GEOS 3010 University of Maine Silicate Minerals and Phase Diagrams Worksheet

GEOS 3010
In-Class Activity #6
Silicate Minerals
Answer the following in groups and turn in one per group:
1. Two chemical analyses for a silicate mineral are provided.
a. Calculate the oxide proportions (OP) for each sample (in the table).
b. Write a general formula for this mineral group.
c. Determine the silicate mineral group and subclass to which these samples belong.
d. Write a specific chemical formula for each analysis.
2. Using the olivine T-X phase diagram:
a. Determine the coexisting mineral and liquid compositions at 1350°C.
b. A liquid of composition 80% Fo begins to crystallize. Using the diagram, determine (1) the
temperature at which the first mineral forms and (2) the composition of the mineral that
coexists with this liquid.
c. A mineral of composition 35% Fa heats up and begins to melt. Using the diagram,
determine (1) the temperature at which melting begins, and (2) the composition of the
liquid that coexists with this mineral.
GEOS 3010
In-Class Activity #6
Names:
Silicate Minerals
1. Mineral Analyses. Below are chemical analyses of two mineral samples of the same group:
#1
#2
(oxide wt%)
(oxide wt%)
58.49
51.06
SiO2
FeO
5.34
34.55
MnO
0.17
0.85
MgO
35.35
11.74
CaO
0.63
1.78
SUM
99.98
99.94
a. Which elements in this mineral are likely to share solid solution? Why?
b. In the first column of the table below, calculate the atomic weight of each oxide.
Example: SiO2 = 28.08 gm/mole Si + 2(16.0) gm/mole O = 60.08 gm/mole SiO2
In the next two columns, calculate the proportions of each oxide (OP) in this mineral by dividing
the oxide wt% (in the table above) by the atomic weight of each oxide.
Example: Sample #1 SiO2 = 58.49 gm / 60.08 g/mol = .973 mol
Finally, sum the proportions of all of the oxides except silica.
Oxide atomic weight
(gm/mole)
SiO2
FeO
MnO
MgO
CaO
Total metal oxides
(add all but SiO2)
#1
OP (moles)
#2
OP (moles)
c. What is the ratio of total metal oxides to SiO2 for each sample in this mineral group?
d. Write a general formula for this mineral group. [HINT: group the solid solution elements
together, add Si, then count up the oxygen and move these to the end of the formula.]
e. Look carefully at the elements making up this mineral, the ratio of metal oxides to SiO2, and the
ratio of Si to O. What silicate subclass and mineral group do these samples belong to?
f. We are almost ready to write a specific (exact) formula for each sample of this mineral. But since
the cation (metal) proportions do not exactly add up to 100%, we have to do a little math-magic
to normalize the proportions in order to write the formula. Let’s also ignore Mn, since it is