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How many molecules?

Learn about chemical fingerprints and isotopic signatures in minerals, including substitution rules, stable isotopes, and radioactive isotopes. Explore how chemical heterogeneity and stoichiometry affect mineral composition.

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How many molecules?

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  1. How many molecules? • Pyrite – FeS2 • Would there be any other elements in there???

  2. 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

  3. 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

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

  5. 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?

  6. Chemical ‘fingerprints’ of minerals • Major, minor, and trace constituents in a mineral • Stable isotopic signatures • Radioactive isotope signatures

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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: • Ca0.004Mg1.859Fe0.158Mn0.003Al0.006Zn0.002Cu0.001Pb0.00001Si0.0985Se0.002O4 • One of the ions is a determined integer, the other numbers are all reported relative to that one.

  15. 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

  16. 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)

  17. 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

  18. Compositional diagrams Fe3O4 magnetite Fe2O3 hematite FeO wustite A Fe O A1B1C1 x A1B2C3 x B C

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

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