Horticultural perlite. Photo by Ragesoss. Reproduced under Creative Commons Licence.
Did you know? ... Perlite
Perlite is the white crunchy material we see commonly in potting mixes. The name refers to the pearly appearance of the product. In the wild, it is a volcanic glass (an aluminosilicate) with a water content of 2% to 6% (the water molecules are trapped in the crystal structure of the glass). Raw perlite is typically greyish or brownish, but can be shades of other colours.
On heating to above 850 °C it melts, and the trapped water expands in a flash, puffing up the glass. A similar process creates popcorn and puffed rice. The myriads of trapped gas bubbles repeatedly reflect the light, giving expanded perlite its brilliant white appearance.
Before expansion, perlite has a bulk density of 2.2 to 2.4 g/cm3. After expansion, its density ranges from 0.055 to 0.3 g/cm3. This represents an increase in volume of anywhere from 7 to 40 times.
Perlite is produced in Australia, principally in south-east Queensland, near Chillagoe. Annual production is about 10 000 t, approximately 1% of the output of Greece, the world’s largest producer.
On account of its high surface area – between 0.46 and 2.6 m2/g (J Aust Ceramic Soc 2009, 45(1): 44–49) – perlite has an excellent water-holding capacity. As a rule of thumb, the finer the particle size, the more water-holding capacity the soil mix has, but the less air-holding capacity it has.
Besides it use in horticulture, perlite finds uses in a variety of industries, including construction (as a lightweight, non-flammable structural material, filler and insulator), furnaces, noise reduction, ceramics and glass manufacture, explosives, filters and cement.
Further information
Australian Perlite Pty Ltd. 2006. Perlite.
British Geological Survey. 2009. World Mineral Production 2003–2007.
Mineral Information Institute. Perlite.
Wikipedia: Perlite.
NSW DPI. 2008. Perlite.