Vermiculite mineral sample. This phyllosilicate mineral contains iron and magnesium, and forms from the alteration of other minerals like biotite or phlogopite. It has a clay-like structure of flakes or plates. Due to the water incorporated in its crystal structure, it swells when heated, leading to applications such as heat insulation, bedding for plants, and lightweight filler in concrete. This sample, from Koratti, in Tamil Nadu, India, is 10 centimetres across.