Geology And Geophysics

Sand Composition uses



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What is sand? A generally excepted definition is that sand is composed of rock and mineral particles in the size range 0.6 to 2 mm. Below 0.6 mm size the material is called silt and above 2 mm size it is gravel.

What is a resource? A resource is something that is useful to man. Sand becomes a resource when we apply technology to it to make something useful, or extract some valuable component. Wealth equals a resource multiplied by technology. If technology is zero then there is no resource and no wealth and you have a primitive society.

Sand is derived from erosion of rocks of all types by moving water and wind, and by chemical attack. Rocks are gradually broken down and the bits transported by rivers and streams to the sea, or lakes, where the fragments are further pounded into smaller and smaller particles on beaches. The movement of water here, and by wind in a desert terrain, causes a separation according to particle size, thus making gravel, sand and silt deposits.

The composition of a beach sand is dependent on the rock types present in the eroding watershed or those exposed along the coastline. There are many types of beach sand which often can be distinguished by color , e.g., white to yellow, orange, reddish and black, and there are mixtures of all types. Rarely do we find a pure white (gypsum), or green (olivine) and pink (garnet) sands that form under special conditions.

Sand is a valuable commodity which is essential for a modern growing economy. Sand has many uses. In fact, a modern city could not exist without sand. For a basic concrete you need about 1 part cement, 2 parts sand and 3 parts gravel. Steel reinforced concrete makes tall buildings  with glass windows and  panels which are composed of 75% silica derived from sand. Today, without using sand, we would have a primitive society and be living in wooden or grass huts.

The mining industry produces all the industrial and mineral sands by the millions of tons required for concrete, glass and paint manufacture, and for specialty ores. Zircon is found in sands derived from the long weathering of granite terrains. On the world market it sells for about $1400 per tonne. It is a zirconium silicate mineral from which is extracted zirconia, which is a refractory used in the steel industry and for other applications. Now nearly every woman has some cubic zirconia jewelry (CZ), a single crystal of zirconia, and an inexpensive diamond simulant.

Titanium oxide is used for making paints and the metal for special high-temperature alloys used in aircraft construction and jet engines. The titanium comes from minerals found in beach sand deposits, of old and recent origin. Rutile is titanium oxide TiO2, is the best, but also mined is ilmenite FeTiO3 and its altered form leucoxene. The rare earths are obtained from sands containing the yellow-brown minerals xenotime and monazite, being respectively phosphates of yttrium and cerium. All these valuable minerals are accessory minerals (1% or less) of granites and gneisses and are released on weathering of the rock and so accumulate in the sands.

Sand mining tends to produce a range of mineral products, including light minerals such as quartz, feldspar and the heavier fractions e.g., rutile, ilmenite, magnetite and apatite. Australia is a major sand miner with an annual output of over 600,000 tonnes of zircon and 250,000 tonnes of ilmenite.

How to find out what a sand is composed of? You have to be a geologist, or mineralogist, either amateur or professional, in order to be familiar with the many (dozens) of mineral possibilities and know how to identify them for sure. It would be easier if human beings were the size of an ant and could walk over the sand grains and examine them at close quarters, but this is not possible. What is required is a stereo zoom binocular microscope that can magnify the grains up to 40 times, together with a good knowledge of mineralogy or gemology. On eBay you can find suppliers of such microscopes, made in China, costing about $650. A fine millimeter scale will allow you to estimate the sand grain size. Also, to obtain heavy concentrates from sands collected you need a gold pan (plastic) and set of sieves of the type used by gemstone fossickers.

In order to describe a sand sample you need to provide as much information as possible using the equipment available. Sand samples examined may be as found, or treated by water panning to give a heavy mineral concentrate, which may be anything from 1% or less of the weight of the origin sample. First let us consider the light fraction of the sample which is normally the bulk of the sand. Usually we are dealing with quartz, limestone or rock fragments having a specific gravity (S.G.) in the range 2.6 – 2.8.

Quartz is a colorless or white robust mineral and is found as small grains in many igneous rocks and in bulk in sandstones and quartzites. It readily survives erosion and often accumulates on a sea beach as a pure white to cream colored sand, although when coated with iron oxide may be red. Quartz is the second most abundant mineral in the earth’s crust. Feldspar is the commonest mineral but it is prone to decomposition in the weathering zone, changing to clay minerals which form silt because of their tiny particle size. Through the microscope, quartz grains may appear rounded and transparent whereas feldspar grains maybe colored, transparent, but blocky with cleavage planes apparent. Each mineral has a characteristic physical appearance that helps in its identification.

Most beach sands have some shell fragments in them. Test the sample in a watch glass with dilute hydrochloric acid and examine it under the microscope. Note which grains are bubbling and giving off carbon dioxide gas. You can estimate say 5% or 50% etc, for shell fragments and the rest may be quartz which is inert. The fragmentation of local limestone rocks and off-shore reefs, such as found around the Caribbean and Yucatan Peninsula, often produces a sand composed almost entirely of tiny shell and coral pieces.

Test for magnetics, which are predominately magnetite. Make a magnet probe by attaching a strong neodymium magnet to the end of a nail. Cover the magnet with tissue and stir up the sample with it to remove all the magnetic grains to be examined separately. You will find them to be tiny black shiny octahedrons of magnetite Fe3O4, either single or twinned crystals, or in aggregates.

Black sand beaches are common in volcanic regions such as around the Pacific Rim, including Japan, California, Hawaii and New Zealand. Black volcanic rocks such as basalt and andesite are robust and erode to give black or greyish rock fragments and black magnetic sands composed largely of magnetite. When abundant these magnetite beach deposits are mined for their iron content to make steel, as in New Zealand. Black sands are good conductors of heat and will make walking bare-footed painful on a sunny day.

Black sands are always exciting because who knows if they might contain a few specks of gold or other exotic and dense minerals. Nature helpfully separates away most of the light minerals like quartz and limestone (S.G. both around 2.7) to leave a heavy concentrate which is usually rich in magnetite (S.G. 5.1). Gold is very dense with a specific gravity of 19, which means it is always left behind in the heavy residue tail when panning off. Transfer the tail to a watch glass by pressing with your finger, or use a syringe to suck it up.

Gold grains once seen are never forgotten. In a sand they may have various forms, as tiny yellow flakes, or nodules, or newly grown crystals. Careful microscopic study of gold grains can provide information as to its origin.

Other valuable heavy sand minerals of interest in prospecting, are cassiterite (black tin stone), and white scheelite (tungsten ore). Both can be identified for sure by microchemical tests using strong acids. Of course, in a mining laboratory a technician could separate the minerals using heavy liquids and use x-ray powder diffraction to obtain a positive identification. The assaying of beach sands for zircon and ilmenite content has standard procedures developed by mining companies

Our present day technology is very dependent on sand as a resource for making concrete and so a wide range of infrastructural items, like bridges and buildings, and for extraction of rare elements needed for special alloys and electronic components. Sand is also useful for lying on at the beach to enjoy a sunny day. The seagulls love it and so do the dogs like to run and play on it. Sand is pretty special stuff.



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