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 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 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 Compositional diagrams FeO wustite Fe3O4 magnetite Fe2O3 hematite A Fe O A1B1C1 A1B2C3 x x B C Si fayalite forsterite enstatite Fe Mg fayalite Fe ferrosilite forsterite Mg Pyroxene solid solution MgSiO3 FeSiO3 Olivine solid solution Mg2SiO4 Fe2SiO4 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 Actinolite series minerals K Na 0 1 Ca Na Fe Mg 2 Mg Fe Al 5 Si Al 8O22 OH 2 Hornblende 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 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 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 Feldspar analysis Ca Na K 1 Fe Al Si 4 O8 Oxide wt in the of moles mineral of oxide in mole of 2 cations in of O determined the oxides in oxide in oxide by analysis mineral the mineral 2 oxide Atomic weight of oxide g mol SiO2 60 08 1 2 65 90 1 09687 73 83 Si Al2 O3 Fe 2O3 CaO Na2O K2O 101 96 159 68 56 08 61 96 94 20 2 2 1 2 2 3 3 1 1 1 19 45 1 03 0 61 7 12 6 20 0 19076 0 00645 0 01088 0 11491 0 06582 12 84 0 43 0 73 7 73 4 43 Al 1 48569 100 SUM Cation moles of moles of O Number of cations contributed moles of in by each ion in the sample cation mineral 4 73 83 147 66 2 95 3 25 68 0 87 0 73 15 47 8 86 38 52 1 30 0 73 7 73 4 43 1 03 0 03 0 03 0 62 0 35 125 44 200 38 Fe3 Ca2 Na K of moles Oxygen choosen 8 Ca0 73 Na15 47 K8 86 Fe 0 87Al25 68 Si73 83O200 38 Ca0 03 Na0 62K0 35 Fe 0 03 Al1 03Si2 95 O8 to get here from formula above adjust by 8 200 38
View Full Document
Unlocking...