Material obtained by the heat treatment at high temperatures of a volcanic siliceous rock of the group of rhyolites. It is delivered in white particles whose dimensions vary between 1.5 and 6 mm, with a low density, generally between 100-120 kg / m3, or even lower. It is used mixed with other substrates such as peat and / or coconut to increase aeration and drainage.
It is a magnesium hydrated silicate that looks like the micas that it obtains by exfoliation (dilation) of the natural mineral at high temperatures. As with perlite, the treated material greatly diminishes its density.
The apparent density is between 50-200 kg / m3, according to their origin and the size of the particles. In horticulture the flakes of between 1-4 mm and density between 85-100 kg / m3 are used.