Morewhatissand

Sand can be divided into three main categories:

Mineral sands are formed by weathering (mechanical and chemical breakdown)
of igneous (plutonic or volcanic), metamorphic or sedimentary rocks.
Weathering serves to free individual mineral grains or rock fragments from the
parent material. Igneous and metamorphic rocks are commonly referred to as
crystalline or hard rocks and sedimentary rocks are called soft rocks.

Interlocked mineral grains hold igneous and metamorphic rocks together. The
weathering of crystalline rocks therefore depends in large part upon the relative
stability of the minerals comprising the rock. As some minerals decompose,
those remaining are eventually liberated from the rock. Minerals such as olivine
and pyroxene form at pressures and temperatures found at substantial depth
winthin the earth's crust. These minerals are some of the earliest to form in
igneous rock bodies because of their high temperatures of crystallization.
Quartz, on the other hand, has a lower temperature of crystallization and is one
of the last minerals to form from molten magma or lava. Some other minerals
formed by igneous or metamorphic processes, which a collector might find in
sand are garnet, mica, hornblende. zircon and magnetite among others.

Because of the different temperature and pressure environments in which
various minerals form, they naturally exhibit different degrees of stability when
exposed to the surface environment of earth by weathering away of overlying
rocks. Olivine, pyroxene and amphibole are relatively unstable under surface
conditions and tend to weather to oxides and silicates of iron, magnesium
and/or calcium. Quartz, by contrast, is the most commonly found mineral on
the surface of earth because it is the most stable under atmospheric
conditions. Still, in the context of a human lifespan, the weathering process
of even the more unstable mineral phases may go practically unnoticed.

Quartz is composed of silica (silicon dioxide) and sands consisting
predominantly of this mineral are quite abundant. Tons of quartz sand are
utilized yearly for the production of computer chips, glass and concrete to
name but a few of its uses

Feldspar is the most common mineral found in the crust. Its structure can
accommodate calcium, sodium and potassium. Calcium and sodium readily
substitute for one another and form a soldi solution series group of feldspar
minerals collectively referred to as plagioclase. Calcium-rich plagioclases
form under conditions similar to those under which olivine and pyroxene form.
Consequently, the more calcic plagioclase minerals are relatively unstable at
the surface. Sodic plagioclase forms under conditions closer to that which
quartz forms and is more stable at the surface. Potassium-rich feldspar is a
relatively late appearing mineral in the crystallization sequence and more
closely approximated the stability characteristics of sodic plagioclase. All
feldspar exposed at the surface eventually weathers to aluminosilicate clay.

In the case of volcanic rocks, the matrix material quite often consists of
inherently unstable volcanic glass. Such matrices tend to devitrify (crystallize
from glass) to form a fine-grained mineral matrix. Fine mineral grains tend to
weather more rapidly than their coarser equivalents. The coarse mineral
grains will therefore be freed from the matrix and become available for
processing by the transport and deposition mechanisms that define the
sedimentary cycle.

There are other factors that contribute to weathering besides relative mineral
stability. Freeze/thaw cycles acting on cracks within the rock contribute to
mechanical disaggregation. The abrasive activity of glaciers produces a range
of fragmented rock particle size fractions, including sand. Additionally,
glaciers and glacial streams provide a mode of transport for abraded material.

Chemical dissolution of minerals acting as cementing agents in sedimentary
rocks promoted separation of the less soluble mineral phases. Once freed,
individual grains are subject to transport by wind, water and gravity.

Biogenic sands are a second category a collector might encounter. These
sands, also known as organic or biological sands, are composed of the
remnants of living organisms and are among the most interesting to many
collectors. They are often call carbonate sands since they are largely
comprised of the mineral calcite (calcium carbonate). When viewing with a
microscope, one clearly understands why such samples are highly prized by
psammophiles (sand collectors) worldwide. Most biogenic sands are composed
of coral, forams, bivalve shells, gastropods, marine worm tubes and, if the
collector is lucky, the rare three-axial sponge spicule.

Biogenic structures can, at times, be found intact or fragmented. Such
structures are a treat to see and provide a wonderful learning experience for the
observer. A tip for distinguishing carbonate sands from quartz sands: add a
drop of household white vinegar or a mild acid such as muriatic acid to a small
amount of sand; if the sample contains calcium carbonate, the grains will react
by forming bubbles of carbon dioxide.

Precipitated sands comprise the third category and are mineral grains formed
from an aqueous solution. Such sands include round oolitic sands that form in
the shallow waters of the Great Salt Lake, the Caribbean, off the Florida Keys
and in the Arabian Sea. Borax from Death Valley, California, salt from the Dead
Sea and the gypsum sands found at the White Sands desert of New Mexico are
examples of evaporite deposits. Evaporites form as salt-rich bodies of saline
waters evaporate, thereby becoming supersaturated and incapable of retaining
the minerals in solution. It is interesting to note that the Death Valley borax and
White Sands gypsum, while originally existing in solution in rather extensive
bodies of water, now survive in arid conditions, a testament to the inevitability of
change in the natural world.