Based on 'Rock glazes of NZ'
Minna Bondy
Sample 77: Wairau river sand
The commonest recognisable grains are of quartz, most of this is silt is probably derived from schist and is probably close in composition to it.
The main reason I was inspired to revisit Bondy's work is that her locations aren't clearly documented. Regional differences in rock change the molecular structure, iron sand, sandstone etc will provide different results from different areas. It's important for me to document the sites well, so these differences can be noted.
The Wairau River is 170km long and runs most of its length along the Wairau Fault, a continuation of the Alpine fault. This fault also separates two distinct geological areas which would contribute to a definite regional mix of rock.
Marlboroughonline.co.nz says "the rock in the Wairau catchment is either hard, medium-grained sandstone 'greywacke', or fine black mudstone 'argillite." Which differs from Bondy's schist and quartz description, so it will be good to look further into its geology.
Most of the Wairau River is rocky and braided, however, the point where I collected the samples, Boyces Road, is a popular swimming hole and sandy beach, near Renwick.
There were two grades of sand a fine silty grain on top and a courser grain beneath. I chose the finer as the closer I can get to a final glaze consistency the better.