Sand is a naturally occurring
granular material composed of finely
divided
rock and
mineral particles.
As the term is used by
geologists, sand
particles range in
diameter from 0.0625 (or
mm, or 62.5
micrometers) to 2
millimeters. An individual particle in this range
size is termed a
sand grain. The next larger size
class above sand is
gravel, with particles
ranging from 2 mm up to 64 mm (see
particle size for standards in
use). The next smaller size class in
geology
is
silt: particles smaller than 0.0625 mm
down to 0.004 mm in diameter. The size specification between
sand and gravel has remained constant for more than a century, but
particle diameters as small as 0.02 mm were considered sand under
the
Albert Atterberg standard in
use during the early 20th century. A 1953 engineering standard
published by the
American Association of State Highway and Transportation
Officials set the minimum sand size at 0.074 mm.
A 1938 specification
of the United States Department of
Agriculture
was 0.05 mm. Sand feels gritty when rubbed
between the fingers (silt, by comparison, feels like flour).
ISO 14688 grades sands as fine, medium and
coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to
2.0 mm. In the United States, sand is commonly divided into
five sub-categories based on size: very fine sand ( - mm
diameter), fine sand ( mm - mm), medium sand ( mm
- mm), coarse sand ( mm - 1 mm), and very coarse
sand (1 mm - 2 mm). These sizes are based on the
Krumbein phi scale, where size in
Φ = -log base 2 of size in mm. On this scale, for sand the value of
Φ varies from -1 to +4, with the divisions between sub-categories
at whole numbers.
Constituents
The most common constituent of sand, in inland continental settings
and non-
tropical coastal settings, is
silica (silicon dioxide, or SiO
2),
usually in the form of
quartz, which, because
of its chemical inertness and considerable hardness, is the most
common
mineral resistant to
weathering.
The composition of sand is highly variable, depending on the local
rock sources and conditions. The bright
white
sands found in tropical and subtropical coastal settings are
eroded
limestone and may contain
coral and
shell fragments
in addition to other organic or organically derived fragmental
material.
The gypsum sand dunes
of the White Sands National Monument
in New
Mexico
are famous for their bright, white color.
Arkose is a sand or
sandstone with considerable
feldspar content, derived from the
weathering and
erosion of
a (usually nearby)
granitic rock outcrop.
Some sands contain
magnetite,
chlorite,
glauconite or
gypsum. Sands
rich in magnetite are dark to black in color, as are sands derived
from volcanic
basalts and
obsidian. Chlorite-glauconite bearing sands are
typically green in color, as are sands derived from basaltic
(
lava) with a high
olivine content. Many sands, especially those found
extensively in
Southern Europe, have
iron impurities within the quartz
crystals of the sand, giving a deep yellow color.
Sand deposits in some areas contain
garnets
and other resistant minerals, including some small
gemstones.
Environments
Sand is transported by wind and water and deposited in the form of
beaches,
dunes,
sand spits,
sand
bars and related features. In environments such as
gravel-bed
rivers and
glacial moraines it often occurs as one of
the many grain sizes that are represented.
Sand-bed rivers, such
as the Platte River in Nebraska
, USA
, have sandy
beds largely because there is no larger source material that they
can transport. Dunes, on
the other hand, are sandy because larger material is generally
immobile in wind, and are a distinctive geographical feature of
desert environments.
Study
The study of individual grains can reveal much historical
information as to the origin and kind of transport of the grain.
Quartz sand that is recently weathered from
granite or
gneiss quartz
crystals will be angular. It is called
grus in geology or
sharp sand in the
building trade where it is preferred for concrete, and in gardening
where it is used as a soil amendment to loosen clay soils. Sand
that is transported long distances by water or wind will be
rounded, with characteristic abrasion patterns on the grain
surface. Desert sand is typically rounded.
People who collect sand as a hobby are known as
arenophiles or psammophiles.
Uses
- Agriculture: Sandy soils are ideal for crops such as watermelons, peaches, and
peanuts and their excellent drainage
characteristics make them suitable for intensive dairy farming.
- Aquaria: Sand makes a low cost aquarium
base material which some believe is better than gravel for home
use.
- Artificial reefs: Geotextile bagged sand can serve as the
foundation for new reefs.
- Beach nourishment: Governments
move sand to beaches where tides, storms or
deliberate changes to the shoreline erode the original sand.
- Brick: Manufacturing plants add sand to a
mixture of clay and other materials for
manufacturing bricks.
- Car Engine Disablement
Sand is also used in addition to Sodium
Silicate to inexpensively, quickly, and permanently disable
automobile engines.
- Cob: Coarse sand makes up as much
as 75% of cob.
- Concrete: Sand is often a principal
component of this critical construction material.
- Glass: Sand is the principal component in
common glass.
- Landscaping: Sand makes
small hills and slopes (for example, in golf
courses).
- Paint: Mixing sand with paint produces a textured finish for walls and ceilings
or non-slip floor surfaces.
- Railroads: Train operators use
sand to improve the traction of wheels on the rails.
- Roads: Sand improves traction (and thus
traffic safety) in icy or snowy conditions.
- Sand animation: Performance artists draw images in sand.
Makers of animated films use the same term
to describe their use of sand on frontlit or backlit glass.
- Sand casting: Casters moisten or
oil molding sand, also known as
foundry sand and then shape it into molds
into which they pour molten material. This type of sand must be
able to withstand high temperatures and pressure, allow gases to
escape, have a uniform, small grain size and be non-reactive with
metals.
- Sand castles: Shaping sand into
castles or other miniature buildings is a popular beach
activity.
- Sandbags: These protect against floods and gun fire. The
inexpensive bags are easy to transport when empty, and unskilled
volunteers can quickly fill them with local sand in
emergencies.
- Sandblasting: Graded sand serves as
an abrasive in cleaning, preparing, and
polishing.
- Water filtration: Media filters use sand for filtering water.
- Zoanthid "skeletons": Animals in this order of marine
benthic cnidarians
related to corals and sea anemones, incorporate sand into their
mesoglea for structural strength, which
they need because they lack a true skeleton.
Hazards
While sand is generally non-toxic, sand-using activities such as
sandblasting require precautions. Bags
of silica sand used for sandblasting now carry labels warning the
user to wear respiratory protection to avoid breathing the
resulting fine silica dust.
Material safety data sheets
(MSDS) for silica sand state that "excessive inhalation of
crystalline silica is a serious health concern".
In areas of high
pore water
pressure sand and salt water can form
quicksand, which is a
colloid hydrogel that
behaves like a liquid. Quicksand produces a considerable barrier to
escape for creatures caught within, who often die from exposure
(not from submersion) as a result.
Environmental Issues
Sand's many uses require a significant dredging industry, raising
environmental concerns over fish depletion, landslides, and
flooding. Countries such as China, Indonesia, Malaysia and Cambodia
ban sand exports, citing these issues as a major factor.
See also
References
- Urquhart, Leonard Church, "Civil Engineering Handbook"
McGraw-Hill Book Company (1959) p.8-2
- Seaweed also plays a role in the formation of
sand
- Importing Sand, Glass May Help Restore Beaches :
NPR
- Simplot
External links
.jpg/180px-Volcanic_sand_(Perissa,_Santorini,_Greece).jpg)
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