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Formula: Mg(CO3)
Carbonate, calcite group
Breunnerite is an iron-bearing variety of magnesite
Crystal System: Trigonal
Specific gravity: 2.98 to 3.02 measured, 3.01 calculated
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).
At the Southern Crude Oil Purchasing Well No. 89-E-1, Winkler county, Texas, USA, euhedral magnesite crystals
occurred in drill cores. The matrix is a dolomitic
limestone containing
chert or flint nodules. Many
of the magnesite crystals are stained with petroleum or asphalt. The crystals originated by replacement of the
dolomite by magnesium-bearing solutions
(AM 15.238).
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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