ALTERATIONS OF CHLORITE. 347 



very abundant secondary constituent in the metamorphosed sedimentary 

 rocks, such as slates, schists, and gneisses. Frequently chlorite may occur 

 more abundantly adjacent to intrusive rocks than elsewhere. The most 

 characteristic associated secondary minerals are epidote, serpentine, talc, 

 zeolites, kaolin, magnesite, iron oxides, aluminum oxides, etc. 



In the discussion of the individual minerals it has been shown that 

 chlorite is one of the abundant derivation products of the following minerals : 

 Almandite, augite, garnet, hornblende, iolite, prehnite, pyrope, staurolite, 

 tourmaline, and vesuvianite. Being essentially a magnesium-aluminum 

 silicate, it is especially likely to be derived from the heavily magnesian 

 minerals, of which the olivine, pyroxene, amphibole, mica, and garnet 

 groups are the more important. As already seen, corundophilite and pro- 

 chlorite may contain a large percentage of iron monoxide, and therefore 

 one would naturally expect these chlorites to form from the minerals which 

 also contain a large percentage of iron monoxide, as, for instance, olivine, 

 axtinolite, etc. In many cases the mineral from which the chlorite is 

 derived does not contain a sufficient amount of magnesium. In such cases 

 this substance is derived from adjacent minerals, or is brought in in solution. 

 It has been supposed in such cases that the magnesium is transported as a 

 carbonate. However, the principles of its develojjment would be in no 

 way changed if any other salt of magnesium, such as magnesium chloride, 

 were substituted for the carbonate. 



In the discussion of the individual minerals it is shown that chlorite 

 develops especially in the upper physical-chemical zone, and particularly in 

 the belt of cementation. Under conditions of quiescence it develops at 

 very considerable depth; but in proportion as interior movement occurs it is 

 likely to develop in smaller quantity or not at all, its place as a metamor- 

 phic mineral being taken by the mag-nesian mica biotite. 



Alterations. — The alterations of chlorite, like those of other minerals, are 

 largely dependent upon the zones or belts in which the mineral is located. 

 The only definite alteration products of chlorite which are recorded are 

 those which Tschermak has called enophite and berlanite. The first is said 

 by him to be a serpentinous variety of chlorite. No formula for either has 

 been determined, and therefore it is not possible to write equations repre- 

 senting the transformation. Rosenbusch says that the last stage of the 

 alteration of chlorite is into an aggregate of limonite, carbonate, and quartz. 

 This degeneration is especially characteristic of the belt of weathering. As 



