Lecture 15 Part 1Systematic 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 Properties – low hardness, high melting

points, poor conductors (except at high temperatures)

Some are soluble in water (Halite, Sylvite)

Common Halides

Halite (NaCl)

Sylvite (KCl)

Fluorite (CaF2)

More on these later.

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

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-

Fluorite

CaF2 Isometric Cubic xtals, but Cleavage {111} perfect

Hydrothermal veins, e.g Franklin marble

Mfg HF and in steel Mfg Low melting point

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

Mineral Groups formed with Anionic Complexes

CarbonatesCarbonates

SulfatesSulfatesSOSO

44----

PhosphatesPhosphates

SilicatesSilicates

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After Thanksgiving

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

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

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.

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

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

Barite Group Structure

Cation of Barium Ba++, of Strontium Sr++, of Lead Pb++ surrounded by 12 oxygens

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.

All photos courtesy of John Betts

Barite BaSO4 Orthorhombic 2/m 2/m 2/m

Mined locally Hopewell

Barite BaSO4

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

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

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

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

Gypsum (continued)

Pictures courtesy John Betts and mindat.org

Monoclinic2/m

Gypsum Structure

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

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

Anhydrite

Photos by Martins da Pedra courtesy Mindat.org

Anhydrite Structure CaSO4

CaSO4 . 2 H2O

CaSO4 . 1/2 H2O

CaSO4

Gypsum

Hemihydrate

Anhydrite

Anhydrite may be formed by the dehydration of Gypsum

95oC

68oC

Evaporite Minerals• Most of Evaporites are Halides and Sulfates

Volume of water

50% calcite precipitates, gypsum at 20% volume, halite at at 10% volume

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

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

Playas (cont'd) –A Playa in Death Valley, California

Evaporite crust

Swimmers in hypersaline Dead Sea

Evaporite deposits indicate high aridity index.

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

Gulf Coast continental margin

Rising Salt Domes

Salt Dome lease 351-17Gulf Of MexicoCourtesy Shell Exploration, HoustonUsed with Permission

For Lab

Halite Sylvite Fluorite

Gypsum Anhydrite