Problem:

1
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?

2
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

3
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

4
Chemical fingerprints of minerals

• Major, minor, and trace constituents in a mineral
• Stable isotopic signatures
• Radioactive isotope signatures

5
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

6
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

7
Major, minor, and trace constituents in a mineral

• A handsample-size rock or mineral has around
  51024 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

8
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

9

• 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
10
Compositional diagrams
Fe3O4 magnetite
Fe2O3 hematite
FeO wustite
A
Fe
O
A1B1C1
x
A1B2C3
x
C
B
11
Pyroxene solid solution ? MgSiO3 FeSiO3
Olivine solid solution ? Mg2SiO4 Fe2SiO4
12
Minor, trace elements

• Because a lot of different ions get into any
  minerals structure as minor or trace impurities,
  strictly speaking, a formula could look like
• Ca0.004Mg1.859Fe0.158Mn0.003Al0.006Zn0.002Cu0.001P
  b0.00001Si0.0985Se0.002O4
• One of the ions is a determined integer, the
  other numbers are all reported relative to that
  one.

13
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 minerals chemical formula
• Technique of relating all ions to one (often
  Oxygen) is called normalization

14
Normalization

• Be able to convert between element weight ,
  oxide weight , and of atoms
• What do you need to know in order convert these?
• Elements weight ? atomic mass (Si28.09 g/mol
  O15.99 g/mol SiO260.08 g/mol)
• Original analysis
• Convention for relative oxides (SiO2, Al2O3,
  Fe2O3 etc) ? based on charge neutrality of
  complex with oxygen (using dominant redox species)

15
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
  oxides molecular weight
• Fudge factor from Perkins Box 1.5, pg 22 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

16
(No Transcript)