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Chapter 22

Chapter 22. Continued. Aragonite Structural Type Aragonite CaCO 3. Orthorhombic Tabular prysmatic or commonly pseudohexonal twinned intergrowths Forms: As precipitates from mineral springs In oxidation zone of sulfide ores and weathering crusts Marine chemical sediments Karst caves

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Chapter 22

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  1. Chapter 22 Continued

  2. Aragonite Structural TypeAragoniteCaCO3 Orthorhombic Tabular prysmatic or commonly pseudohexonal twinned intergrowths Forms: As precipitates from mineral springs In oxidation zone of sulfide ores and weathering crusts Marine chemical sediments Karst caves Part of mother-of-pearl in mollusk shells

  3. Aragonite Structural TypeWitheriteBaCO3 Isomorphic series with aragonite and strontianite Low T hydrothermal mineral associated with strontianite and barite

  4. Aragonite Structural TypeStrontianite SrCO3 End-member of isomorphic series with aragonite, and with witherite (BaCO3) Low T hydrothermal mineral Concretions in sedimentary rocks

  5. Aragonite Structural Type CerussitePbCO3 Oxidation product from lead sulphide ore Adamantine luster on faces, greasy luster on uneven or shell-like fracture surfaces High S.G. Lead ore, but not very NB

  6. Occur in oxidation zone of chalcopyrite and other Cu-sulphides Both pseudomorphs of copper and cuprite Malachite Green Green paint Rare semi-precious stone Azurite: Blue Blue paint The Malachite GroupMalachite Cu2(CO3)(OH)2Azurite Cu3(CO3)2(OH)2

  7. Bastnaesite groupBastnaesite – CeCO3F Rare earth element carbonates Ce, La, Sr, Ba Form when REE silicates decomposes in pegmatite veins Other REE carbonate minerals: Parisite, rontgenite, synchisite, carbocernaite, ancylite, lanthanite

  8. Formation conditions Sedimentary rocks – major occurrence Carbonatites – rare rock type A carbonate rock of igneous origin Usually forms dikes or volcano plugs Hydrothermal deposits – carbonates common Medium and low T conditions Surface conditions Caves: stalactites and stalagmites Weathering crusts of ore deposits Metamorphic rocks From sedimentary rocks: limestones If limestone pure calcite – forms marble If calcite, dolomite and quartz – new silicate minerals CaCO3 + SiO2  CaSiO3 + CO2 Calcite Quartz Wollastonite

  9. Carbonates in sedimentary rocks Carbonate rocks = ¼ of all sedimentary rocks Limestone, dolostone (less common) and rarely carbonate evaporites In solution in seawater deeper than: 4.2 – 4.7km Therefore only precipitates in shallow water environments In sediments of continental shelf and coral reefs Shallow oceans of intermediate, tropical zones Dolomite more rarely precipitated than calcite

  10. Carbonates in sedimentary rocks

  11. Carbonates in sedimentary rocks Continental carbonate sediments Clastics, surface crusts, soil, lakes, evaporites Carbonates in oceans Clastic material transported from coast Remnants of skeletons from living organisms Authigenic minerals Forms in place: reactions of primary minerals with porous solutions during diagenesis

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