298 A TREATISE ON METAMORPHISM. 



Haiiynite and noselite are common in certain igneous rocks, especially 

 those which contain nephelite and leucite. Neither of these minerals is 

 known in the schists and gneisses derived from the sedimentary rocks. 



Alterations. — The minerals alter to zeolites, especially to natrolite (ortho- 

 rhombic; sp. gi\ 2.20-2.25), stilbite (monoclinic; sp. gr. 2.094-2.205), and 

 chabazite (rhombohedral ; sp. gr. 2.08-2.16). Simultaneously with certain 

 of these alterations calcite (rhombohedral; sp. gr. 2.713-2.714) also forms. 



Noselite passes into natrolite according to the following reaction : 



( 1 ) 2Na 4 ( NaS0 4 . Al ) Al 2 Si 3 12 + C0 2 + 7H,0 = 



2( H 4 Na,Al 2 Si 3 12 ) +2A1 ( OH ) 3 + 2Na 2 S0 4 +Na 2 C0 3 +k. 



It appears that the change requires the formation of gibbsite (mono- 

 clinic; sp. gr. 2.3-2.4) or diaspore (orthorhombic; sp. gr. 3.3-3.5), although 

 these minerals are not recorded as forming contemporaneously with the 

 natrolite. Supposing the gibbsite to separate as a solid, and the sodium 

 sulphate and sodium carbonate to be taken into solution, the decrease in 

 volume is 16.44 per cent. 



The parallel reaction for the passage of haiiynite into natrolite and 

 gibbsite is as follows: 



(2) 2Na 2 Ca(NaS0 4 .Al)Al 2 Si 3 12 +2C0 2 +8H 2 = 



2 ( H.Na, Al 2 Si 3 12 ) + 2 Al ( O H) 3 +2CaC0 3 +2NaHS0 4 + k. 



Supposing the gibbsite and calcite to remain as solids with the natrolite, but 

 the sodium acid sulphate to pass into solution, the increase in volume is 4.99 

 per cent. 



As stilbite is a calcium-bearing silicate, it may be assumed that this 

 forms from haiiynite rather than noselite. The reaction is as follows: 



( 3 ) 6Na 2 Ca ( NaS0 4 . Al) Al 2 Si 3 12 +36H 2 + 6C0 2 = 



Ca 3 Al 6 (Si 3 8 ) 6 .18H 2 0+12Al(OH) 3 +3CaC0 3 +6Na 2 S0 4 +3Na 2 C0 3 H-k. 



It appears that the reaction for the formation of stilbite thus requires the 

 formation of calcite, and also of gibbsite or diaspore. The equation is 

 written for the former mineral, but could readily be chauged to the latter. 

 Supposing the calcium carbonate and the gibbsite, as well as the stilbite, to 

 be solids, and the other compounds to be taken into solution, the increase 

 in volume is 0.460 per cent. 



