Lines work for depicting solid solutions with two end members but not for solutions, like feldspars, that have three. The numbers 0.029, 0.617, and 0.354 are called mole fractions they are equivalent to percent divided by 100.Ībove we plotted olivine compositions on a line – on a binary diagram. If we ignore the small amount of Fe that is present, the Grurud feldspar composition can be described as: Any discrepancy is due to analytical imprecision. So, ideal feldspar formulas have stoichiometry described by (Ca,Na,K)(Al,Si) 4O 8 the Grorud feldspar comes very close to this. The parentheses remind us which elements substitute for each other.Īs pointed out earlier in this chapter, most feldspars are primarily solutions of three end members: anorthite (CaAl 2Si 2O 8), albite (NaAlSi 3O 8), and orthoclase (KAlSi 3O 8). The values in column J give us the normalized formula for the feldspar (Ca 0.029Na 0.617K 0.354)(Fe 0.035Al 1.025Si 2.948)O 8. So, in the last column (J) we have multiplied all the atom numbers in columns H and I by a constant factor so that the number of atoms of oxygen is exactly 8. Using the numbers in column H and the oxygen total of 2.977, we could write the formula of the feldspar as Ca 0.011Na 0.230K 0.132Fe 0.013Al 0.382Si 1.097O 2.977, but normal feldspar analyses are written with 8 atoms of oxygen. Columns H and I give the results of these calculations. To calculate moles of oxygen, we multiply the values in column G by the number of O in each oxide (column F). To calculate relative numbers of cations, we multiply the values in column G (moles of oxides) by the number of cations in each oxide (column E). The values in column G are relative values only they total to 1.486, which has no scientific meaning. To convert from % grams of oxides to moles of oxides, we divide the oxide weight % values (column B) by oxide atomic weight (column C) column G shows the results. Columns C, D, E, and F give chemical data for the oxides. Column A lists the oxides and column B gives the results of the analysis. The table below gives the chemical analysis of a feldspar from Grorud, Norway. Fortunately there are Excel spreadsheets and programs to do it for you. Normalization is a tedious but straightforward arithmetical operation when done by hand. Normalization is the process of converting an analysis into a formula. We normally report mineral analyses in values of oxide weight %, but mineral formulas are written in terms of numbers of atoms.

Plot ternary diagram of feldspars