368 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



very fine grained with an almost or quite clayey consistency in the 

 hand specimen and is not casually distinguishable from other clay 

 minerals except when coarse enough to have a distinctly silky luster. 



As a rock making mineral sericite is a major constituent of many 

 sericite or sericite-quartz schists making up the so-called hydro-mica 

 schists and many of what have until recently been called talc schists. 



Sericite is a mineral commonly formed by hydrothermal altera- 

 tion of igneous rocks and occurs as a replacement of the wall rocks of 

 many metalliferous veins, the replacement usually beginning with 

 the feldspars and proceeding in some cases until the whole mass of 

 the rock is composed largely of sericite. The replacement by sericite 

 is often accompanied by the deposition of secondary silica and dis- 

 seminated crystals of pyrite. Rock alteration of this type is, as a 

 rule, not confined to the immediate vicinity of the veins, but extends 

 outward with gradually diminishing intensity in a broad halo sur- 

 rounding the center of ore-deposition and hydrothermal activity. 

 This type of alteration is accomplished by hot alkaline waters and 

 it is such waters which have, in most cases, deposited metalliferous 

 ores. This is a generalization of great practical importance to geo- 

 logist and prospector alike in the location of valuable ore bodies. 

 The sericitization causes a bleaching of the rock, especially if this 

 l>e a moderately coarse lava, a granitic intrusive, or a porphyry. 

 The bleaching is accompanied by blurring and disappearance of the 

 outlines of the original minerals, loss of coherence, assumption of a 

 greasy or soapy feel, especially when moist, and the accession of 

 small crystals of pyrite. 



This alteration of "porphyry," which term, in western mining 

 parlance indicates almost any igneous rock occurring in association 

 with ore deposits, serves as a most important guide to the prospector, 

 even though he never recognizes sericitic alteration as such. An 

 experienced prospector will pass over miles of country with scarce 

 a glance at the outcrops because it is, to his eye, what he describes 

 as "hungry-looking country" — in reality because the rocks are hard 

 and fresh. Upon entering into an area characterized by sericitic 

 alteration he will, on the other hand, stop and search each foot of 

 the way for metalliferous veins because, in his terms, the "rotten 

 rock" is "likely-looking country" in which to expect metalliferous 

 veins. 



The sericitic replacement is usually especially vigorous in its 

 attack upon the crushed rock which occupies the fissure forming 

 the channel for the solutions and for this reason many of the so- 

 called "clay gouges" are nearly pure sericite. 



Under the microscope sericite appears as brightly polarizing 

 scales and flakes aggregated into confused masses which often, in 

 thin section, have a fibrous appearance. It often resembles talc, 



