Vermiculite is a mineral closely related to mica, primarily mined in Montana and South Carolina. Consisting of silica, magnesium dioxide, aluminum oxides, and other minerals, vermiculite expands when heated to form a lightweight material with insulating properties. There are two types of vermiculite: untreated and treated. The treated material is coated with asphalt to make it water-repellent for use in high-moisture areas. Untreated vermiculite absorbs water and, once wet, dries very slowly.