OCCURRENCE AND ALTERATIONS OF ARAGONITE. 245 



in the belt of weathering. The alteration to hematite occurs as a somewhat 

 deeper seated change, usually in the belt of cementatiou of the zone of 

 katamorphism. The alteration to magnetite is especially characteristic of 

 the zone of anamorphism, but it can not be asserted not to take place in 

 the belt of cementation. The alteration to gTiinerite occurs under deep- 

 seated conditions, and is in its heat and volume relations a characteristic 

 reaction of the lower zone. Magnetite and griinerite often form simul- 

 taneously. (See p. 284 ) The series of changes from siderite are very 

 interesting, in that the volume changes are all diminutions, and therefore,, 

 so far as this factor is concerned, might take place in either zone. The first; 

 three reactions (equations 1, 2, and 3) liberate heat, and hence these reac- 

 tions in their physical-chemical relations are similar to those of dolomite, 

 discussed on pages 182, 240, and may take place in both zones. But the 

 reaction of equation (4) probably absorbs heat, and that of (5) certainly does. 

 Magnetite having the origin represented by equation (4) is probably, and 

 griinerite is certainly, confined to the zone of anamorphism, where pressure 

 is a controlling factor. 



ARAGONITE GROUP. 



The only important rock-making member of this group is aragonite. 



A11AUO.MTK. 



Aragonite: 

 CaC0 3 . 



Orthorhombic. 

 Sp. gr. 2.93-2.95. 



. — A chief source of aragonite is as an organic precipitate. 

 It occurs intimately associated with calcite in numerous marine shells.' 

 While abundant, it is very subordinate to calcite as an organic deposit. A 

 second abundant source of aragonite is as a chemical precipitate, frequently 

 in association with beds of iron carbonate and gypsum. It also occurs as 

 a chemical precipitate from ground-water solutions, in openings in rocks, 

 especially at places where the temperature of the solutions is from 30° to 

 100° C. or more. Aragonite is not mentioned as an alteration product 

 of other minerals. 



Alterations. — The chief change of aragonite is to calcite (rhombohedral ; 

 sp. gr. 2.713-2.714). This is a change involving recrystallization, increase 

 of symmetry, and lowering of specific gravity. The increase in volume is 

 8.35 per cent. The heat effect of the change has not been found; but it 



