Clays allogenic

Chamosite appears to be the finest grained and most abundant mineral in this group. It occurs in lateritic clay deposits (Brindley, 1951), both as oolites and matrix in sedimentary ironstones (Hallimond, 1925), in hydrothermal deposits (Ruotsala et al., 1964), in shales (Drcnnan, 1963), in Recent shallow-marine deposits (Porrenga, 1966) and in estuarine sediments (Rohrlich et al., 1969). It is probable that chamosite is more abundant than commonly realized however, Drennan (1963) has pointed out that it is extremely unstable in a leached and oxidized environment and is not likely to persist as an allogenic mineral. 

Clays introduced shortly after deposition occur as burrows lining or filling, grain coatings, and randomly distributed flocculated aggregates. Figure 1 depicts various forms of allogenic clays in sedimentary rocks. 

Figure 1. Modes of occurrence of allogenic clay in sandstones.

Red clays are mainly allogenic in origin (Glasby 1991). In the Pacific, this allogenic component is dominantly eolian dust. The high input of dust from... 

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