Science corner

5th May 2006 at 01:00
Ray Oliver sees the world in grains of sand

Since the two commonest elements in the Earth's crust are oxygen and silicon, it is not surprising that sand is so familiar. Sand is mostly silicon dioxide and is one of the most resistant rock-forming minerals.

When rocks decay, often it is only sand that remains. We use it to make glass and mortar and to sandblast old buildings. The abrasive in sandpaper, despite its name, is more likely to be made from an even harder mineral, corundum.

Make a collection of sand samples but don't use the sand in the fire bucket. If children have a sandpit at home, or have any building work in progress, they should be able to supply the tablespoonful of sand required for study. Anyone with access to the coast can add beach sand to the collection. When seen together, it becomes obvious that sands differ in two key ways: the shape of the grains; and the inclusion of other minerals both cause variation and give clues to the origin of the sand.

Close examination of the grains can be made using a hand lens, a simple microscope or, best, a projection microscope. Children should look to see if the sand grains are angular or rounded. When they are rounded, is it only the largest grains that are smooth? When sand forms in a desert, such as the Sahara, the movement caused by the wind wears the grains away. The result is rounded grains, sometimes called millet-seed grains. If the grains are very angular, it is probably beach sand. A mixture of partly rounded and angular grains suggests a marine origin. Grains in the sea acquire a coating of water which prevents them from wearing completely smooth. Make a sand display by writing a label, eg "beach", using a glue stick and pour sand on top.

Some sand deposits contain other minerals, such as tin in Cornwall or diamonds in Africa. British beach sands may contain traces of crystalline minerals such as tourmaline or rutile, from which white emulsion paint is derived. You may even make your fortune studying sand, though diamonds are still a rare find.

Ray Oliver teaches science at St Albans Girls' School, Hertfordshire

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