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Chemical Formulas

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)SiO 4 – Olivine group – mineral composed of 0-100 % of Mg, 100-Mg% Fe. Stoichiometry.

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Chemical Formulas

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  1. 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

  2. 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

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

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

  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. 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

  8. 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)

  9. 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’ 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

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