Major Clay Minerals

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Major Clay Minerals

• Kaolinite Al2Si2O5(OH)4
• Illite K1-1.5Al4(Si,Al)8O20(OH)4
• Smectites
• Montmorillonite (Ca, Na)0.2-0.4(Al,Mg,Fe)2(Si,Al
  )4O10(OH)2nH2O
• Vermicullite - (Ca, Mg)0.3-0.4(Al,Mg,Fe)3(Si,Al)4O
  10(OH)2nH2O
• Swelling clays can take up extra water in their
  interlayers and are the major components of
  bentonite (NOT a mineral, but a mix of different
  clay minerals)

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Phyllosilicates
SiO4 tetrahedra polymerized into 2-D sheets
Si2O5 Apical Os are unpolymerized and are
bonded to other constituents
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Phyllosilicates
Tetrahedral layers are bonded to octahedral
layers (OH) pairs are located in center of T
rings where no apical O
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Phyllosilicates
Octahedral layers can be understood by analogy
with hydroxides
Brucite Mg(OH)2 Layers of octahedral Mg in
coordination with (OH) Large spacing along c due
to weak van der waals bonds
c
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Phyllosilicates
a2
a1
Gibbsite Al(OH)3 Layers of octahedral Al in
coordination with (OH) Al3 means that only 2/3
of the VI sites may be occupied for
charge-balance reasons Brucite-type layers may
be called trioctahedral and gibbsite-type
dioctahedral
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Phyllosilicates
T O - T O - T O
Yellow (OH)
vdw
Kaolinite Al2 Si2O5 (OH)4 T-layers and
diocathedral (Al3) layers (OH) at center of
T-rings and fill base of VI layer ?
vdw
weak van der Waals bonds between T-O groups
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Phyllosilicates
T O - T O - T O
Yellow (OH)
vdw
Serpentine Mg3 Si2O5 (OH)4 T-layers and
triocathedral (Mg2) layers (OH) at center of
T-rings and fill base of VI layer ?
vdw
weak van der Waals bonds between T-O groups
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Clay building blocks
11 Clay

• Kaolinite micelles attached with H bonds many H
  bonds aggregately strong, do not expend or swell

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Clay building blocks
21 Clay

• Slightly different way to deal with charge on the
  octahedral layer put an opposite tetrahedral
  sheet on it
• Now, how can we put these building blocks
  together

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Calcite vs. Dolomite

• dolomite less reactive with HCl calcite has lower
  indices of refraction
• calcite more commonly twinned
• dolomite more commonly euhedral
• calcite commonly colourless
• dolomite may be cloudy or stained by iron oxide
• Mg ? spectroscopic techniques!
• Different symmetry ? cleavage same, but easily
  distinguished by XRD

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Calcite Group

• Variety of minerals varying by cation
• Ca ? Calcite
• Fe ? Siderite
• Mn ? Rhodochrosite
• Zn ? Smithsonite
• Mg ? Magnesite

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Dolomite Group

• Similar structure to calcite, but Ca ions are in
  alternating layers from Mg, Fe, Mn, Zn
• Ca(Mg, Fe, Mn, Zn)(CO3)2
• Ca ? Dolomite
• Fe ? Ankerite
• Mn ? Kutnahorite

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Aragonite Group

• Polymorph of calcite, but the structure can
  incorporate some other, larger, metals more
  easily (Pb, Ba, Sr)
• Ca ? Aragonite
• Pb ? cerrusite
• Sr ? Strontianite
• Ba ? Witherite
• Aragonite LESS stable than calcite, but common in
  biological material (shells.)

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Carbonate Minerals
Calcite Group(hexagonal)       Calcite Group(hexagonal)       Dolomite Group(hexagonal)     Dolomite Group(hexagonal)     AragoniteGroup(orthorhombic)         AragoniteGroup(orthorhombic)        
mineral formula mineral formula mineral formula
Calcite CaCO3 Dolomite CaMg(CO3)2 Aragonite CaCO3
Magnesite MgCO3 Ankerite Ca(Mg,Fe)(CO3)2 Witherite BaCO3
Siderite, FeCO3 Kutnohorite CaMn(CO3)2 Strontianite SrCO3
Rhodochrosite MnCO3

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Carbonate Minerals
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Sulfate Minerals

• More than 100 different minerals, separated into
  hydrous (with H2O) or anhydrous (without H2O)
  groups
• Gypsum (CaSO42H2O) and anhydrite (CaSO4) are the
  most common of the sulfate minerals
• Gypsum typically forms in evaporitic basins a
  polymorph of anhydrite (g-CaSO4) forms when the
  gypsum is later dehydrated)

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Gypsum
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• Gypsum formation can demarcate ancient seas that
  dried up (such as the inland seas of the Michigan
  basin) or tell us about the history of current
  seas which have dried up before (such as the
  Mediterranean Sea)

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Halide Minerals

• Minerals contianing halogen elements as dominant
  anion (Cl- or F- typically)
• Halite (NaCl) and Sylvite (KCl) form in VERY
  concentrated evaporitic waters they are
  extremely soluble in water, indicate more
  complete evaporation than does gypsum
• Fluorite (CaF2) more typically occurs in veins
  associated with hydrothermal waters (F- in
  hydrothermal solutions is typically much higher
  leached out of parent minerals such as biotites,
  pyroxenes, hornblendes or apatite)

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Halite Structure

• NaCl ? Na (gray) arranged in CCP with Cl- (red)
  at edges and center (in octahedral cavities)

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Flourite structure

• CaF2 ? Ca2 (gray) arranged in CCP, F- ions (red)
  inside cage

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Sulfate Minerals II

• Barite (BaSO4), Celestite (SrSO4), and Anglesite
  (PbSO4) are also important in mining.
• These minerals are DENSE ? Barite 4.5, Anglesite
  6.3 (feldspars are 2.5)

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Barite, Celestite, Anglesite

• Metals bond with sulfate much more easily, and
  thus are generally more insoluble they do not
  require formation in evaporitic basins
• What do they indicate then?

Lots of SO42- Not very much Ba, Sr, Pb
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Just silica

• Chert extremely fine grained quartz
• Forms as nodules in limestone, recrystallization
  of siliceous fossils
• Jasper variety with hematite inclusions ? red
• Flint variety containing organic matter ?
  darker color
• Chalcedony microcrystaliine silica (very
  similar to low quartz, but different it is yet
  uncertain how different) ? typically shows
  banding, often colored to form an agate (rock
  formed of multiple bands of colored chalcedony)
• Jasper variety colored with inclusion of
  microcrystsalline oxides (often iron oxides
  red)
• Opal a hydrogel (a solid solution of water in
  silica) forms initially as water silica
  colloids, then slowly the water diffuses into the
  silica ? making it amorphous (no XRD pattern!)
• Some evidence opal slowly recrystallizes to
  chalcedony

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Opal - Gemstone
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Agates
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Oxides - Oxyhydroxides

• FeOOH minerals ? Goethite or Limonite (FeOOH) ?
  important alteration products of weathering
  Fe-bearing minerals
• Hematite (Fe2O3) ? primary iron oxide in Banded
  Iron Formations
• Boehmite (AlOOH) ? primary mineral in bauxite
  ores (principle Al ore) which forms in tropical
  soils
• Mn oxides ? form Mn nodules in the oceans
  (estimated they cover 10-30 of the deep Pacific
  floor)
• Many other oxides important in metamorphic rocks

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Mn oxides - oxyhydroxides

• Mn exists as 2, 3, and 4 oxide minerals are
  varied, complex, and hard to ID
• Wad ? soft (i.e. blackens your fingers),
  brown-black fine-grained Mn oxides
• Psilomelane ? hard (does not blacked fingers)
  gray-black botroyoidal, massive Mn oxides
• XRD analyses do not easily distinguish different
  minerals, must combine with TEM, SEM, IR
  spectroscopy, and microprobe work

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Mn Oxide minerals (not all)

• Romanechite Ba.66(Mn4,Mn3)5O101.34H2O ?
  Psilomelane
• Pyrolusite MnO2
• Ramsdellite MnO2
• Nsutite Mn(O,OH)2
• Hollandite Bax(Mn4,Mn3)8O16
• Cryptomelane Kx(Mn4,Mn3)8O16
• Manjiroite Nax(Mn4,Mn3)8O16
• Coronadite Pbx(Mn4,Mn3)8O16
• Todorokite (Ca,Na,K)X(Mn4,Mn3)6O123.5H2O
• Lithiophorite LiAl2(Mn2Mn3)O6(OH)6
• Chalcophanite ZnMn3O73H2O
• Birnessite (Na,Ca)Mn7O142.8H2O
• Vernadite MnO2nH2O
• Manganite MnOOH
• Groutite MnOOH
• Feitknechtite MnOOH
• Hausmannite Mn2Mn23O4
• Bixbyite Mn2O3
• Pyrochroite Mn(OH)2

Wad
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Iron Oxides

• Interaction of dissolved iron with oxygen yields
  iron oxide and iron oxyhyroxide minerals
• 1st thing precipitated ? amorphous or extremely
  fine grained (nanocrystaliine) iron oxides called
  ferrihydrite

O2
Fe2
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Ferrihydrite

• Ferrihydrite (Fe5O7OHH2O Fe10O159H2O ? some
  argument about exact formula) a mixed valence
  iron oxide with OH and water

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Goethite

• Ferrihydrite recrystallizes into Goethite
  (a-FeOOH)
• There are other polymorphs of iron oxyhydroxides
• Lepidocrocite g-FeOOH
• Akaganeite b-FeOOH

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Iron Oxides

• Hematite (Fe2O3) can form directly or via
  ferrihydrite ? goethite ? hematite
• Red-brown mineral is very common in soils and
  weathering iron-bearing rocks

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• Magnetite (Fe3O4) Magnetic mineral of mixed
  valence ? must contain both Fe2 and Fe3 ? how
  many of each??
• Spinel structure 2/3 of the cation sites are
  octahedral, 1/3 are tetrahedral

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Banded Iron Formations (BIFs)

• HUGE PreCambrian formations composed of
  hematite-jasper-chalcedony bands
• Account for 90 of the worlds iron supply
• Occur only between 1.9 and 3.8 Ga ? many sites
  around the world ? Hammersley in Australia,
  Ishpeming in Michigan, Isua in Greenland, Carajas
  in Brazil, many other sites around the world