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Carbonates and Bicarbonates

Carbonates and Bicarbonates. GLY 4200 Lab 8 - Fall, 2012. Carbonate Stability. Carbonates are minerals consisting of metallic elements bonded to the carbonate group, CO 3 2- Although anionic groups are usually stable, one exception to this is the reaction of a carbonate with acid:. Fizz.

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Carbonates and Bicarbonates

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  1. Carbonates and Bicarbonates GLY 4200 Lab 8 - Fall, 2012

  2. Carbonate Stability • Carbonates are minerals consisting of metallic elements bonded to the carbonate group, CO32- • Although anionic groups are usually stable, one exception to this is the reaction of a carbonate with acid:

  3. Fizz • The carbon dioxide is a gas and bubbles (or "fizzes") off • This provides a good qualitative test for the presence of carbonate minerals in most samples • Most carbonate minerals will fizz when a small (!) drop of acid is placed on the mineral • However, it may be necessary to listen for the fizz and a few minerals react so slowly that the reaction is not detectable

  4. Hydrous Carbonates • Hydrous carbonates are often soluble in water • Hydrous carbonates are frequently weathering products • Some, like azurite and malachite, are indicator minerals for possible ore deposits • Both are idiochromatic

  5. Azurite • Azurite = Deep blue

  6. Malachite • Malachite = green • Secondary mineral in the oxidized zones of copper ore deposits • This picture shows a botryoidal specimen

  7. Botryoidal • From the Greek botrys, meaning bunch or cluster of grapes

  8. Banding in Malachite

  9. Calcite Group • Consists of divalent cations bonded to the carbonate anionic group • Structure is a distorted NaCl type, compressed along a three-fold axis, and CN = VI • All members of the calcite group belong to class bar3 2/m • Cations and carbonate anionic groups form alternate layers

  10. Dolomite Group • The dolomite group is similar to calcite except that two distinct divalent cations are present in the structure • These cations occupy separate layers • For example in dolomite itself there is a layer of Mg2+ ions, a layer of carbonate ions, a layer of Ca2+ ions, a layer of carbonate ions, a layer of Mg2+ ions, a layer of carbonate ions, etc • The ordering of the cations results in dolomite group minerals belonging to class bar3

  11. Aragonite Group • Minerals in this group have large divalent cations and each cation is coordinated to nine oxygens • Calcium is borderline between the ratio required for CN = VI or IX so occurs in both the calcite and aragonite structures • Aragonite is denser and is stable at higher pressures • The aragonite group minerals are orthorhombic, 2/m 2/m 2/m

  12. Bicarbonates • Bicarbonates are minerals with the bicarbonate anion, HCO3- • Trona is a typical saline lake deposit

  13. Text Reference • See chapter 17 in the text for more information on carbonates and bicarbonates, pp. 399-403 and 407-416

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