Greywackes of the
greater Auckland Region (Brynderwyn Hills to Hamilton), when considered on a regional scale,
have characteristic ranges of physical, mineralogical and other properties. Some
properties (eg density, porosity, crushing resistance, clay index) generally
cluster and correlate with the basement greywacke facies, regions and terranes
recognised by geologists, and their diagenetic/metamorphic level. Other local
factors, particularly variations in mineralogical composition, nature of matrix
cement, grain size, and degree of weathering may produce aberrant properties.
Some of these local variations, when extreme, have the potential to affect the
aggregate's performance in roading or the concrete industries.

Smectite and the
zeolite laumontite are the major deleterious minerals occurring in fresh
greywacke rocks of the Auckland region, although weathered rocks may
contain halloysite, an expanding member of the kaolin clay mineral group. These
minerals, when found in abnormally high levels in greywackes cause expansion
effects in the aggregate. Currently there are inconsistencies in the detailed
correlation of shrinkage, clay index, clay mineral content and intrinsic
expansion data that need further research to resolve them.