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Systematic Overview of Halides and Sulfates: Properties and Evaporite Deposits

This lecture explores the systematic description of minerals, focusing on halides, sulfates, and evaporite deposits. Halides are characterized by isometric structures and predominantly ionic bonding with low hardness and high melting points, including important minerals like Halite (NaCl) and Sylvite (KCl). The lecture also discusses sulfate minerals, notably Gypsum and Barite, with emphasis on their chemical structures, coordination, and industrial applications. Students will gain insights into the unique properties and uses of these mineral groups.

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Systematic Overview of Halides and Sulfates: Properties and Evaporite Deposits

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

  2. 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)

  3. Common Halides Halite (NaCl) Sylvite (KCl) Fluorite (CaF2) More on these later.

  4. 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 Cubic xtals, cubic cleavage

  5. Octahedral Packing CN 6in Halite NaCl Sodium cations Na+ filling all of the octahedral holes in a lattice of cubic closest packed chloride Cl- Sylvite KCl is isostructural

  6. Fluorite • CaF2 Isometric Cubic xtals, but Cleavage {111} perfect • Hydrothermal veins, e.g Franklin marble • Mfg HF and in steel Mfg • Low melting point

  7. Coordination in Fluorite CaF2 F- (blue) are in Tetrahedral coordination with FCC Ca++ cations. Each cation layer alternates vertical positions and cations occupy half of the possible positions

  8. NEXT: Mineral Groups formed with Anionic Complexes Carbonates Sulfates SO4-- Phosphates Silicates Next time Today After Thanksgiving

  9. The Sulfate Ion has Sulfur in tetrahedral coordination with 4 oxygens. Sulfur, element 16, with 1S2 2S2 2P6 3S2 3P4 electrons, so 6 electrons in the outer shell of the neutral atom. These 6 electrons shared with the Oxygens,1S2 2S2 2P4, leaving the Sulfate Ion SO4 with a charge of -2, SO4 -- Two double bonds and two single bonds Notice BOTH S and O have 6 valence electrons

  10. Pp 60-61 Electronegativities: S 2.4, O 3.5 Difference 1.1, only 25% ionic K&D Fig 3.21 So mostly covalent, i.e. electrons spend almost as much time near Sulfur Residual charge on each Oxygen = -1/2 x 4 Oxygens = -2 Amount of residual charge (charge not neutralized) indicates relative strength available for bonds with cations, reflected in the hardness of the mineral Figure 17.1 b Sulfate Ion SO4-- We consider the S valence +6 (as if ionic) O valence always -2

  11. Sulfates: Metals + Sulfate Ion Again: The Sulfate Ion: strong covalent bonds, acts as a unit. The Sulfate Ion has Sulfur in tetrahedral coordination with 4 Oxygens. Sulfur, element 16, with 1S2 2S2 2P6 3S2 3P4 electrons, so 6 electrons in the outer shell of the neutral atom. These 6 electrons shared with the Oxygens, leaving the Sulfate Ion SO4 with a charge of -2, SO4 --

  12. Common Sulfates HYDROUS ANHYDROUS H: 2 SG: 2.32 H: 3-3.5 SG: 2.9 Gypsum – CaSO4·2H2O Anhydrite – CaSO4 H: 3-3.5 SG: 4.5 Barite – BaSO4 and similar

  13. Sulfate Ion Radius = ~1.49 Angstroms S in SO4 TETRAHEDRAL [4] Coordination w Oxygen In all three, cation in 12-fold coordination w Oxygens O— e.g. in Barite, each BaO12 group is bonded to seven individual (SO4)-2 tetrahedra Cation Val. Cation Radii (Angstroms) Barium Ba+2 1.68 [12] Barite Strontium Sr+2 1.48 [12] Celestite Lead Pb+2 1.57 [12] Anglesite XSO4 Solid solutions limited within these species. Why? These three very difficult to distinguish in hand specimens

  14. Barite Group Structure Cation of Barium Ba++, of Strontium Sr++, of Lead Pb++ surrounded by 12 oxygens

  15. 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.

  16. Barite BaSO4Orthorhombic 2/m 2/m 2/m All photos courtesy of John Betts Mined locally Hopewell

  17. http://rruff.geo.arizona.edu/doclib/cm/vol15/CM15_522.pdf Barite BaSO4

  18. Other Sulfates in the Barite Group Celestite SrSO4 Orthorhombic 2/m 2/m 2/m Cleavage {001} perfect, {210} good H 3-3.5 G 3.95-3.97 Also called Celestine Often light sky blue in color

  19. Other Sulfates in the Barite Group Anglesite PbSO4 Orthorhombic 2/m 2/m 2/m Cleavage {001} good, {210} imperfect H 3 G 6.2-6.4 Much greater SG

  20. 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

  21. Gypsum (continued) Monoclinic 2/m Pictures courtesy John Betts and mindat.org

  22. Gypsum Structure From Klein and Hurlburt Manual of Mineralogy 20th Edition. Looking down along c-axis. Note perfect (010) cleavages. `

  23. 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

  24. Anhydrite Photos by Martins da Pedra courtesy Mindat.org

  25. Anhydrite Structure CaSO4

  26. CaSO4. 2 H2O Gypsum 68oC CaSO4. 1/2 H2O Hemihydrate 95oC CaSO4 Anhydrite Anhydrite may be formed by the dehydration of Gypsum

  27. Evaporite Minerals • Most of Evaporites are Halides and Sulfates Volume of water 50% calcite precipitates, gypsum at 20% volume, halite at at 10% volume

  28. Evaporites Discussion: Dead Sea, Playas Messinian Crisis • 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

  29. 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

  30. Playas (cont'd) –A Playa in Death Valley, California Evaporite crust

  31. Swimmers in hypersaline Dead Sea Evaporite deposits indicate high aridity index.

  32. 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

  33. Gulf Coast continental margin Rising Salt Domes

  34. Salt Dome lease 351-17 Gulf Of Mexico Courtesy Shell Exploration, Houston Used with Permission

  35. For Lab • Halite • Sylvite • Fluorite • Gypsum • Anhydrite

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