The word "sand" is used by most of us to mean the fine, somewhat gritty, stuff under our feet on a beach. That's also generally what scientists would call "sand". A geologist's definition would be that sand is unconsolidated sedimentary mineral material with a particle size between 1/16 mm and 2 mm. Silt and clay are progressively finer materials. Granules, pebbles, and boulders are progressively coarser particles that make up sediments called gravel.

Sands on beaches and shorelines can be divided according to their compositions into two major groups. One is the carbonate sands, sands made of particles of CaCO₃ (calcium carbonate). The other is commonly called the "siliciclastic sands", where "silici-" refers to a chemical composition rich in silicate material and "-clastic" refers to the origin of the grains as clasts or fragments of silicate rocks.

Siliciclastic or non-carbonate sands

        As noted just above, "siliciclastic sands" are defined as sands consisting of grains that originated as clasts or fragments of silicate rocks. They thus typically consist of silicate minerals, such as quartz (SiO₂), feldspars (KAlSi₃O₈, NaAlSi₃O₈, and CaAl₂Si₂O₈,) and micas such as muscovite (KAl₂(AlSi₃O₁₀)(OH)₂) and biotite ((K(Fe,Mg)₃(AlSi₃O₁₀)(OH)₂). Other dark-colored to black Mg- and Fe-bearing silicates (mafic silicates) like hornblende, pyroxene, and even olivine can be present. Silicates such as zircon (ZrSiO₄) and titanite or sphene (CaTiOSiO₄) are commonly present, but usually in minor amounts.

        Weathering of silicate rocks also releases non-silicate minerals, such as magnetite (Fe₃O₄), ilmenite (FeTiO₃), and rutile (TiO₂). Weathering and hydrologic sorting can produce black sands very rich in the latter minerals, so that such sands are "clastic" but not "silicic". These sands require one to use a definition of "siliciclastic" not as "silicate clasts" but as "originating as clasts from silicate rocks". If one uses a strictly compositional approach, all these "siliciclastic" sands must simply be called "non-carbonate" to include the magnetite-, ilmenite-, and rutile-rich sands weathering from silicate rocks.

        Much research has been dedicated to connecting compositions of siliciclastic sands either to specific rocks types from which they might be derived or to general tectonic settings from which they might be derived. In the former case, one can readily envision that a quartz-rich sand might be derived from weathering of sandstones, which are commonly rich in quartz, whereas sands rich in K-spar and quartz might commonly be derived from granites, which are igneous rocks rich in K-feldspar and quartz. This line of reasoning is attractive, but it proves difficult to use. That's because many rocks can produce sands of similar composition, and because variation in weathering environments (for example, variation in rainfall, temperature, slope, and plant cover) are also a major control on weathering and thus on the composition of sands produced by weathering.

        Attempts to link sand compositions and plate-tectonic settings have been more successful. For example:

Along passive continental margins (continental margins at which no subduction occurs, as in eastern North America), relatively abundant sedimentary rocks provide much quartz, and low topographic relief allows extensive chemical weathering that destroys silicate minerals other than quartz. The result is quartz-rich sands.

Along active continental margins (continental margins at which subduction occurs, as in western North America and South America), diverse rock types originating from volcanism and emplacement of plutonic rocks provides diverse minerals among which quartz is one component. Higher topographic relief allows erosion without extensive weathering, so that silicate minerals in addition to quartz survive weathering. The result is diverse sands with quartz, feldspars, other silicate minerals, and rock fragments (grains of multiple minerals).

Along island arcs (arc-like chains of volcanoes at which ocean crust is subducted under ocean crust, as in Tonga and the Marianas Islands in the western Pacific), mafic volcanic rocks are the dominant bedrock. Sands thus contain a large proportion of dark mafic minerals and dark fragments of basalt.

At ocean islands (volcanic islands not over subduction zones, of which the Hawaiian Islands are the classic example), the bedrock is entirely mafic volcanic rock (basalt). The result is sands that are dark-colored to black. Oxidation of some of those sands to yield rusty-looking coatings of iron oxides produces red sands on some of these islands too.

Carbonate sands

        Most carbonate sands consist of CaCO₃ (calcium carbonate) that has formed recently from the ocean. Their grains are thus generally younger than siliciclastic grains, most of which formed in the geological past. The grains of most carbonate sands also have formed at or very near where they are presently found, unlike siliciclastic grains that have formed on the continents and thus at locations perhaps thousands of kilometers away.

        CaCO₃ commonly occurs as two minerals, aragonite and calcite. Carbonate sands can consist of grains of either mineral, or of mixtures of both. Formation of aragonite is favored by higher temperatures, and so aragonite-rich sands are more common in lower latitudes, whereas calcitic sands are more common at higher latitudes. Many exceptions nonetheless exist.

        The CaCO₃ grains of carbonate sands are most commonly generated by organisms but can precipitate by a largely, if not entirely, abiotic process. Many organisms produce shells, tests, or skeletons of CaCO₃, and the entirety or pieces of their remains can be sand-sized. Carbonate sands can thus consist of foraminiferal tests, tiny snail and clam shells, pieces of snail and clam shelves, echinoderm fragments, pieces of red aglae and green algae, and fragments of coral - or mixtures of these. Abiotic precipitaton (precipitation of CaCO₃ not caused or induced by organisms) produces ooids, spherical concentrically-zoned grains of CaCO₃. Carbonate sands thus feature a host of different grain shapes ranging from rounded grains to distinctly spiky irregular grains.

        The previous three paragraphs pertain to carbonate sands formed recently in the ocean. It is also possibe for the weathering and erosion of limestone to generate sand consisting of ancient CaCO₃ (always calcite) and/or CaMg(CO₃)₂ (dolomite). Such sands are "clastic", just as siliciclastic sands are clastic, but come from carbonate rather than silicate rocks. They are thus clastic or detrital carbonate sands.

Mixed sands

        Finally, there are sands consisting of mixtures of siliciclastic and carbonate grains. These sands commonly occur in environments transitional between shorelines to which weathered sediments deliver siliciclastic sediments and shorelines along which CaCO₃-generating organisms are sufficiently abundant to make sands. Our page on the transition from siliciclastic to carbonate sands along the southeastern coast of the United States includes examples of such sands. Another transition, but with a very different siliciclastic component, can be seen in our page of sands from Hawaii.

Summary

        The considerations discussed above mean that there is a huge variation among beach and shoreline sands. We have tried to present this variation both in informative scientific contexts and in just-for-fun comparisons. We hope you'll go away with a better appreciation of what sands tell you about their origins and with a better appreciation of the variability of sands around the world.

Back to the index page for the images of sand
The title page and table of contents for this set of web pages
e-mail to Railsback (rlsbk@uga.edu)
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