Magnesite

magnesite

sellaite

enstatite

serpentine

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Formula: Mg(CO3)
Carbonate
Breunnerite is an iron-bearing variety of magnesite
Specific gravity: 2.98 to 3.02
Hardness: 3½ to 4½
Streak: White
Colour: Colourless, white, greyish-white, yellowish, brown, faintly pink, lilac-rose; colourless in transmitted light
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acids. Slightly affected by cold acids. Readily soluble in warm hydrochloric acid with effervescence. Slightly soluble in water with the solubility increasing with the presence of NaCl, Na2SO4, or CO2
Common impurities: Fe,Mn,Ca,Co,Ni,ORG
Environments:

Metamorphic environments
Hydrothermal environments
Evaporite deposits

Magnesite is an evaporite mineral that may be associated with serpentine.

Localities

At Hohe Tauern mountains, Salzburg, Austria, magnesite pseudomorphs after calcite have been found (KL p156).

At Brumada, Bahia, Brazil, there is a magnesite mine where the rare halide sellaite occurs in vugs associated with magnesite and quartz (Dana).

Alteration

antigorite and magnesite to forsterite, CO2 and H2O
Mg3Si2O5(OH)4 + MgCO3 → 2Mg2SiO4 + CO2 + 2H2O
(DHZ 1A p263)

forsterite and CO2 to enstatite and magnesite
Mg2SiO4 + CO2 ⇌ MgSiO3 + MgCO3
(DHZ 2A p105)

magnesite and H2O to hydromagnesite Mg5(CO3)4(OH)2.4H2O and CO2
5MgCO3 + 5H2O → Mg5(CO3)4(OH)2.4H2O + CO2
Magnesite may react with water to form hydromagnesite or other low-temperature hydrated carbonates (R&M 90.6.521).

olivine and CO2 to enstatite- ferrosilite and magnesite- siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3
(DHZ 2A p139)

serpentine (chrysotile) and CO2 to talc, magnesite and H2O
2Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3MgCO3 + 3H2O
serpentine (chrysotile) is not stable in the presence of carbon dioxide and reacts with it according to the above equation (R&M 90.6.521).

talc and CO2 to magnesite, quartz and H2O
Mg3Si4O10(OH)2 + 3CO2 → 3Mg(CO3) + 4SiO2 + H2O
At low temperatures talc is unstable in the presence of excess CO2, and is replaced by magnesite (DHZ 3 p128).

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