.59 per cent. 



The reactions above given, except the last, are those of hydration, 

 and the second and thh'd are those of carbonation also. They are, 

 therefore, reactions which are to be expected in the zone of katamorphism. 

 The change of leucite into orthoclase and nephelite gives decrease in 

 volume, with neither hydration nor dehydration, carbonation nor silication. 

 It is, therefore, to be expected that the change is one which takes place in 

 the zone of anamorphism. 



The artificial transformation of leucite into aualcite by treatment 

 with soda solutions, and the reverse alteration of analcite into leucite 

 by treatment with potassium solutions, as shown by Lemberg, a is an 

 excellent illustration of the law of mass actioD and proves the importance 

 of this principle under natural conditions. 



PYROXENE GROUP. 



ORTHORHOMBIC PYROXENES. 



ENSTATITE, BROXZITE, AXD HTPERSTHENE. 



Enstatite: 



MgSiO,. 



Orthorhombic. 



Sp. gr. 3.1-3.3. 

 Bronzite: 



(MgFe)Si0 3 where Mg:Fe : : 8:1, 6:1, and 3:1. 



Orthorhornbic. 



Sp. gr. 3.2-3.3. 

 Hypersthene: 



(MgFe)Si0 3 where Mg:Fe : : 3:1, nearly to 1:1. 



Orthorhombic. 



Sp. gr. 3.4-3.5. 



«Lemberg, J., Ueber Silicatumwandlungen : Zeitschr. Deutsch. geol. Gesell., vol. 28, 1876, pp. 

 536-545. 



