I was surfing the net and found this reference to Vermiculite
ALSO KNOWN AS: Horticultural vermiculite
DESCRIPTION: A white, extremely light-weight granular substance, with
grains about 1/16" in
diameter (No. 3 grade). It is formed when mica chips are heated and expanded
to many times the
original size. Contains some potassium, magnesium, and calcium which are
slowly released in
soil. Construction grade vermiculite is larger, and is used for loose fill
USE: Used in soil mixes to increase the water and fertilizer retention of
the mixture, as well
as to lighten and open it up. Considered a soil amendment and a rooting
medium as well as a
mulch (Chapter 2).
USE TIP: It is a very light material, lighter than sand, which can be an
to container soil mixes. Avoid adding to clayey soils.