ALTERATIONS OF ANKERITE AND PARANKERITE. 243 



clinic; sp. gr. 3.25-3.4) and to actinolite (monoclinic ; sp. gr. 3.00-3.20). 

 Supposing that the magnesium and iron are present in equal quantity in 

 the sahlite, the reaction in the case of normal ankerite is : 



(1 ) CaFeC 2 6 .CaMgC 2 6 -HSi0 2 =MgFeCa 2 Si 4 12 -HC0 2 --k. 



Supposing the silica to be present as a solid, the decrease in volume is 

 37.27 per cent. In the formation of actinolite from normal ankerite, on 

 the supposition that the iron and magnesium are present in equal quantity 

 in the actinolite, the reaction is: 



(2) 3CaFeC 2 6 .CaMgCA+8Si0 2 =Mg 3 Fe 3 Ca 2 Si 3 24 -HCaC0 3 -i-8C0 2 ---k. 



The decrease in volume, supposing the silica to be present as a solid and 

 the CaC0 3 as a solid, is 22.62 per cent. Of course, if the ferro-dolomite 

 were one in which the calcium carbonate is not so plentiful, being replaced 

 in equal molecular parts by magnesium and iron, it would not be necessary 

 for any calcium carbonate to form as a result of the reaction. For instance, 

 if the ferro-dolomite were CaFesC^O^.CaMgsdOjs the reaction would be as 

 follows : 



(3) CaFe 3 C 4 12 .CaMg 3 C 4 I2 +8Si0 2 =Mg 3 Fe 3 Ca 2 SiA4+SCO,-k. 



Using the specific gravity of normal ankerite, the decrease of volume of 

 the actinolite as compared with the ankerite and quartz is 32.72. 



Sahlite and actinolite are both known to form abundantly in the zone 

 of anamorphism. Sahlite is found in the marbles of eastern United States. 

 Actinolite is very abundant in the iron-bearing formations of the Lake 

 Superior region. The development of these silicates may be taken as 

 typical illustrations of the reaction of silication in the lower physical- 

 chemical zone, with condensation of volume and absorption of heat. 



MAGNESITE. 



occurrence. — Magnesite may be a product of the alteration of any of the 

 heavily magnesian rocks. It is especially prevalent in the olivinitic 

 rocks and the chloritic, serpentinous, and talcose schists and gneisses, 

 being a product which is produced by the alteration of original minerals 

 simultaneously with the formation of chlorite, serpentine, and talc. It 

 is also found in dolomite. The more important minerals from which it is 

 recorded as forming are common garnet, olivine, pyrope, and serpentine. 



Alterations. — No alterations are recorded for magnesite. There is, how- 

 ever, no doubt that this compound does break up in the zone of anamor- 



