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Lecture 15 Part 1Systematic Description of Minerals
Part 2a:Halides, Sulfates, and a discussion of
Evaporite Deposits
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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)
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Common Halides
Halite (NaCl)
Sylvite (KCl)
Fluorite (CaF2)
More on these later.
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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
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Octahedral Packing CN 6in Halite NaCl
Sylvite KCl is isostructural
Sodium cations Na+ filling all of the octahedral holes
in a lattice of cubic closest packed chloride Cl-
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Fluorite
CaF2 Isometric Cubic xtals, but Cleavage {111} perfect
Hydrothermal veins, e.g Franklin marble
Mfg HF and in steel Mfg Low melting point
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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
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Mineral Groups formed with Anionic Complexes
CarbonatesCarbonates
SulfatesSulfatesSOSO
44----
PhosphatesPhosphates
SilicatesSilicates
NEXT:
Next time
Today
After Thanksgiving
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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
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Figure 17.1 b Sulfate Ion Figure 17.1 b Sulfate Ion SOSO
44-- -- We consider theWe consider the
S valence +6 (as if ionic)S valence +6 (as if ionic)O valence always -2O valence always -2
Amount of residual charge Amount of residual charge (charge not neutralized) (charge not neutralized) indicates relative strength indicates relative strength available for bonds with available for bonds with cations, reflected in the cations, reflected in the hardness of the mineralhardness of the mineral
Pp 60-61Electronegativities: S 2.4, O 3.5Difference 1.1, only 25% ionicK&D Fig 3.21 So mostly covalent, i.e. electrons spend almost as muchtime near Sulfur
Residual charge on each Oxygen = -1/2 x 4 Oxygens = -2
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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 SO4 with a charge of -2, SO4 --
Again: The Sulfate Ion: strong covalent bonds, acts as a unit.
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Common SulfatesHYDROUS ANHYDROUS
Gypsum – CaSOGypsum – CaSO44·2H·2H22OO
Barite – BaSOBarite – BaSO4 4 and similar
Anhydrite – CaSOAnhydrite – CaSO44
H: 3-3.5H: 3-3.5SG: 4.5SG: 4.5
H: 3-3.5H: 3-3.5SG: 2.9SG: 2.9
H: 2H: 2SG: 2.32SG: 2.32
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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 BaO12 group is bonded to seven individual (SO4)-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
XSO4
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Barite Group Structure
Cation of Barium Ba++, of Strontium Sr++, of Lead Pb++ surrounded by 12 oxygens
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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.
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All photos courtesy of John Betts
Barite BaSO4 Orthorhombic 2/m 2/m 2/m
Mined locally Hopewell
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Barite BaSO4
http://rruff.geo.arizona.edu/doclib/cm/vol15/CM15_522.pdf
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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
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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
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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 massiveSource for drywall (gypsum board, sheetrock) and
Plaster of Paris
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Gypsum (continued)
Pictures courtesy John Betts and mindat.org
Monoclinic2/m
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Gypsum Structure
From Klein and Hurlburt Manual of Mineralogy 20th Edition.Looking down along c-axis. Note perfect (010) cleavages. `
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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
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Anhydrite
Photos by Martins da Pedra courtesy Mindat.org
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CaSO4 . 2 H2O
CaSO4 . 1/2 H2O
CaSO4
Gypsum
Hemihydrate
Anhydrite
Anhydrite may be formed by the dehydration of Gypsum
95oC
68oC
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Evaporite Minerals• Most of Evaporites are Halides and Sulfates
Volume of water
50% calcite precipitates, gypsum at 20% volume, halite at at 10% volume
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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
Discussion: Dead Sea, Playas Messinian Crisis
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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
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Playas (cont'd) –A Playa in Death Valley, California
Evaporite crust
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Swimmers in hypersaline Dead Sea
Evaporite deposits indicate high aridity index.
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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
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Gulf Coast continental margin
Rising Salt Domes
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Salt Dome lease 351-17Gulf Of MexicoCourtesy Shell Exploration, HoustonUsed with Permission
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For Lab
Halite Sylvite Fluorite
Gypsum Anhydrite