Talc is a watery mineral composed of magnesium, silicon and oxygen. But it is not a major component of the earth's crust.
But when present in sufficient quantity it can, at sufficiently high temperatures, forestall a dangerous build-up of pressure, say geologists Diane Moore and Michael Rymer from the US Geological Survey.
Talc has a hardness rating of 1, the lowest possible, while diamonds have a rating of 10.

Talc is held together by weakly-bonded microscopic platelets that slide past one another, giving talc its soft, almost oily feel.

The mineral is used in the production of ceramics, paint, paper and a number of building materials, and is also present is some food and pharmaceutical products.