346 A TREATISE ON METAMORPHISM. 



Proehlorite: 



H 40 Mg 23 Al 14 Si I3 O 90 to H 20 Mg 12 Al 6 Si 7 O 45 . 



Monoclinic. 



Sp. gr. 2.78-2.96. 



Clinochlore: 



H 20 Mg 12 Al 6 Si,O 4 5 to H 8 Mg 5 Al 2 Si 3 18 . 



Monoclinic. 



Sp. gr. 2.65-2.78. 



Penninite: 



H 8 Mg 5 Al,Si 3 18 to H 20 Mg 13 Al 4 Si 8 O 45 . 

 Pseudorhombohedral and monoclinic. 

 Sp. gr. 2.60-2.85. . 



A considerable part of the magnesium, as shown by the analyses, 

 may be replaced by iron, the analyses of corundophilite showing as high 

 as 15 per cent of monoxide of iron ; of proehlorite, from 15 to 25 per cent, 

 running even higher than the magnesia. The percentage of monoxide of 

 iron in clinochlore and penninite is usually much less. The alumina may 

 be replaced in part by sesquioxide of iron, although the proportion of this 

 replacement is not nearly so great as that of the magnesia by the iron mon- 

 oxide, the iron sesquioxide generally not running beyond 2 or 3 per cent. 



It is apparent that the specific gravities of the chlorites do not regularly 

 grade from lower to higher, as in the feldspars. Doubtless such a regular 

 gradation would occur provided the chlorites were pure magnesium min- 

 erals, corresponding to the formulae above given. The high specific gravities 

 which the intermediate minerals in the group, corundophilite and proehlo- 

 rite, may have are doubtless explained by their frequent high content of 

 iron monoxide. 



occurrence. — Chlorite is the most abundant and widespread of all the 

 secondary silicates. As a secondary mineral it is probably subordinate 

 only to quartz. Chlorite is nowhere known as a pyrogenic constituent of 

 igneous rock. It is very abundant in many of the altered igneous rocks, 

 both plutonic and volcanic, including lavas and tuffs, being especially 

 abundant in the so-called green-schists. Also it is very abundant in many 

 amphibolites. In the altered igneous rocks which are changed under mass- 

 static conditions it is one of the most abundant secondary constituents, 

 being especially prevalent in the basic rocks, such as the greenstones. It 

 is also found in the acid rocks. Chlorite occurs as a plentiful allogenic 

 constituent in all kinds of mechanical sedimentary rocks. It develops as a 



