Thompson AFM Projection

Shale is the most abundant sedimentary rock, so metamorphosed shale is the most common metasedimentary rock. More than 90% of the chemical composition of an average shale is in the 6-component system: SiO2, Al2O3, FeO, MgO, K2O, and H2O. This is too many components to include in a composition diagram. Quartz is always present in metamorphosed shales and water is likely to be present during metamorphism because of the hydrous minerals. Therefore, projection from Qz and from water is a good strategy for reducing the number of components that must be plotted in assemblage diagrams for metamorphosed shales. Projection from Qz and from water is also easy to do, as SiO2 and H2O are used as components in standard chemical analyses. Projection is accomplished by ignoring the SiO2 and H2O values, leaving the 4-component system Al2O3, FeO, MgO, and K2O. However, 4-component assemblage diagrams are still hard to work with because 3-dimensions are needed.

J.B. Thompson (1957)

Figure 19. Al2O3-FeO-MgO-K2O Compsition Diagram. Click on the image to see a larger version with more information.

recognized that metamorphosed shales typically also have muscovite (Ms), along with Qz and water, and that most other minerals present plot on or close to the Al2O3-FeO-MgO plane. You can see this in Figure 19, a 3-D representation of common metamorphic minerals in the Al2O3-FeO-MgO-K2O system. Click on the diagram and then move the slider to see graphically where the minerals plot and how biotite (Bt) and K-feldspar (Kfs) are projected from Ms onto the AFM plane.

To do the projection algebraically, Thompson recast the remaining 4-"old" components (Al2O3, FeO, MgO, and K2O) into 4 "new" components (FeO, MgO, KAl3O5, and Al2O3-3K2O) that included as one of the components the composition of muscovite, which is KAl3O5 (with SiO2 and H2O removed). Projecting from muscovite is accomplished by ignoring the KAl3O5 and plotting the other 3-components. The result is the Thompson AFM projection. Figure 20 is an example of a Thompson AFM equilibrium mineral assemblage diagram.

Figure 20. Sample Thompson AFM Assemblage Diagram - Muscovite Projection. Click on the image to see a larger version with more information.

Figure 21. Sample Thompson AFM Assemblage Diagram - K-feldspar Projection. Click on the image to see a larger version with more information.

At high temperatures and low pressures of metamorphism, muscovite may break down (react) to form K-feldspr and other products. If muscovite is not present in the mineral assemblage, it is not valid to project from muscovite. J.B. Thompson (1957) was aware of this possibility and showed that an alternative projection from K-feldspar insted of muscovite could be used. To project from K-feldspar, Thompson recast the remaining 4-component system Al2O3, FeO, MgO, and K2O into 4 "new" components (FeO, MgO, KAlO2, and Al2O3-K2O) that included as one of the components the composition of K-feldspar, which is KAlO2 -- with SiO2 removed. Projecting from K-feldspar is accomplished by ignoring the KAlO2 and plotting the other 3-components. Figure 21 is an example of a Thompson AFM equilibrium mineral assemblage diagram.

Several of the minerals shown on the Thompson AFM diagrams are solid solution minerals. This means that many of the equilibrium mineral assemblages have only 2 minerals (in addition to Ms, Qz, and Water), which is shown on the AFM diagrams of Figure 20 and Figure 21 with red tie lines connecting the compositions of the 2 minerals. The compositions of a solid solution mineral depends on the composition(s) of the other mineral(s) in its equilibrium mineral assemblage. The presence of solid solution minerals make possible another type of chemical reaction.



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