The structures are similar to the brucite structure. It occurs in nature as the mineral brucite. Gibbsite's structure is closely related to the structure of brucite, Mg(OH). A fibrous variety of brucite is called nemalite. Natural magnesium hydroxide (brucite) is used commercially as a fire retardant. The formula can be thought of as four olivine (MgSiO), plus one brucite (Mg(OH)). These brucite particles encrusts microbes growing inside the coral skeleton. It readily alters to brucite under near surface conditions. Younger chimneys are primarily brucite and aragonite, being white and flaky in appearance. The exact magnitude of impact that brucite has over cement paste is still debatable. The compound adopts the brucite structure, as do several other metal dihydroxides. Associated minerals include brucite, hydromagnesite, pyroaurite, chrysotile, aragonite, calcite, dolomite and magnesite. As a consequence, chlorite interlayers are cemented by brucite and cannot swell nor shrink. Synthetic brucite is mainly consumed as a precursor to magnesia (MgO), a useful refractory insulator. Brucite may also crystallise in cement and concrete in contact with seawater.