I think "silica sand" is recommended in British books to steer people
away from some other sources that might be bad for plants. I don't
know what those are. However, ocean beach sand should be avoided. You
also wouldn't want sand derived from ultramafic ("serpentine") rocks,
which are toxic to some plants.
The sand I use is a mixture of various minerals, primarily quartz
along with basalt and other volcanic material. The important thing is
that the particles be of various sizes, as Bob Nold mentioned, and
that they be angular rather than rounded. Some sand is loaded with
organic fines and has to be washed before being used with plants that
don't tolerate that. The sand I use comes from a quarry on a river
close to the Cascade range and the fines appear to be more mineral
than organic. You can improve the consistency of lowland river sand
by mixing in some kind of angular grit, such as quarter-ten crushed
rock or ground pumice. If you use crushed rock, it should not have
any fines in it (i.e., don't use the kind intended for compacting
into paths, etc.).
Different kinds of sand provide different levels of water retention
and more or less air space in the mix. Unwashed sand direct from a
quarry may also contain a significant amount of mineral nutrients
useful to plants, especially plants that naturally grow in soils low
in organic content.
Jane McGary
Portland, Oregon, USA
with apologies for mentioning our profligate use of pumice