MINERALS OF GARNET GROUP. 299 



The reaction for the formation of chabazite from haiiynite is — 



(4) 4Na 2 Ca(jS T aS0 4 .Al)A] 2 Si 3 12 +24H 2 + 6CO,= 



Ga 8 Al 6 (8i0 4 ),(Sij0 8 ),.18H 2 0+4Al(OH),+CaS0 4 +Al,(S0 4 ),+6N , a s OO,+k. 



This reaction again requires the formation of gibbsite or diaspore. Sup- 

 posing the compound to be gibbsite, and it and the chabazite to remain as 

 solids, and the other compounds to be taken into solution, the decrease in 

 volume is 7.46 per cent. 



The alterations of haiiynite and noselite to the zeolites, calcite, and 

 gibbsite or diaspore are all reactions of hydration and carbonation and 

 liberation of heat. If the readily soluble compounds are dissolved, as is 

 probable, the volume is decreased in most instances. The reactions are 

 therefore characteristic of the zone of katamorphism. 



GARNET GROUP. 

 (JROSSULARITE, PTROPE, ALMAXDITE, SPESSARTITE, MELAXITE, AXD ITVAROVITE. 



The garnet group includes the following rock-making species: 



Grossuldrite: 



Ca 3 Al 2 Si 3 12 . 



Isometric. 



Sp. gr. 3.55-3.66. 



Pyrope: 



Mg 3 Al 2 Si 3 12 . 



Isometric. 



Sp. gr. 3.70-3.75. ' 



Almandite: 



Fe 3 Al 2 Si 3 12 . 

 Isometric. 

 Sp. gr. 3.9-4.2. 



Mn 3 Al 2 Si 3 12 . 



Isometric. 



Sp. gr. 4.00-4.30. 



Melanite: 



Ca 3 Fe 2 Si 3 12 . 



Isometric. 



Sp. gr. 3.80-3.90. 



Uvarovite: 



Ca 3 Cr 2 Si 3 I2 . 



Isometric. 



Sp. gr. 3.41-3.52. 



