[mag-nuh-sahyt] 
| a mineral, magnesium carbonate, MgCO3, having a characteristic conchoidal fracture and usually occurring in white masses. |
| mag·ne·site (māg'nə-sīt') n. A white, yellowish, or brown mineral, magnesium carbonate, MgCO3, used in the manufacture of magnesium oxide and carbon dioxide. |
magnesite
the mineral magnesium carbonate (MgCO3), a member of the calcite group of carbonate minerals that is a principal source of magnesium. The mineral has formed as an alteration product from magnesium-rich rocks or through the action of magnesium-containing solutions upon calcite. Notable deposits are those at Radenthein, Austria; the Liaotung Peninsula, Liaoning Province, China; and Clark County, Nev., U.S. Iron is usually present, and a complete chemical substitution series exists between magnesite and siderite in which iron replaces magnesium. Magnesite is used as a refractory material, a catalyst and filler in the production of synthetic rubber, and a material in the preparation of magnesium chemicals and fertilizers. For detailed physical properties, see carbonate mineral (table)
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nəˌsaɪt