Magnesite

n. mineral comprised of magnesium carbonate

Magnesite is not to be confused with Magnetite or Magnemite. Magnesite is magnesium carbonate, MgCO3. Iron (as Fe2+) substitutes for Mg with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts. Dolomite, (Mg,Ca)CO3, is almost indistinguishable from magnesite.
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Noun
1. a black magnetic mineral consisting of magnesium carbonate; a source of magnesium
(hypernym) mineral
(substance-meronym) magnesium, Mg, atomic number 12

n. magnesite, mineral comprised of magnesium carbonate

(n.)
Native magnesium carbonate occurring in white compact or granular masses, and also in rhombohedral crystals.