238 A TREATISE ON METAMOEPHISM. 



phyllite, aragonite, augite, diopside, dolomite, epidote, fluorite, garnet, 

 grossularite, gypsum, haiiynite, hornblende, noselite, parankerite, pyrope, 

 sahlite, scapolites, tremolite, and zoisite. 



While the abundant direct sources of calcite are (1), (2), and (3) 

 above, the indirect and ultimate source which has probably furnished the 

 great quantity of calcium carbonate is the carbonation of the silicates. (See 

 pp. 473-480.) This process occurs on a great scale in the zone of kata- 

 morphism, especially in the belt of weathering. It is a reaction which 

 takes place with liberation of heat and increase of volume in case the 

 replaced silica separates as quartz in situ. Many of the individual carbo- 

 nation reactions of the silicates, as, for instance, wollastonite, diopside, etc., 

 are given under that class of minerals. 



Alterations. — The first of the alterations of calcite is recrystallization. Cal- 

 cite is the most mobile of the abundant rock-making- minerals. It responds 

 readily to changes of physical conditions, and is very susceptible to weak 

 chemical agents. A slight stress may produce in it twinning structure. A 

 state of unequal strain favors its solubility. Where the pressure increases, 

 solution increases; where pressure is lessened, deposition takes place. 

 Increase of temperature greatly increases its solubility, and vice versa. 

 The increase of carbon dioxide in water greatly increases its solubility, 

 and vice versa. Tims it happens that in rocks where the calcite is almost 

 constantly subjected to changing pressure, temperature, and varying 

 amounts of carbon dioxide it is constantly being taken into solution 

 and, after a greater or less journey, being- deposited from solution or 

 carried to the sea to be ultimately precipitated by organic agents. The 

 recrystallization of great masses of calcite, the solution of calcite in the 

 belt of weathering and its partial deposition in the belt of cementation, the 

 formation of caves, cave deposits, etc., are considered later. 



The second important change of calcite is partial replacement of cal- 

 cium by magnesium, often producing dolomite (rhombohedral; sp. gr. 

 2.8-2.9). The generalized reaction is: 



(1) 2CaC0 3 +Mg=CaMg(C0 3 ) 2 +Ca+k. 



Supposing- the calcium to be present as a carbonate, and supposing the 

 added magnesium to be a chloride — and this is believed to be a very 

 common case — the reaction would be: 



(2) 2CaC0 3 +MgCl 2 =CaMgCA+CaCL+k. 



