stoichiometry some minerals contain varying amounts of 2+ elements which substitute for each other...
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Stoichiometry• Some minerals contain varying amounts of
2+ elements which substitute for each other
• Solid solution – elements substitute in the mineral structure on a sliding scale, defined in terms of the end members – species which contain 100% of one of the elements
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Chemical Formulas
• Subscripts represent relative numbers of elements present
• (Parentheses) separate complexes or substituted elements– Fe(OH)3 – Fe bonded to 3 separate OH
groups
– (Mg, Fe)SiO4 – Olivine group – mineral composed of 0-100 % of Mg, 100-Mg% Fe
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• KMg3(AlSi3O10)(OH)2 - phlogopite
• K(Li,Al)2-3(AlSi3O10)(OH)2 – lepidolite
• KAl2(AlSi3O10)(OH)2 – muscovite
• Amphiboles:
• Ca2Mg5Si8O22(OH)2 – tremolite
• Ca2(Mg,Fe)5Si8O22(OH)2 –actinolite
• (K,Na)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2 - Hornblende
Actinolite series minerals
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Compositional diagrams
Fe O
FeOwustite
Fe3O4
magnetiteFe2O3
hematite
A1B1C1
xA1B2C3
A
CB
x
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Fe Mg
Si
fayalite forsterite
enstatite ferrosilite
Pyroxene solid solution MgSiO3 – FeSiO3
Olivine solid solution Mg2SiO4 – Fe2SiO4
Fe Mg
forsteritefayalite
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Minor, trace elements
• Because a lot of different ions get into any mineral’s structure as minor or trace impurities, strictly speaking, a formula could look like:
• Ca0.004Mg1.859Fe0.158Mn0.003Al0.006Zn0.002Cu0.001Pb0.000
01Si0.0985Se0.002O4
• One of the ions is a determined integer, the other numbers are all reported relative to that one.
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Normalization• Analyses of a mineral or rock can be reported in
different ways:– Element weight %- Analysis yields x grams element in
100 grams sample– Oxide weight % because most analyses of minerals and
rocks do not include oxygen, and because oxygen is usually the dominant anion - assume that charge imbalance from all known cations is balanced by some % of oxygen
– Number of atoms – need to establish in order to get to a mineral’s chemical formula
• Technique of relating all ions to one (often Oxygen) is called normalization
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Normalization• Be able to convert between element weight
%, oxide weight %, and # of atoms• What do you need to know in order convert
these?– Element’s weight atomic mass (Si=28.09
g/mol; O=15.99 g/mol; SiO2=60.08 g/mol)– Original analysis– Convention for relative oxides (SiO2, Al2O3, Fe2O3
etc) based on charge neutrality of complex with oxygen (using dominant redox species)
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Normalization example
• Start with data from quantitative analysis: weight percent of oxide in the mineral
• Convert this to moles of oxide per 100 g of sample by dividing oxide weight percent by the oxide’s molecular weight
• ‘Fudge factor’ is process called normalization – where we divide the number of moles of one thing by the total moles all species/oxides then are presented relative to one another
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Feldspar analysis(Ca, Na, K)1(Fe, Al, Si)4O8
oxide
Atomic weight
of oxide (g/mol)
# cations in oxide
# of O2-
in oxide
Oxide wt % in the
mineral (determined by analysis)
# of moles of oxide in
the mineral
mole % of oxides in
the mineral Cation
moles of cations
in sample
moles of O2-
contributed by each cation
Number of moles of ion in the mineral
SiO2 60.08 1 2 65.90 1.09687 73.83 Si4+73.83 147.66 2.95
Al2O3 101.96 2 3 19.45 0.19076 12.84 Al3+25.68 38.52 1.03
Fe2O3 159.68 2 3 1.03 0.00645 0.43 Fe3+ 0.87 1.30 0.03CaO 56.08 1 1 0.61 0.01088 0.73 Ca2+ 0.73 0.73 0.03Na2O 61.96 2 1 7.12 0.11491 7.73 Na+ 15.47 7.73 0.62
K2O 94.20 2 1 6.20 0.06582 4.43 K+ 8.86 4.43 0.35
SUM 1.48569 100 125.44 200.38
# of moles Oxygen choosen: 8
Ca0.73Na15.47K8.86Fe0.87Al25.68Si73.83O200.38
Ca0.03Na0.62K0.35Fe0.03Al1.03Si2.95O8
to get here from formula above, adjust by 8 / 200.38
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Mineral assembly
• Most minerals will deal with ionic bonds between cations and anions (or anionic subunits which are themselves mostly covalent but do not dissociate)
• Assembly of minerals can be viewed as the assembly of individual ions/subunits into a repeatable framework
• This repeatable framework is a crystal or crystalline material
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Mineral Assembly
• Isotropic – same properties in every direction
• Anisotropic- different properties in different directions most minerals are this type
• Assembly of ions from melts, water, or replacement reactions which form bonds
• The matrices the ions are in always contain many different ions – different conditions of formation for the same mineral creates differences…
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Polymorphs• Two minerals with the same chemical formula but
different chemical structures• What can cause these transitions??
•sphalerite-wurtzite•pyrite-marcasite •calcite-aragonite•Quartz forms (10)•diamond-graphite
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Complexes Minerals• Metals in solution are coordinated with ligands
(Such as H2O, Cl-, etc.)• Formation of a sulfide mineral requires direct
bonding between metals and sulfide – requires displacement of these ligands and
deprotonation of the sulfide
• Cluster development is the result of these requirements
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Mineral growth
• Ions come together in a crystal – charge is balanced across the whole
• How do we get large crystals??– Different mechanisms for the growth of
particular minerals– All a balance of kinetics (how fast) and
thermodynamics (most stable)
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Crystal Shapes
• Shape is determined by atomic arrangements
• Some directions grow faster than others
• Morphology can be distinct for the conditions and speed of mineral nucleation/growth (and growth along specific axes)
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Ostwald Ripening
Larger crystals are more stable than smaller crystals – the energy of a system will naturally trend towards the formation of larger crystals at the expense of smaller ones
In a sense, the smaller crystals are ‘feeding’ the larger ones through a series of dissolution and precipitation reactions
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Figure 3-17. “Ostwald ripening” in a monomineralic material. Grain boundaries with significant negative curvature (concave inward) migrate toward their center of curvature, thus eliminating smaller grains and establishing a uniformly coarse-grained equilibrium texture with 120o grain intersections (polygonal mosaic). © John Winter and Prentice Hall
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Small crystals…
• In the absence of ripening, get a lot of very small crystals forming and no larger crystals.
• This results in a more massive arrangement
• Microcrystalline examples (Chert)
• Massive deposits (common in ore deposits)
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Topotactic Alignment•Alignment of smaller grains in space – due to magnetic attraction, alignment due to biological activity (some microbes make a compass with certain minerals), or chemical/ structural alignment – aka oriented attachment