Sheet Silicates

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• Sheet Silicates aka Phyllosilicates

• Si2O52- Sheets of tetrahedra
  Phyllosilicates
• micas talc clay minerals serpentine
• Clays ? talc ? pyrophyllite ? micas
• Display increasing order and lower variability of
  chemistry as T of formation increases

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Clays

• Term clay ALSO refers to a size (lt 1mm lt10-6 m)
• Sheet silicates, hydrous some contain up to 20
  H2O ? together with a layered structure and weak
  bonding between layers make them SLIPPERY WHEN
  WET
• Very complex (even argued) chemistry reflective
  of specific solution compositions

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Major Clay Minerals

• Kaolinite Al2Si2O5(OH)4
• Illite K1-1.5Al4(Si,Al)8O20(OH)4
• Smectites
• 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)
• 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
• Mixed-layer clays (I/S illite/smectite layers)

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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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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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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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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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Calcite vs. Dolomite

• dolomite less reactive with HCl calcite has lower
  indices of refraction
• calcite more commonly twinned
• dolomite more commonly euhedral
• calcite commonly colorless
• 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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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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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