What Is Sand?

What Is Sand?

Sand means different things to different people. A genteral, non-technical dictionary definition for sand is “loose
particles of hard broken rock”. More restrictive definitions als exist which depend upon the frame of reference or academic discipline to which the meaning of sand is to be applied. For example, to a sedimentologist sand is an unconsolidated (loose), rounded to angular rock fragment or minteral grain having a diameter in the range of 1/16 to 2 mm (0.0025 to 0.08 in.). An engineer on the other hand may restrict the meaning of sand to include only rounded framents having a diameter of 0.074 mm (retained on U.S. standard sieve no. 200) to 4.76 mm (passing U.S. standard sieve no. 4). Practically, sand may be considered to consist of small detrital fragments (rock or minteral particles liberated by mechanical disintegration of parent rock material), biogenic particles (shells or shell fragments) or chemical precipitates (evaporites or oolites) occurring in nature and distinguishable by the naked eye.

A sand sample can be described in terms of grain size, color, composition, morphology (angularity and shape) and surface texture. Grain size is a result of several factors, including composition, durability, severity of weathering conditions, transport distance from its site of origin, and physical sorting by wind and/or water currents.

A variety of scales exist for measuring grain size. One which nearly all sand collectors use, and the easiest to remember, is the “Wentworth Article Size Classification for Sand”, which uses a geometric interval of 1/2 to define the limits of each size fraction. In descending order, the Wentworth sand classification scheme is as follows:

Size Class Millimeters
Boulders 250-1000
Gravel
a. Cobbles
b. Pebbles
c. Granules
-
65-250
4-65
2-4
Sand
a. Very coarse sand
b. Coarse sand
c. Medium sand
d. Fine sand
e. Very fine sand
-
1-2+0
5-1 (1/2-1)
0.25-0.5 (1/4-1/2)
0.125-0.25 (1/8-1/4)
0.0625-0.125 (1/16-1/8)
Muds
a. Coarse silt
b. Medium silt
c. Fine silt
d. Very fine silt
e. Clay
f. Dust particles
-
0.031-0.0625
0.0156-0.031
0.0078-0.156
0.0039-0.0078
below 0.0039
As small as 0.05 microns (0.0005 mm)

A population of sand grains examined at low magnification is often not uniform as revealed by the occurrence of a mixture of grain colors, morphologies and sizes. Whenever a wide range of grain sizes occurs withins a sample, the
sample is regarded as being “very poorly sorted.” If there exists a very limited variation in grain size within a population, the sample may be described as “very well sorted.” those samples have sorting characteristics between the extremes will be graded as well sorted, moderately sorted or poorly sorted depending on the range and relative percentages of grain size fractions.

Degree of angularity, also referred to as roundness, is a property that reflects the formative processes (weathering, transport, abrasion and deposition) involved in
forming a sand deposit. Generally speaking, the more well rounded the individual grain, the greater the energy involved in transport or the longer the duration of transport. Sand grains in beach environments are subjected to high-energy abrasion involving collisions with numberous other grains due to the action of waves. Aeolian (eolian is an appropriate alternative spelling), are wind transported sands also commonly well rounded by high energy collisions and abrasion. Particles transported long distances by rivers or streams may also achieve a high level of roundedness. Variations in the durability of different
mineral types also affect rounding.

Rounded particles may also form by precipitation in near shore ocean currents. Ooliths are rounded grains formed by concentric precipitation of calcium carbonate around a pre-existing particle such as a small shell fragment or quartz grain that acts as a site of nucleation. They commonly form in areas where strong bottom currents exist, such as tidal channels or near shore sites where strong currents exist.

Degree of rounding or angularity defines one aspect of the shape of a sand particle. A more complete characterization of particle shape requires consideration of the way size differs (or remains the same) according to direction within individual grains. Particles, which are tabular or shaped like a disk, are said to be oblate. Equdimensional particles are called equant. Elongated and somewhat flattened particles are bladed. Rod–shaped particles are prolate. Surface texture refers to small-scale features of a sand grain’s surface which have no apparent bearing on its overall shape. Examples of surface textures include polished and frosted. A polished texture is commonly observed on grains which have been transported by water, and correspondingly are typically rounded. Frosting is produced by collision with other sand grains and is commonly observed in aeolian sands.

Angularity (Shape of grains)
Six Classes of Particle Roundness (visual estimation)

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.

Prepared by: Thomas J. Hopen, ISCS Microscopy Advisor