2. 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?

3. Goldschmidt’s rules of Substitution
The ions of one element can extensively replace those of another in ionic crystals if their radii differ by less than about 15%
Ions whose charges differ by one may substitute readily if electrical neutrality is maintained – if charge differs by more than one, substitution is minimal

4. Goldschmidt’s rules of Substitution
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
Substitution may be limited when the electronegativities of competing ions are different, forming bonds of different ionic character

5. FeS2 What ions would substitute nicely into pyrite?? S- radius=219 pm
Fe2+ radius=70 pm

7. Chemical ‘fingerprints’ of minerals
Major, minor, and trace constituents in a mineral
Stable isotopic signatures
Radioactive isotope signatures
Forensic mineralogy intro – specific chemistry can delineate location of rock, sediment, soil…

8. 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

9. 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

10. Compositional diagrams
Fe3O4
magnetite
FeO
wustite
Fe2O3
hematite A Fe O
A1B2C3
C=50%, B=35%, C=15%
A1B1C1 x
A1B2C3 x B C

11. Fe Mg Si
fayalite
forsterite
enstatite
ferrosilite Fe Mg
forsterite
fayalite
Pyroxene solid solution  MgSiO3 – FeSiO3
Olivine solid solution  Mg2SiO4 – Fe2SiO4

12. 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
Fractionation discussion – go through mathematics of Rayleogh fractionation

13. 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