Triphormic clay minerals from the lower hunter permian succession of new South Wales

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Abstract
Triphormic clay minerals from the Permian sediments of the Lower Hunter Valley fall into two categories—mixed‐layer mica‐montmorillonites and mont‐morillonites. The montmorillonites from epiclastic sedimentary environments contain appreciable interlayer potassium and show “abnormal” (mica‐like) layer characteristics. The mixed layer minerals have mica‐montmorillonite ratios ranging from about 8:2 to 1:1. These structures are also mica‐like, but are usually richer in interlayer K+ and show more extensive tetrahedral Al for Si substitution than the fully expandable types. A common mica parentage is implied for these mixed‐layer and abnormal montmorillonite species. Mixed‐layer minerals from the coarser rock types invariably show more highly‐ordered interstratification than those from the lutites. The trend in ordering is toward maximum alternation and predominance of ABABAB. . . . type regularity. Strongest development is generally observed in the medium‐grained arenites. It is concluded that the ordering phenomenon is probably due to “unmixing” of interlayer cations in irregularly‐interstratified mica‐montmorillonite detritals, and may have been developed during recrystallisation of the matrix minerals. Fully‐expandable structures from the bentonitic clays of the Tomago and Newcastle Coal Measures are authigenic alteration products of volcanic glass. They are beidellites with low tetrahedral layer charges and alkali contents. The dominant interlayer components in these structures are Mg2+, H+, and Al3+.

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