[Clayart] Sand

[Joseph Herbert]

Hello,

First, “sand” is a particle size designation that says nothing about
composition.  There are five divisions of sand size in the Wentworth Scale
- very course to very fine, 1-2 mm to 0.0625-0.125mm . Sieves 10 through 18
comprise very course sand and sieves 120 through 230 for very fine.

Natural sands, especially finer sizes, tend to be largely composed of
quartz (silica) because quartz is so hard.  Other rock forming minerals are
not as hard or as immune to chemical weathering as is quartz.

There are notable exceptions, like the green olivine beaches in Hawaii and
rutile sands mined as titanium ore. So “sand” can have nearly any mineral
composition.

A famous high purity silica sand is from Ottawa Illinois where the St.
Peters sand stone is mined and processed.  The sand grains are very
rounded.  According to some sedimentologists, this roundness and the
absence of other composition particles indicates a long distance of travel
from the parent igneous rock.  Perhaps these grains from the St Peter
sandstone were previously deposited in another sedimentary rock, re-eroded
(maybe more than once), and deposited as the St Peter.  This idea derived
from the estimated distance of travel for the rounded grains being
significantly farther than the length of any existing rivers.

Conversely, sands found near their source, like beach sands of the Great
Lakes, will have many non-quartz grains present making them more
interesting for some purposes.  Fatally flawed for others.

One of our correspondents described “cubic” grains of quartz.  Quartz is in
the trigonal crystal class and displays no cleavage.  Conchoidal fracture
is typical in broken quartz crystals, looking much like broken glass.
Equant grains, perhaps; cubic grains, like table salt, no so much.

Because many sand suppliers only care about the size of what they sell, the
user needs to be more aware and demanding to get sand that meets their
needs.

Joe
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