1. Lecture 15 Part 1 Systematic 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 (CaF 2 ) 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 6 in Halite NaCl Sylvite KCl is isostructural Sodium cations Na+ filling all of the octahedral holes in a lattice of cubic closest packed chloride Cl-

6. Fluorite  CaF 2 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 CaF 2 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. Mineral Groups formed with Anionic Complexes CarbonatesSulfates SO 4 -- PhosphatesSilicates NEXT: 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 SO 4 with a charge of -2, SO 4 -- Two double bonds and two single bonds Notice BOTH S and O have 6 valence electrons

10. Figure 17.1 b Sulfate Ion SO 4 -- We consider the S valence +6 (as if ionic) O valence always -2 Amount of residual charge (charge not neutralized) indicates relative strength available for bonds with cations, reflected in the hardness of the mineral 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

11. Sulfates: Metals + Sulfate Ion 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 SO 4 with a charge of -2, SO 4 -- Again: The Sulfate Ion: strong covalent bonds, acts as a unit.

12. Common Sulfates HYDROUS ANHYDROUS Gypsum – CaSO 4 ·2H 2 O Barite – BaSO 4 Barite – BaSO 4 and similar Anhydrite – CaSO 4 H: 3-3.5 SG: 4.5 H: 3-3.5 SG: 2.9 H: 2 SG: 2.32

13. Sulfate Ion Radius = ~1.49 Angstroms 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 In all three, cation in 12- fold coordination w Oxygens O— e.g. in Barite, each BaO 12 group is bonded to seven individual (SO 4 ) -2 tetrahedra Solid solutions limited within these species. Why? S in SO4 TETRAHEDRAL [4] Coordination w Oxygen These three very difficult to distinguish in hand specimens XSO 4

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

15. Barite  BaSO 4 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 muddrilling mud  Absorbs x-rays in medical diagnostic studies of the digestive tract. Absorbs x-rays

16. All photos courtesy of John Betts Barite BaSO 4 Orthorhombic 2/m 2/m 2/m Mined locally Hopewell

17. Barite BaSO 4 http://rruff.geo.ari zona.edu/doclib/ cm/vol15/CM15_ 522.pdf

18. Other Sulfates in the Barite Group Celestite SrSO 4 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 PbSO 4 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 CaSO 4  CaSO 4. 2H 2 O 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) Pictures courtesy John Betts and mindat.org Monoclinic 2/m

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

23. Anhydrite  CaSO 4 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 H 2 SO 4

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

25. Anhydrite Structure CaSO 4

26. CaSO 4. 2 H 2 O CaSO 4. 1/2 H 2 O CaSO 4 Gypsum Hemihydrate Anhydrite Anhydrite may be formed by the dehydration of Gypsum 95 o C 68 o C

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  Precipitate when concentration in water at T o 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 (CaSO 4 -2H 2 O) and/or Anhydrite (CaSO 4 ). T o dependant. Halite (i.e. common salt, NaCl) Potassium and magnesium salts KCl, KNO 3, MgSO 4 ·7H2O Discussion: Dead Sea, Playas Messinian Crisis

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