Siim

Sand samples of volcanic origin are of two dominant types: clastic detritus eroded from a volcanic terrain and products of explosive volcanic eruptions (pyroclastic sediments, also known as tephra). The first group of volcanic sand is similar to most other types of sand in terms of genesis. However, they generally have different composition from the so-called normal continental quartz-rich sand. Such volcanic sand is typically immature because most of its components are susceptible to weathering. This type of sand is often composed mostly of rock fragments rather than individual mineral crystals.

The second group consists of volcanic ash. This material is composed of volcanic glass, rock fragments, and crystals in all possible proportions. Lithified volcanic ash is tuff. Lithified deposit of a pyroclastic flow is ignimbrite.

Olivine, pyroxene, and magnetite are the most characteristic minerals of most volcanic sand deposits. Most volcanic beach sands are dark-colored. Hence, they are often named black sand. Black color is given to these sands by minerals augite (pyroxene), magnetite, and sometimes hornblende. These minerals contain lots of iron in their crystal structure which often gives them black color and reddish rust-colored hue when weathered.

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Volcanic sand composed of basaltic rock fragments. Kahena Beach, Hawaii. Width of view 10 mm.

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Volcanic ash collected one day after the eruption of Mount St. Helens in 1980. Width of the view is 4 mm.

Heavy mineral sand contains lots of heavy minerals, hence the name. The density of these minerals is generally above 2.9 grams/cm³. Important heavy minerals are magnetite, garnet, ilmenite, zircon, spinel, augite, staurolite, hornblende, rutile, kyanite, tourmaline, biotite, titanite, apatite, etc.

These minerals often occur in sand, but usually in low quantities (less than 1 percent). Sometimes they get concentrated in beaches or river bottoms to form beautiful and unusual-looking sand samples. The composition of these sand samples is highly variable. The most common constituents of heavy mineral sands are garnet, magnetite, ilmenite, and epidote. Most heavy mineral sand samples contain quartz as well, but it is not as important as usual.

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Beach sand from Calvert Cliffs State Park, Soloman Islands, Maryland. Black mineral is ilmenite, yellow spots on some ilmenite grains are composed of leucoxene, pink mineral is almandine garnet.

The source materials of heavy mineral sand are metamorphic and igneous rocks. Metamorphic rocks give rise to sand containing kyanite and garnet, for example, although the latter may crystallise from magma as well. Magnetite is mostly of igneous origin.

Heavy mineral sand is a natural mineral concentrate which is often mined commercially. Zircon, gold, ilmenite, rutile, monazite, and cassiterite are some noteworthy heavy minerals that are often extracted from sand. Heavy mineral sand is not rare, but its area of occurrence is usually limited.

Magnetite grains from a heavy mineral sand from Guam (Talofofo Beach). These magnetite grains are aligned in the presence of a strong external magnetic field. Width of view 10 mm.

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Metamorphic boulder on the coast in northern Norway (Varanger peninsula). It is composed of magnetite, garnet and quartz. These are very common constituents of heavy mineral sands and the rock may be a metamorphosed placer deposit. Width of sample 36 cm.

Hematitic sand and sandstones are mainly consisting of quartz but these quartz grains have an intense pink, orange or red color. The coloring agent in this sand is volumetrically insignificant – it is hematite (oxide of iron) that forms very thin rust-colored pigment on larger silicate grains.

However, its influence on the appearance of sand or sandstone is very strong. A well-known example of such sand is the Coral Pink Sand Dunes in the State of Utah, USA. Devonian sandstones (Old Red) in many places are often even darker (brick red). These sandstone formations are known throughout the world as “red beds”.

Hematite is not a detrital mineral (formed as a result of disintegration of a parent rock) in this sand type. It is mainly groundwater that carries iron and forms ferric gel on silicate grains. Complete dewatering of such gel results in a thin layer of iron oxide. Hematitic sand is very common in deserts.

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Sand dunes in Sahara (Morocco) are not gray or white as one would expect pure quartz sand to be. Instead they are orange. This color is a combination of white quartz and reddish hematite.

Continental sand, as its name says, is the most dominant sand type on the continental beaches. We are probably all (with few exceptions) very familiar with this buff grainy material that is mostly composed of quartz but may also contain other minerals like feldspar, mica, and biogenic grains. However, none of them are not nearly as abundant as quartz.

This sand is usually yellow or light brown and often speckled with dark grains. It is common along passive continental margins (without active volcanism).

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Continental sand from Botany Bay, Sydney, Australia.

Quartz is the most important sand-forming mineral and occurs in very many sand types but usually not exclusively. In this sand type (which is aptly named quartz sand), quartz is almost the sole component of sand. Similar sandstones are called quartz arenites or orthoquartzites.

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Fine-grained quartz sand from the St Peter formation (Ordovician sandstone) from Minnesota, USA. Sand from this formation is extensively used in hydraulic fracturing (fracking). Width of view 7 mm.

Quartz is the most important sand-forming mineral because it is resistant to both physical and chemical weathering. Sand that is enriched in quartz is likely old (mature) and has traveled far from the source area, sometimes thousands of kilometers. Long journey is required to allow weathering to break down weaker minerals that were initially present in the rocks.

Mineral grains that make up this kind of sand can be very old. They may have broken away from their source rocks millions or even hundreds of millions of years ago and have perhaps seen several lithification and weathering cycles.

Good examples of quartz sand can be found in Florida. Sand from Siesta Key beach is sometimes called the whitest in the world although it has competitors. Quartz sand are found on the coasts of passive continental margins (margins of the continents without active volcanism).

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Siesta Key beach sand in Florida is composed almost exclusively of quartz grains.