Geol 2311
Lab 7 – Calculating and Plotting Mineral Compositions
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9/26/06
Additional lecture on mineral calculations
Overview of lab exercise (may work on at home)
Prepare for Non-silicate Mineral Quiz
Calculating Mineral Formulas and Plotting Compositional Components
Total Points: 20
Due Date: Thurs, October 5. (EMAIL YOUR Excel SPREADSHEET!)
In the table below are chemical analyses of plagioclase and olivine from different zones in the
Layered Series of the Duluth Complex at Duluth (DLS). These analyses were acquired by
averaging about 10 spot analyses of each mineral by electron microprobe. The analytical results
are reported in weight percent oxide in Table 1. For these data, do the following:
1. Convert the weight % oxide of each analysis into mole% oxide by constructing a spreadsheet
similar to the “mineral calculation.xls” file (see orthoclase calculations) on the class website.
Record your results in Table 2 below.
Table 1: Analyses of olivine and plagioclase from the Duluth Complex Layered Series at
Duluth (values in weight %)
Mineral
DLS Unit*
Olivine
TZ
Olivine
CZ
Olivine
GZ
Plag.
TZ
Plag.
CZ
SiO2
36.7
34.3
32.8
52.2
55.8
TiO2
Al2O3
30.4
27.1
FeO
33.3
42.9
52.7
0.32
0.42
MnO
0.4
0.55
0.67
MgO
29.4
21.8
13.2
CaO
0.06
0.07
0.07
13.1
10.9
Na2O
3.9
5.1
K2O
0.34
0.61
Total
99.9
99.6
99.5
100.3
99.9
* DLS Units: TZ – troctolite zone, CZ – cyclic zone, GZ – gabbro zone
Plag.
GZ
58.1
26.2
0.21
7.84
6.48
1.10
99.9
Table 2. Analyses of olivine and plagioclase converted to mole% oxide
Mineral
DLS Unit*
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Olivine
TZ
Olivine
CZ
Olivine
GZ
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
Plag.
TZ
Plag.
CZ
Plag.
GZ
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
2. Using your spreadsheet, calculate the anorthite:albite:orthoclase (An:Ab:Or) components for
plagioclase (see Mineral Calculations.xls file for definitions) and the fosterite:fayalite
(Fo:Fa*) components for olivine from the mole % oxide data. Record the data in Table 3
below.
(* Fo = MgO/(MgO+FeO)*100; Fa = FeO/(MgO+FeO)*100; oxides as mole%)
Table 3. Oxide components of olivine and plagioclase
Mineral
DLS Unit*
Fo
Fa
Olivine
TZ
Olivine
CZ
Olivine
GZ
_____
_____
_____
_____
_____
_____
An
Ab
Or
Plag.
TZ
_____
_____
_____
Plag.
CZ
Plag.
GZ
_____
_____
_____
_____
_____
_____
3. Plot (and label) the An:Ab:Or compositions of plagioclase on the ternary diagram below and
the Fo:Fa composition of olivine on the linear graph. Answer the questions below.
Or
Ab
An
Fa
Fo
With the TZ sample taken ¼ up into the DLS, the CZ sample taken ½ up into the DLS, and the GZ
sample taken ¾ up into the DLS, what can you say about the upsection compositional changes of
these two solid solution minerals in the DLS?
_____________________________________________________________________________
_____________________________________________________________________________
In the plagioclase feldspars, does the Or component have limited or complete solid solution ?
________________________
How does the Or component change as the plagioclase becomes more albitic?
____________________________________________________________________________
4. Plot An vs. Fo for the olivine and plagioclase compositions from each zone of the DLS and
answer the questions below.
Do these components of olivine and plagioclase correlate with one another? _______
Is the correlation positive or negative? _________________
Knowing the stratigraphic position of the different zones in the DLS and that the DLS
progressively crystallized from the bottom up, would you expect that Fo and An are the high or
low temperature end members of the respective solid solution series of olivine and plagioclase?
Fo – High or Low
An – High or Low
(circle one)
5. Calculate (in your spreadsheet) the cation proportions of three plagioclase analyses from the
DLS based on 8 oxygens. Record the cation formulas of the plagioclase in the table below
(up to 2 decimal places). Using normal valence states for the 5 main cations, calculate the
total amount of + charge contributed by all the cations. Is each plagioclase close to being
charge balanced?
Balanced?
TZ Plag - Ca ____ Na____ K____ Al ____ Si____O8
______total + vs. 16 total - ______
CZ Plag - Ca ____ Na____ K____ Al ____ Si____O8
______total + vs. 16 total -
______
GZ Plag - Ca ____ Na____ K____ Al ____ Si____O8
______total + vs. 16 total -
______
Plagioclase is known for having coupled substitution. What does that mean and how is it
demonstrated by your calculations above?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
6. Pyroxene is common in all gabbroic rocks as well and shows solid solution between Ca, Fe,
and Mg. However, with cooling, Ca ions are less easily accommodated in the crystal
structure which forces the crystal to exsolve into two types of phases: high-Ca clinopyroxene
and low-Ca orthopyroxene. In Table 4 below are analyses (in weight %) two phases of
pyroxene from a gabbroic rock in the DLS. The clinopyroxene (augite) occupies about 80
vol % of the crystal and would be considered the “host” phase to orthopyroxene exsolution
lamellae, which occupy about 20 vol % of the crystal.
You can calculate the weight % concentration of any element or oxide in the original
(unexsolved) pyroxene crystal by simple mass balance. To find the concentration (X) of an
oxide/element “a” in the total rock (or in this case, exsolved mineral), which is composed of
minerals 1, 2, 3…, mass balance is defined as:
Xa (total) = Xa(Min 1)*Vol%(Min1) + Xa(Min 2)*Vol%(Min2) + Xa(Min 1)*Vol%(Min1) …
A. Set up a mass balance worksheet in your spreadsheet, calculate the original composition of the
unexsolved pyroxene, and record your results in column 3 of Table 4.
Table 43: Analyses of exsolved pyroxene (in weight %) recombined and recalculated to
mole%.
Augite
Host
80%
Hypersth.
Lamellae
20%
Original
Pyroxene
Augite.
Host
Mole %
Hypersth.
Lamellae
Original
Pyx
SiO2
51.1
53.3
______
______
______
______
TiO2
0.71
0.28
______
______
______
______
Al2O3
1.92
0.79
______
______
______
______
FeO
10.8
22.5
______
______
______
______
MnO
0.26
0.44
______
______
______
______
MgO
13.5
21.9
______
______
______
______
CaO
21.2
1.2
______
______
______
______
Na2O
0.26
---
______
______
______
______
K2O
Total
--99.1
--100.4
_______
_______
_______
_______
Oxide Components
En
_____
_____
_____
Fs
_____
_____
_____
Wo
_____
_____
_____
B. Using your mineral formula calculation spreadsheet, convert the weight % oxide into mole %
oxide for all three pyroxene compositions and record this in Table 4.
C. Calculate the En:Fs:Wo (Enstatite:Ferrosilite:Wollastinite) end member oxide components
from the mole% compositions of each of the three pyroxenes. Record these values in Table
4. The definition of each component is:
En = MgO/(MgO+FeO+CaO)*100
Fs = FeO/(MgO+FeO+CaO)*100
Wo = CaO/(MgO+FeO+CaO)*100
D. Plot (and label) the En:Fs:Wo compositions of the three pyroxene compositions on the ternary
diagram below.
Wo
En
Fs
E. Draw a line between the two exsolved end member pyroxene compositions. Does the original
composition lie on or close to this line? ________________
Measure the length of the line (in cm); measure the length of the line from augite to the
original pyroxene, measure the length of the line from hypersthene to the original pyroxene.
Record these below and calculate the proportion of each smaller segment to the overall line.
1) Overall line _____cm
2) Aug-Orig _______cm
% of Aug-Orig (2/1*100) = _____%
3) Hypersth-Orig_________cm
% of Hypersth-Orig (2/1*100) = _____%
If your calculations are correct, you should get results that approximately agree with volume
% of host augite and exsolved hypersthene in the original pyroxene. This graphical test is
called the lever rule.