OCCURRENCE OF HAUYNITE AND NOSELITE. 297 



The reaction for the alteration of sodalite to muscovite (monoclinic; 

 sp. gr., 2.76-3) and kaolinite (monoclinic; sp. gr., 2.6-2.63), supposing 

 potassium to replace one-fourth of the sodium of the silicate, would be — 



(7) 2(4NaCl.K 3 Na 9 Al I2 Si 12 J8 )+12H 2 0+9C0 2 = 



6KH 2 Al 3 Si 3 O 12 +3H 4 Al 2 Si 2 O 9 -|-8NaCl+9Na|0O3-)-k. 



Provided the sodium chloride and sodium carbonate are dissolved, the 

 decrease in volume is 37.07 per cent. 



As in the case of nephelite, it is suspected that sodalite may pass into 

 albite or other feldsjjar. However, as this change is conjectural, no reaction 

 will be written. 



The various reactions above given are analogous, both from a physical- 

 chemical point of view and from a geological point of view, with the corre- 

 sponding reactions in the case of nephelite. Hence it need only be said 

 that the changes written are those occurring in the zone of katamorphism, 

 in which rock fracture occurs and ground solutions are active. These 

 ground solutions by the changes become bearers of sodium chloride and 

 sodium carbonate. 



The relations between the alterations of nephelite and sodalite illustrate 

 very well the law of mass action. In the laboratory, if nephelite be exposed 

 to the " slow action of fused sodium chloride with the addition of vaporized 

 NaCl " it is changed into sodalite." On the contrary, however, in nature, 

 where water is abundant and the amount of sodium chloride is small, the 

 reverse reaction takes place, and sodium chloride is abstracted. Probably 

 at the same time the nephelite molecule is altered as above indicated. 

 Thus, while observation does not as yet .record nephelite as an alteration 

 product of sodalite, it is believed to be highly probable that this mineral is 

 really formed as a stage in the process of alteration of sodalite. 



HAUYNITE AXI) NOSELITE. 



-Haiiynite is sodium-calcium-aluminum silicate with some 

 sulphate. Noselite is sodium-aluminum silicate with some sulphate. 



«Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. S. Dana, Wiley & Sons, New 

 York, 6th ed., 1892, p. 430. See also Eosenbusch, Mikroskopische Physiographie, Stuttgart, 1SS5, p. 284. 



