Composition

Heavy minerals (dark) in a quartz beach sand (Chennai, India).

Sand from Coral Pink Sand Dunes State Park, Utah. These are grains of quartz with a hematite coating
providing the orange color.

Sand from Pismo Beach, California. Components are primarily quartz, chert, igneous rock, and shell fragments.

The exact definition of sand varies. The scientific Unified Soil Classification System used in
engineering and geology corresponds to US Standard Sieves, [5] and defines sand as particles with a
diameter of between 0.074 and 4.75 millimeters. By another definition, in terms of particle size as
used by geologists, sand particles range in diameter from 0.0625 mm (or 1⁄16 mm) to 2 mm. An
individual particle in this range size is termed a sand grain. Sand grains are between gravel (with
particles ranging from 2 mm up to 64 mm by the latter system, and from 4.75 mm up to 75 mm in the
former) and silt (particles smaller than 0.0625 mm down to 0.004 mm). 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. The grains of sand in Archimedes' The Sand Reckoner written around 240 BCE, were
0.02 mm in diameter. A 1938 specification of the United States Department of Agriculture was
0.05 mm.[6] A 1953 engineering standard published by the American Association of State Highway
and Transportation Officials set the minimum sand size at 0.074 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 (1⁄16 – 1⁄8 mm diameter), fine sand (1⁄8 mm – 1⁄4 mm), medium sand (1⁄4 mm – 1⁄2 mm),
coarse sand (1⁄2 mm – 1 mm), and very coarse sand (1 mm – 2 mm). These sizes are based on
the Krumbein phi scale, where size in Φ = -log2D; D being the particle 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.

Close up of black volcanic sand from Perissa, Santorini, Greece

The most common constituent of sand, in inland continental settings and non-tropical coastal
settings, is silica (silicon dioxide, or SiO2), 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 mineral 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, suggesting that sand formation depends on living
organisms, too.[7] The gypsum sand dunes of the White Sands National Park in New Mexico are
famous for their bright, white color. Arkose is a sand or sandstone with
considerable feldspar content, derived from 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.

Sources
Rocks erode or weather over a long period of time, mainly by water and wind, and
their sediments are transported downstream. These sediments continue to break apart into smaller
pieces until they become fine grains of sand. The type of rock the sediment originated from and the
intensity of the environment gives different compositions of sand. The most common rock to form
sand is granite, where the feldspar minerals dissolve faster than the quartz, causing the rock to
break apart into small pieces. In high energy environments rocks break apart much faster than in
more calm settings. For example, Granite rocks this means more Feldspar minerals in the sand
because it wouldn't have had time to dissolve. The term for sand formed by weathering is epiclastic.
[8]

Sand from rivers are collected either from the river itself or its flood plain and accounts for the
majority of the sand used in the construction industry. Because of this, many small rivers have been
depleted, causing environmental concern and economic losses to adjacent land. The rate of sand
mining in such areas greatly outweighs the rate the sand can replenish, making it a non-renewable
resource.[9]
Sand dunes are a consequence of dry conditions or wind deposition. The Sahara Desert is very dry
because of its geographic location and is known for its vast sand dunes. They exist here because
very little vegetation is able to grow and there's not a lot of water. Over time, wind blows away all the
fine particles, such as clay and dead organic matter, leaving only sand and larger rocks. Only 15% of
the Sahara is sand dunes, while 70% is bare rock. [10] The wind is responsible for creating these
different environments and shaping the sand to be round and smooth. These properties make desert
sand unusable for construction. [11]
Beach sand is also formed by erosion. Over thousands of years, rocks are eroded near the shoreline
from the constant motion of waves and the sediments build up. Weathering and river deposition also
accelerate the process of creating a beach, along with marine animals interacting with rocks, such
as eating the algae off of them. Once there is a sufficient amount of sand, the beach acts as a
barrier to keep the land from eroding any further. This sand is ideal for construction as it is angular
and of various sizes.[12]
Marine sand (or ocean sand) comes from sediments transported into the ocean and the erosion of
ocean rocks. The thickness of the sand layer varies, however it is common to have more sand closer
to land; this type of sand is ideal for construction and is a very valuable commodity. Europe is the
main miners of marine sand, which greatly hurts ecosystems and local fisheries. [9]

Study

Scanning electron micrograph showing grains of sand

The study of individual grains can reveal much historical information as to the origin and kind of
transport of the grain.[13] 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. Organisms that thrive in sandy
environments are psammophiles.[14]