Anorthosite Thin Sections (cont.)
2.2 Secondary Minerals
Quartz can be considered as a secondary mineral in anorthosite, since it represents less than 5% on the QAPF diagram.
Figure 2-7. A large quartz crystal in an Archean meta-anorthosite from Minnesota.
Both microphotographs are in crossed polarized light (XPL). This crystal has been heavily deformed and for that reason shows undulose extinction at all angles. Hem: hematite. Pl: plagioclase. Qtz: quartz. Magnification: 40x. Field of view: 3 mm.
The thin sections in plane-polarized light (PPL) and cross-polarized light (XPL) reveal an opaque mineral. It is hematite (Fe2O3), as shown in reflected light on p. 1 (click here to go to that page). Hematite crystals have grown in micro-fractures around and across plagioclase crystals and originated in hydrothermal alteration.
Figure 2-8. Hematite in anorthosite.
Hematite in the same meta-anorthosite from Minnesota, which developed in fractures around and inside plagioclase (the large crystals with albite twinning). Hematite is isotropic, so it's opaque (black) in both PPL and XPL. Note the euhedral hematite crystal at bottom center. Left: PPL. Right: XPL. Hem: hematite. Pl: plagioclase. Ser: sericite. Small circles are cement bubbles. Magnification: 40x. Field of view: 3 mm.
Figure 2-9. Sericite in meta-anorthosite (plane-polarized view).
In the same Archean meta-anorthosite, sericite fills cracks around and across plagioclase crystals. Hem: hematite. Pl: plagioclase. Ser: sericite. Small circles are cement bubbles. Magnification: 40x. Field of view: 3 mm.
Like hematite, sericite also fills the microfractures in plagioclase, but it does it in elongated crystals, unlike the rather equant hematite crystals. Sericite is a fine-grained variety of muscovite, with the same composition KAl 2(AlSi3O10)(OH)2. It usually forms by hydrothermal alteration of K-feldspars (e.g., microcline), which provide the necessary potassium (K). It grows in pre-existing microfractures where the fluids can penetrate, or in fractures created by the fluid pressure. Here (figs. 2-9 and 2-10), in a K-devoid anorthosite, it could only have formed with a K-rich fluid. In the same way the presence of hematite proves that the fluid also contained iron (Fe).
Figure 2-10. Sericite in meta-anorthosite.
Same view as figure 2-9 in crossed polarizers. Notice the fine-grained sericite with bird's eye extinction pattern. Field of view: 3 mm.