Home About Us Our Services Publications Testing Submissions Contact Us Links

Did you know ...? – Vermiculite

SESL clients will be most familiar with vermiculite as a horticultural soil component: those shiny bronze flakes that can be easily compressed between the fingers.

Vermiculite is a clay mineral with the chemical formula (Mg,Fe,Al)₃(Al,Si)₄O₁₀(OH)₂·4H₂O, making it a magnesium-aluminium-iron silicate. It is a weathered mica in which the K⁺ ions between the molecular sheets are replaced by Mg²⁺ and Fe²⁺ cations.

Little worms

When vermiculite is heated rapidly, the water between the layers turns to steam, which expands the vermiculite into long wormlike strands. The word comes from the Latin for “little worms”.

The expansion increases the volume typically by 8 to 12 times (up to 30 times), and decreases the bulk density from 640–1120 kg/m³ to 64–160 kg/m³.

Horticulture

Being a mineral, vermiculite won’t rot. Being heat-expanded, it is initially sterile. With a pH of around 7, it is neutral. Its high cation exchange capacity – 100–150 meq/100 g – means it will retain soil cations against leaching during watering.

Potting soils made of equal parts vermiculite and an organic material such as coco peat retain moisture and nutrients, while at the same time allowing good aeration of roots.

Production

South Africa is the world’s largest producer of vermiculite, annually producing over 200 000 tonnes in 2005. World production amounts to over 520 000 tonnes a year. Australia produces about 12 000 tonnes a year, all from a single deposit near Alice Springs.

Further information

The Vermiculite Association: http://www.vermiculite.org/

Download this article as a PDF