A
survey has been undertaken of the petrography of North Island greywackes with a particular focus
on the occurrence in them of smectite (swelling clay) and chlorite, two minerals
that are closely associated, chemically similar, and commonly interlayered with
each other. Smectite is the mineral which is a major cause of greywacke
aggregates' poor performance. Smectite occurs in post-metamorphic shear zones
and in surface weathering profiles over greywackes. This study focuses on the
petrography of fresh greywacke rocks.

The
occurrence and abundance of smectite and chlorite and their relationship in
greywackes is largely a function of the chemistry of the sediment and the
maximum temperature that the sediment has been subjected to during diagenesis
and/or very low grade (anchizone) metamorphism. Rock chemistry, particularly
total iron and MgO, determines the smectite and/or chlorite content in
fine-grained greywackes (argillites and siltstones) but chemistry is less
influential in sandstones, where at low grades of metamorphism much of the Fe
and Mg of the rock is held in unreacted detrital minerals and the mineral
constituents of lithic grains.

Four
zones which represent different levels of diagenesis and very low grade
metamorphism, based on the crystallinity of illite and chlorite in the
greywackes, and the proportion of smectite interlayered with chlorite, have been
mapped in the North Island. In the anchizone smectite does not appear as a
discrete mineral, it is always interlayered with chlorite. In the upper
anchizone chlorite contains approximatelyten per centof smectite
interlayered with chlorite. In the lower anchizone about 30per
centsmectite can occur in chlorite.

In
addition to the smectite and chlorite inherent in the greywackes as the result
of the thermal overprint, smectite as a discrete mineral and smectite-chlorite
interlayered minerals with high levels of smectite, occur in abundance on shear
zone surfaces and in alteration zones adjacent to shear zones; these are the
main contributers to the high levels of smectite recorded in many greywacke
aggregates.