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Silicate Mineral Chemistry and Phase Diagrams Name___________________________
GEOS 3010 Take Home Assignment #4
Consider the following chemical analyses of some greenish minerals.
Oxides Sample #1 Sample #2 Sample #3 Sample#4
SiO240.99 38.11 33.72 31.85
FeO 8.58 31.48 47.91 58.64
MnO 0.2 0.22 0.41 0.85
MgO 50.00 30.50 18.07 8.49
Totals –> 99.77 100.31 100.11 99.83
Each of the bottle green minerals are found as well formed, Orthorhombic crystals in dense
phaneritic (igneous) rocks.
Let us determine the chemical formula for these minerals (all are in the same mineral group!!!).
These data illustrate a significant limitation with analyses of oxygen-containing compounds (compared to
the analyses we worked with for sulphide minerals). There is no convenient method of analyzing for
oxygen (O) so that the analyses are expressed as oxides of each element instead of the weight percent of
the elements themselves.
Instead of converting the analyses to gram atomic weight proportions we convert to gram
molecular weight proportions. This is convenient because the various silicate mineral subclasses (neso,
soro , ino, cyclo, phyllo, and tecto- silicates) have specific ratios of metals to silica (as we will see). The
conversion to gram molecular proportions (gmp) is accomplished by dividing the weight percent oxide by
the gram formula weight of each oxide {i.e. for SiO2: (40.99)/(28 + (2*16)) = 0.68}
Determine the gram molecular proportions (gmp) for each oxide for each sample in the table below
Oxide #1 (gmp) #2 (gmp) #3 (gmp) #4 (gmp)
FeO
MnO
MgO
Total Metals (TM)
SiO2
TM/SiO2
1. What is the ratio of molecular proportions of total metal oxides to silica (SiO2), that is molecular
proportion of total metals/molecular proportion of SiO2 in each sample? (This is a WHOLE
NUMBER ! for these samples, analytical error may result in slight variation from WHOLE
NUMBER RATIOS)
