Pyrite FeS2 - University of Vermontgdrusche/Classes/GEOL 110 - Earth... · Goldschmidt’s rules of...
Transcript of Pyrite FeS2 - University of Vermontgdrusche/Classes/GEOL 110 - Earth... · Goldschmidt’s rules of...
How many molecules?
• Pyrite – FeS2
• Would there be any other elements in
there???
Goldschmidt’s rules of Substitution
1. The ions of one element can extensively
replace those of another in ionic crystals
if their radii differ by less than about 15%
2. Ions whose charges differ by one may
substitute readily if electrical neutrality is
maintained – if charge differs by more
than one, substitution is minimal
3. When 2 ions can occupy a particular
position in a lattice, the ion with the
higher charge density forms a stronger
bond with the anions surrounding the site
4. Substitution may be limited when the
electronegativities of competing ions are
different, forming bonds of different ionic
character
Goldschmidt’s rules of Substitution
FeS2• What ions would
substitute nicely
into pyrite??
• S- radius=219
pm
• Fe2+ radius=70
pm
Problem:
• A melt or water solution that a mineral
precipitates from contains ALL natural
elements
• Question: Do any of these ‘other’ ions get
into a particular mineral?
Chemical ‘fingerprints’ of minerals
• Major, minor, and trace constituents in a
mineral
• Stable isotopic signatures
• Radioactive isotope signatures
Major, minor, and trace constituents
in a mineral• A handsample-size rock or mineral has around 5*1024
atoms in it – theoretically almost every known element is somewhere in that rock, most in concentrations too small to measure…
• Specific chemical composition of any mineral is a record of the melt or solution it precipitated from. Exact chemical composition of any mineral is a fingerprint, or a genetic record, much like your own DNA
• This composition may be further affected by other processes
• Can indicate provenance (origin), and from looking at changes in chemistry across adjacant/similar units - rate of precipitation/ crystallization, melt history, fluid history
Stable Isotopes• A number of elements have more than one naturally
occuring stable isotope.– Why atomic mass numbers are not whole they
represent the relative fractions of naturally occurring stable isotopes
• Any reaction involving one of these isotopes can have a fractionation – where one isotope is favored over another
• Studying this fractionation yields information about the interaction of water and a mineral/rock, the origin of O in minerals, rates of weathering, climate history, and details of magma evolution, among other processes
Radioactive Isotopes• Many elements also have 1+ radioactive isotopes
• A radioactive isotope is inherently unstable and through radiactive decay, turns into other isotopes (a string of these reactions is a decay chain)
• The rates of each decay are variable – some are extremely slow
• If a system is closed (no elements escape) then the proportion of parent (original) and daughter (product of a radioactive decay reaction) can yield a date.
• Radioactive isotopes are also used to study petrogenesis, weathering rates, water/rock interaction, among other processes
Chemical heterogeneity
• Matrix containing ions a mineral forms in contains many different ions/elements –sometimes they get into the mineral
• Ease with which they do this:
– Solid solution: ions which substitute easily form a series of minerals with varying compositions (olivine series how easily Mg (forsterite) and Fe (fayalite) swap…)
– Impurity defect: ions of lower quantity or that have a harder time swapping get into the structure
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
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
• 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
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:
• Ca0.004Mg1.859Fe0.158Mn0.003Al0.006Zn0.002Cu0.001Pb
0.00001Si0.0985Se0.002O4
• One of the ions is a determined integer, the
other numbers are all reported relative to that
one.
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
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)
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
• ‘O factor’ from page 204: 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
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 0.03
CaO 56.08 1 1 0.61 0.01088 0.73 Ca2+ 0.73 0.73 0.03
Na2O 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
Compositional diagrams
Fe O
FeO
wustite
Fe3O4
magnetiteFe2O3
hematite
A1B1C1
xA1B2C3
A
CB
x
Fe Mg
Si
fayalite forsterite
enstatite ferrosilite
Pyroxene solid solution MgSiO3 – FeSiO3
Olivine solid solution Mg2SiO4 – Fe2SiO4
Fe Mg
forsteritefayalite