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Halides, Sulfates, and Borates

Halides, Sulfates, and Borates. GLY 4200 Lab 7 - Fall, 2012. Halides. Minerals contain the anions F - , Cl - , Br - , or I - More than one hundred forty halides are known – few are important Bonding is usually nearly purely ionic Anion is spherical, so structures have high symmetry

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Halides, Sulfates, and Borates

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  1. Halides, Sulfates, and Borates GLY 4200 Lab 7 - Fall, 2012

  2. Halides • Minerals contain the anions F-, Cl-, Br-, or I- • More than one hundred forty halides are known – few are important • Bonding is usually nearly purely ionic • Anion is spherical, so structures have high symmetry • If the cation is small and highly polarizing, symmetry will be reduced (cryolite, Na3AlF6 is monoclinic)

  3. Halide Properties • Ionic bonding makes them highly soluble • Usually found in evaporite deposits in arid regions • Some are quite insoluble • Isometric halides have low hardness and moderate to high melting points • They are poor conductors of heat and electricity except near the melting point

  4. Taste • Halite (NaCl) and sylvite (KCl) are similar in appearance • One method of distinguishing them is taste • Halite = salty • Sylvite = bitter

  5. Sulfates • There minerals are characterized by cations bonded to the SO42- group • This group is tetrahedral • The sulfur-oxygen bonds are strong, and the anionic group does not break up during most chemical reactions • Exception: Anaerobic bacteria are capable of breaking the sulfur-oxygen bond

  6. Sulfate Groups • There are two major groups of sulfate minerals • Hydrous Sulfates • Barite (anhydrous)

  7. Barite Group • Large divalent cations in XII coordination • Similar structures in manganates (MnO42-) and chromates (CrO42-) with large cations • Barite is usually found in hydrothermal veins, often as a gangue mineral with ores of Ag, Pb, Cu, Co, Mn, and Sb • Used as drilling mud because of its high density

  8. Anhydrite • Anhydrite has a much smaller cation, Ca2+ • It has a CN of VIII • Found in locations similar to gypsum • Cap rock of salt domes • In limestones • In amygdaloidal cavities of basalts, as a secondary mineral

  9. Gypsum Group • Gypsum is hydrous • Has a layered structure, with layers of sulfate bonded to calcium • Sheets of water molecules separate the Ca-SO4 layers • Bonding between water molecules is weak, producing excellent cleavage

  10. Varieties of Gypsum • Satin spar – fibrous, silky luster • Alabaster – fine-grained, massive • Selenite – broad, usually colorless and transparent, cleavage folia • Twinning is common • Occurrence: Often in thick-bedded sedimentary deposits, may be interstratified with limestone and shale • Often under beds of rock salt (halite) in evaporite deposits

  11. Borates • Forms extended polymer structures • Trigonal planar, BO33- • Tetrahedral, BO45- • Extended network of BO3 triangles is possible, but has low stability • Make it useful in glass preparation for light weight, highly transparent glasses

  12. Borate Occurrence • Many borates are found in evaporite deposits in arid regions • Most are hydrous, and will be destroyed if completely dehydrated • May be deposited from brine solutions

  13. Text Reference • See chapter 16 in the text for more information on halides, pp. 374-375 and 393-397; see chapter 17 in the text for more information on borates, pp. 403 and 416-419; see chapter 17 in the text for more information on sulfates, pp. 403-405 and 420-427

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