pumice - Natural minerals and crystals

A pumice or, more usually, in apposition, a pumice stone, is a very porous volcanic rock and of low density. Pumice stone is formed at temperatures of the order of 500 to 600 ° C. The lava projected into the air cools very quickly and the drop in pressure causes degassing which forms bubbles, hence the porosity and low density of the stone. It is formed from fragments of rhyolite, dacite or andesite. It is considered volcanic glass because it does not have a crystalline structure. It can be of different colors, including white, yellowish, gray, brown or pale red. Its average density is 910 kg / m3, which is less than that of water, which allows it to float on water. The Roman city of Pompeii is covered with phonolite pumice.