Geol 2311 9/26/06 Lab 7 – Calculating and Plotting Mineral Compositions • 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 Plag. GZ SiO2 36.7 34.3 32.8 52.2 55.8 58.1 TiO2 Al2O3 30.4 27.1 26.2 FeO 33.3 42.9 52.7 0.32 0.42 0.21 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 7.84 Na2O 3.9 5.1 6.48 K2O 0.34 0.61 1.10 Total 99.9 99.6 99.5 100.3 99.9 99.9 * DLS Units: TZ – troctolite zone, CZ – cyclic zone, GZ – gabbro zone Table 2. Analyses of olivine and plagioclase converted to mole% oxide Mineral DLS Unit* Olivine TZ Olivine CZ Olivine GZ Plag. TZ Plag. CZ Plag. GZ SiO2 _____ _____ _____ _____ _____ _____ TiO2 Al2O3 _____ _____ _____ FeO _____ _____ _____ _____ _____ _____ MnO _____ _____ _____ MgO _____ _____ _____ CaO _____ _____ _____ _____ _____ _____ Na2O _____ _____ _____ K2O _____ _____ _____ Total _____ _____ _____ _____ _____ _____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* Olivine TZ Olivine CZ Olivine GZ Plag. TZ Plag. CZ Plag. GZ Fo _____ _____ _____ Fa _____ _____ _____ An _____ _____ _____ Ab _____ _____ _____ Or _____ _____ _____ 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