348 A TREATISE ON METAMORPH1SM. 



usual, no attempt is made to write equations for these degenerative changes; 

 but if one knew definitely the composition of the original mineral and that 

 of the minerals which were produced in a given case, it would be easy to 

 write equations for the change and to calculate the volume relations. 



While the alterations of chlorite in the zone of anamorphism are not 

 recorded, it is certain that the chlorite of chloritic rocks under the condi- 

 tions of the lower physical-chemical zone pass into other constituents, since 

 chlorite is almost always rare or absent in both the sedimentary and the 

 igneous rocks which have recrystallized in the lower zone and have not 

 been later affected by changes in the upper zone. 



Therefore in the lower zone chlorite and some of the material of the 

 associated minerals recombine and reproduce minerals from which chlorite 

 was originally derived, or other minerals. There is little doubt that chlorite 

 furnishes a considerable part of the elements for such minerals as the micas, 

 feldspars, amphiboles, pyroxenes, and even the olivines, which develop in 

 the zone of anamorphism, and also it is probable that the chlorite furnishes 

 a part of the constituents for certain of the heavy metamorphic minerals, 

 such as garnet, clintonite, staurolite, tourmaline, etc, 



SERPENTINE-TALC GROUP. 

 SERPENTINE AKD TALC. 



The serpentine-talc group includes: 



Serpentine: 



H 4 Ma 3 Si 2 9 . ( A part, of the Mg may be replaced by Fe, and where the amount of Fe is 



considerable this mineral is called bastite. ) 

 Monoclinic. 

 Sp. gr. 2.50-2.65. 



Talc: 



H 2 Mg 3 Si 4 12 - 



Orthorhombic or monoclinic. 

 Sp. gr. 2.7-2.8. 



Serpentine and talc, like chlorite, are both hydrous magnesium silicates. 

 Indeed, as has been pointed out, Tschermak regards the serpentine mole- 

 cule with the amesite molecule (H 4 Mg 2 Al 2 Si0 9 ) in variable proportions to 

 constitute the chlorites. Serpentine is more hydrous and more basic than 

 talc. Since the serpentine molecule is similar to some of the chlorites, 

 one would expect that the occurrence of the two would be very similar, 

 and such is the fact. 



