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Lecture 15 Part 1 Systematic Description of Minerals

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

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Lecture 15 Part 1 Systematic Description of Minerals

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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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