Question

Using the Electron Microprobe: Mineral Stoichiometry For the following mineral chemical analyses, determine the mineral formula. Express cation formulae to three decimal places for a specific (integer) number of anions (oxygens). Example: (Mg_0.891Fe_0.109)Si_1. 000 Forsterite Olivine - 4 oxygens Oxide Wt% Mol. Weight Mol. Prop Cation Prop Oxygen Prop Cations/0 Si02 42.7 60.086 MgO 57.3 40.312 Mole ratios Mg: Si: O Albite Feldspar - 8 oxygens Oxide Wt% Mol. Weight Mol. Prop Cation Prop Oxygen Prop Cations/O Si02 68.74 60.086 AI203 19.44 101.963 Na20 11.81 61.9796 Mole ratios Na:Al:Si:0 pyroxene-6 oxygens Oxide wt% Mol. Weight Mol. Prop Cation Prop Oxygen Prop Cations/O Si02 52.65 60.086 FeO 11.65 71.8464 MgO 11.13 40.312 CaO 24.57 55.96 Mole ratios Na:AI:Si:0

Answer 1

Below are some rules which we will be using for calculating the mineral formula:

1. Molar properties of oxide is equal to the wt % divided by the mol. wt. of oxide

2. Cation property of oxide is equal to the number of cations in the oxide multiplied by its molar property.

3. Oxygen property of oxide is equal to the number of oxygens in the oxide multiplied by its molar property.

4. Number of cations per oxygen (Cation/O) is calculated by multiplying the cation property by a correction factor x.

Correction factor x is calculated by dividing the total number of oxygens in the mineral by the sum of molar properties of oxygen in all the oxides.

Applying these rules to the given questions:

a.

b.

c.