CARBONATES (p. 404)
(CO3)2- groups are triangular (coplanar) and do not share oxygen atoms with each other
CALCITE GROUP
- NaCl type structures with Cl- replaced by CO32- and Na by Ca.
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Calcite - hex 3 2/m - CaCO3
- limestones, marbles, cements in other sedimentary rocks
- igneous-carbonatites + syenites
- hydrothermal alteration of basalts
- use: cements
Magnesite - MgCO3
- alteration of Mg-rich igneous + metamorphic rocks.
- MgCO3 commonly occurs associated with serpentine + talc
- Use: fire-bricks, Mg metal, flux in iron ore pellets
Siderite - FeCO3
- in iron stones with clays, magnetite, hematite
- ore of Fe
Rhodochrosite - MnCO3 complete S.S. with siderite
- in hydrothermal deposits with Ag, Pb, Cu
- with rhodonite
- minor ore of Mn
Smithsonite - ZnCO3
- Oxidized supergene zones of Zn-Pb deposits with sphalerite, galena, cerussite, malachite, azurite.
ARAGONITE GROUP
Aragonite CaCO3 - orthorhombic 2/m 2/m 2/m
- occurrence: some hot springs, some marine animals, high-P metamorphic rocks with other blueschist minerals
Witherite BaCO3
- associated with galena
Strontianite SrCO3 with up to 20% Ca
- Sr for fireworks
- with barite, celestite (SrSO4), calcite as a result of hydrothermal fluids in limestones
Cerussite PbCO3
- supergene zones of lead deposits
- pseudohexagonal twins = look like drill bits
DOLOMITE GROUP
- structure = calcite, but with Ca and Mg alternating in layers along the C-axis
- useful as a geothermometer
Dolomite CaMg(CO3)2 hex. 3
- sedimentary environments
- replacement of limestone by Mg-rich hydrothermal brines (basinal fluids)
- also in met . limestones
- use: cement, refractory, building stone
Ankerite CaFe(CO3)2 Kutnahorite CaMn(CO3)2
- Precambrain iron formations
- Hydrothermal deposits
Malachite CuCO3 (OH)2
- Supergene Cu ore is oxidizes Cu deposits w/native Cu, cuprite, azurite
- Cu ore, gem
Azurite Cu3 (CO3)2 (OH)2
- same occurrence as malachite
- Cu ore, gem