mineralogy 23.10.08(class1)
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LECTURE OUTLINE
Minerals: Building blocks of rocks
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RULE 1: The anions around a cation define a coordination polyhedron.
The best-known example is the silica tetrahedron. The distance between
cations and anions is determined by the sum of their ionic radii. The
ratio of their radii determines the coordination number, or number of
anions surrounding the cation.
RULE 2: Electrostatic valency principle: The total strength of the valency
bonds that reach an anion from all its neighboring cations is equal to the
charge of the anion.
EXAMPLE:
For example, in olivine, (Mg,Fe)2SiO4, the charge on the Si ion is +4, on Mg
or Fe +2 and on O -2. Since 4 oxygens surround a silicon, there must be
a charge of -1 on each oxygen to be balanced by the Mg and Fe atoms.
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RULE 3: Cation coordination polyhedra tend to be linked (share anions) at
corners first, then edges, and faces last of all because of the electrostatic
repulsion between cations. The repulsion is especially great for cations with
large charge and small coordination number, like Si. Silica tetrahedra almostnever link any other way than at their corners.
RULE 4: In a crystal containing different cations, cations with large charge
and small coordination number tend not to share polyhedral elements.
Think of Bowen's Series: among the ferromagnesian minerals, olivine
(isolated tetrahedra) tends to form first, followed by pyroxene (single chains),
amphibole (double chains) and biotite (sheets). The silica tetrahedra don't
link until they have no other choice.
RULE 5: In crystals the number of structurally distinct sites tends to
be small
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CN Polyhedron Example
3 Triangle C in Calcite
4 Tetrahedron Si in Quartz
6 Octahedron Mg in Forsterite
8 Cube Ca in Fluorite
12 Dodecahedron Native gold
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PACKING OF ATOMS IN CRYSTALS
Most compact lowest porosity Relatively less compact higher porosity
CUBIC Close Packing :
Spinel(MgAl2O4), Halite(NaCl)HEXAGONAL close packing:
Corundum, ilmenite
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Minerals: Building blocks of rocks By definition a mineral is
Naturally occurring
Homogenous solid
Definite chemical
composition
Ordered internal
molecular structure
Inorganic solid
Rock
A solid aggregate of minerals
Granite
Quartz Biotite
Feldspar
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C lassification of Minerals
± Oxides CHROMITE, CORRUNDUM
± Sulfides CHALCOPYRITE, PYRITE
± Sulfates GYPSUM
± Native Elements GOLD, SILVER
± Carbonates CALCITE, DOLOMITE
± Halides GYPSITE, HALITE
± Phosphates APATITE,
Minerals are classified on their chemistry, particularly on the anionic elementor polyanionic group of elements that occur in the mineral.
NON-SILICATES
SILICATES
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LINKING OF SiO4 TETRAHEDRA
Linking of SIO4 Tetrahedra
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Mineral Groups
± Silicates ±
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LINKING OF SiO4 TETRAHEDRA AND CLEAVAGE
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Neso slicate: Olivine- M2SiO4, e.g. Forsterire(Mg2SiO4) 1:4
Si :O ratio
Soro silicate : Epidote - Ca2(Fe+3, Al) Al2(SiO4)(Si2O7)(OH) 2:7
Cyclosilicate: Ber yl - Be3 Al2Si6O18 1:3
Ionosilicate(single): Pyroxene- M2M1Si2O6, e.g. Diopside(CaMgSi2O6) 1:3
Ionosilicate (Double): Amphibole- Tremolite(CaMg5Si8O22(OH)2) 4:11
Phyllosilicate: Mica, Chlorite- e.g. Annite(KFe3 AlSi3O10(OH)2) 2:5
Tectosilicate: Quartz, Feldspar- Albite(Na AlSi3O8) 1:2
SILICATES
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The olivine structure is based on isolated SiO4 tetrahedra
(blue) which link chains of (Fe,Mg)O6 octahedra. There
are two octahedral cation sites: M1 (yellow) and M2
(orange). Both sites accomodate Fe2+ and Mg2+ cations
and there is complete disorder of Fe and Mg over the M1
and M2 sites. Olivine is orthorhombic and therefore will
show parallel or symmetric extinction under crossed
polarized light. M1-O distance~2.101 A, M2-O~2.135A.
M1 radius=0.781A, M2 radius=0.812A
Mg(M1)Mg(M2)SiO4
(Forsterite) Mg=0.72A
Fayalite(2Fe2Mg)
Fe=0.78A
Tephroite (2Mn2Mg)
Mn=0.83A
Motecellite (CaMg)
Ca=1.0A
K irschsteinite (CaFeSiO4),
Ni2SiO4
Isomorphous substitutions and speciation of
orthorhombic (EOLIVINE
NESOSILIC ATE (OLIVINE)
Glaucochroite (CaMnSiO4)
M1= Fe,Mg, Ni; M2 = Ca,Mn
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Si
OFe,Mg
STRUCTURE OF OLIVINE
SiO4
Tetrahedra
(Mg,Fe)O6
Octahedra
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OLIVINE
GARNET
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.
The single chain silicates have a basic structural unit
consisting of linked SiO4 tetrahedra that each share 2of their oxygens in such a way as to build long chains
of SiO4. The basic structural group is thus Si2O6
with an Si:O ratio of 1:3. The most important
inosilicates are the pyroxenes. These have a general
structural formula of: XYZ2O6
where X = Na+, Ca+2, Mn+2, Fe+2, or Mg+2 fillingoctahedral sites called M2
Y = Mn+2, Fe+2, Mg+2 , Al+3, Cr+3, or Ti+4 filling
smaller octahedral sites called M1
Z = Si+4 or Al+3 in tetrahedral coordination
SINGLE CHAIN SILICATES : THEIR PHYSICAL AND CHEMICAL PROPERTIES
C axis
PyroxenequadrilateralCALCIC GROUP
NON CALCIC
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Basal Section with intersecting cleavages at 87-88o
Relationship among the optical and cr ystallographic
planes in the oriented pyroxene cr ystals
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Projection along
the C-axis
Structure of AMPHIBOLE
SiO4
tetrahedra
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Pyroxene
Amphibole
CLEAVAGE IN INOSILICATES
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Sheet Silicates: the Mica's and
Clay Minerals Mica and clay minerals are P hyllosilicates
± Sheet or layered
silicates with ± Two dimensional
polymerization of
silica tetrahedra
± Common structure isa Si205 layer
sheets of silica tetrahedraSi2O5Phyllosilicates
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1-direction
of cleavage
Muscovite
non-ferromagnesian
Mica Group and Clay Minerals
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Structure of Phyllosilicates
Octahedral layer
± Layer of octahedral
coordinated
magnesium (brucitelayer) or
Aluminum (gibbsite
layer)
± Makes up the other basic structural unit
Kaolinite: Al2Si2O5(OH)41:1 tetrahedral ² octahedralsheets
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The Major Clay Mineral Groups
Kaolinite group:
± 1:1 TO clay minerals
Mica (illite) group:
± 2:1 TOT clay minerals ± Expandible clays:
Smectite- montmorillonite
complex 2:1 clay minerals
Chlorite ± Fe- and Mg-rich TOT clays
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All the corners of a SiO4 tetrahedron are linked with the corners of the
neighboring tetrahedron resulting in a Si : O = 1:2
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Name Crystal System Density (g/cm 3)Refractive Index
(mean)
Stishovite (Si in 6-fold
coordination)Tetragonal 4.35 1.81
Coesite Monoclinic 3.01 1.59
Low (E) Quartz Hexagonal 2.65 1.55
High ( F) Quartz Hexagonal 2.53 1.54
Kaetite (synthetic) Tetragonal 2.50 1.52
Low (E) Tridymite Monoclinic or Orthorhombic 2.26 1.47
High ( F) Tridymite Hexagonal 2.22 1.47
Low (E) Cristobalite Tetragonal 2.32 1.48
High ( F) Cristobalite Isometric 2.20 1.48
E Ftransformation involves small
change in structural and physical
properties and hence is ³displacive´
transformation
All other transformations involve large
change in structural and physical
properties and hence are
³reconstructive´ transformation.
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IMMISCIBLE
Movement
direction of the
boundar y at high
temperature
Feldspar Group (Si+4 replaced by Al+3 and the charge is balanced by
Ca+2, K+1 or Na+1)
Alkali feldspar : Solid solution between K AlSi3O8 (Sanidine, orthoclase,
microcline) and Albite (Na AlSi3O8)Plagioclase : Solid solution between Albite (Na AlSi3O8) and Anorthite
(Ca Al2Si2O8)