Lecture 15 Part 1 Systematic Description of Minerals Part 2a: Halides, Sulfates, and a discussion of Evaporite Deposits


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Lecture 15 Part 1 Systematic Description of Minerals

  • Part 2a:

  • Halides, Sulfates, and a discussion of Evaporite Deposits


Halides: you studied their structures in foam atom labs

  • Simple compounds with large halogen anions (Cl, Br, F, I)

  • Typically isometric

  • Dominantly ionic bonding

  • Properties – low hardness, high melting points, poor conductors (except at high temperatures)

  • Some are soluble in water (Halite, Sylvite)



Common Halides

  • Halite (NaCl)

  • Sylvite (KCl)

  • Fluorite (CaF2)



Halite NaCl Isometric

  • Source of Sodium to make NaOH and soap mfg, baking soda, sodium carbonate, and Chlorine (HCl, bleach, paper mfg, water purification) and as NaCl in food.

  • An evaporite mineral, mainly from Salt Domes



Octahedral Packing CN 6 in Halite NaCl



Fluorite

  • CaF2 Isometric Cubic xtals, but Cleavage {111} perfect

  • Hydrothermal veins, e.g Franklin marble

  • Mfg HF and in steel Mfg

  • Low melting point



Coordination in Fluorite CaF2



Mineral Groups formed with Anionic Complexes







Sulfates: Metals + Sulfate Ion



Common Sulfates



Sulfate Ion Radius = ~1.49 Angstroms



Barite Group Structure



Barite

  • BaSO4 Orthorhombic 2/m 2/m 2/m

  • H 3 - 3.5 G 4.5

  • Gangue in hydrothermal veins, w/ Ag, Pb, Cu. Often near hot springs.

  • Use for heavy drilling mud

  • Absorbs x-rays in medical diagnostic studies of the digestive tract.







Other Sulfates in the Barite Group



Other Sulfates in the Barite Group



Gypsum – Hydrous CaSO4

  • CaSO4 . 2H2O Monoclinic 2/m

  • {010} perfect H 2, G 2.3

  • Luster vitreous to pearly

  • Varieties Selenite clear simple 2/m

  • Satin Spar fibous

  • Alabaster fine-grained massive

  • Source for drywall (gypsum board, sheetrock) and Plaster of Paris



Gypsum (continued)



Gypsum Structure



Anhydrite

  • CaSO4 Orthorhombic 2/m 2/m 2/m

  • Xtals rare {010} {001} {100} tabular

  • H 3-3.5, G ~3

  • Luster vitreous to pearly on cleavage

  • Used in making H2SO4



Anhydrite



Anhydrite Structure CaSO4





Evaporite Minerals

  • Most of Evaporites are Halides and Sulfates



Evaporites

  • Precipitate when concentration in water at To reaches their saturation limit.

  • Minerals precipitate in reverse order of their solubilities,

  • Order of precipitation from sea water is:

    • Calcite (first, but very little volume)
    • Gypsum (CaSO4-2H2O) and/or Anhydrite (CaSO4). To dependant.
    • Halite (i.e. common salt, NaCl)
    • Potassium and magnesium salts KCl, KNO3, MgSO4·7H2O


Conditions for precipitation

  • Seawater influx into confined basins

    • New rift, shallow bay, playa, isolated basin in dry climate.
  • Freshwater evaporates away

  • Lower freshwater influx than evaporates

  • Saturated solution exists



Playas (cont'd) – A Playa in Death Valley, California



Swimmers in hypersaline Dead Sea



Salt Domes: evaporites rise

  • Mainly NaCl Halite as evaporite deposits, very low density.

  • Rising columns deflect sediments

  • Forms hydrocarbon traps that make petroleum affordable

  • Gulf Coast Jurassic rifting confined basin



Gulf Coast continental margin





For Lab

  • Halite

  • Sylvite

  • Fluorite

  • Gypsum

  • Anhydrite




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