ALTERATIONS OF DOLOMITE. 241 



a previously formed magnesium limestone or dolomite which is in the 

 belt of weathering. Dolomite produced by the carbonation of the silicates 

 or by solution of dolomitic formations is an important chemical precipitate 

 in caves and small crevices in the rocks, the same as calcite. 



In summary, dolomite is chiefly derived as a secondary mineral from 

 ankerite, calcite, and parankerite. 



Alterations. — An important alteration of dolomite is to diopside (mono- 

 clinic; sp. gr. 3.2-3.38). This alteration is a typical example of silication. 

 (See p. 205.) The most probable reaction is: 



(1 ) MgCaC 2 O 6 +2SiO 2 =Mg0aSi 2 O 6 +2CO 2 -k. 



The decrease in volume is 40. 1 1 per cent, provided all of the silica entering 

 into the combination was a solid. In case all of the silica were introduced 

 through water solutions there would be an increase in volume of 2.03 per 

 cent. More important alterations of dolomite are into tremolite (mono- 

 clinic; sp. gr. 2.9-3.1) and calcite (rhombohedral ; sp. gr. 2.713-2.714), 

 or into tremolite and wollastonite (monoclinic; sp. gr. 2.8-2.9). In the first 

 case the reaction is: 



(2) 3CaMgC 2 6 +4Si0 2 =Mg 3 CaSi 4 12 -r-2CaC0 3 +4C0 2 -k. 



The decrease in volume, provided the silica is present as a solid, the 

 calcite remains as a solid, and the carbon dioxide escapes, is 25.20 per cent. 

 However, the excess of calcium carbonate may simultaneously change to 

 wollastonite. In this case the reaction would be: 



(3) 3CaMgC 2 6 +6Si0 2 =Mg 3 CaSi i O I:! +2CaSi0 3 +6C0 2 -k. 



The decrease in volume as compared with the dolomite and quartz of the 

 tremolite and wollastonite is 33.09 per cent. In both of the changes, if a 

 portion of the silica be supposed to be introduced from an outside source 

 the decrease in volume would be lessened, and if all of it were thus sup- 

 posed to be introduced there would be an increase in volume from the 

 solid dolomite of 9.89 per cent in the case of tremolite and calcite, and 14 

 per cent in the case of tremolite and wollastonite. 



The space once occupied by dolomite, like that occupied by calcite, 

 may be taken by other carbonates or by various oxides. The most impor- 

 tant of these are carbonate of iron and oxide of iron. The carbonate may 

 be a replacement, or possibly a substitution, of the iron of some other iron 

 mon xlvii — 04 16 



