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How Many Molecules?

How Many Molecules?. Pyrite Cube weighs 778 g – how many molecules is that?? About 4,000,000,000,000,000,000,000,000 Are they ALL Iron and Sulfur?. Stoichiometry. Some minerals contain varying amounts of 2+ elements which substitute for each other

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How Many Molecules?

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  1. How Many Molecules? • Pyrite Cube weighs 778 g – how many molecules is that?? About 4,000,000,000,000,000,000,000,000 Are they ALL Iron and Sulfur?

  2. Stoichiometry • Some minerals contain varying amounts of 2+ elements which substitute for each other • Solid solution – specific elements substitute for each other in the mineral structure, defined in terms of the end members – species which contain 100% of one of the elements

  3. Chemical Formulas • Subscripts represent relative numbers of elements present • (Parentheses) separate complexes or substituted elements that go into a particular place • 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

  4. 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

  5. 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

  6. 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

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

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

  9. 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.

  10. 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

  11. 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)

  12. 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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