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Optical mineralogy
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Some intragranular textures are produced as a result of solid state deformation of the mineral. Under crustal conditions, it is common to observe undulose extinction in quartz grains, due to strain in the mineral lattice, when the angle of extinction changes across a single crystal as the stage is rotated (e.g. 56, 137). Subgrains produce patchy extinction with distinctly bounded areas within a grain with slightly different orientations, but where the misorientation is not high enough (<10º) to form distinct grain boundaries (27). With increasing defomation this process can lead to increase in misorientation until the boundaries can be regarded as grain boundaries and the mineral has recrystallised to form a finer grained aggregate.
Provenance of the Pakawau Group and Farewell Formation (Late Cretaceous – Paleocene), Taranaki Basin, northwest Nelson, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2020
Sarah L. Smithies, Kari N. Bassett, Greg H. Browne, Alexander R. L. Nichols
The metasedimentary clasts of the North Cape and Farewell formations are highly quartzose, plotting as quartzarenites and subfeldsarenites (Figure 5) (Folk et al. 1970). They contain quartz with undulose extinction, minor alkali feldspar with microcline twinning, minor plagioclase, chert lithics, opaques, detrital muscovite, and rare high birefringence heavy minerals. The majority of the samples (c. 40%) are well-sorted very fine to fine sandstones with sutured quartz grain boundaries (Figure 6(A)). Some (c. 30%) of the metasedimentary clasts are bimodal, with grain-supported very fine to fine sand in a silt matrix (Figure 6(B)). Rare (c. 15%) sub-angular, very well-sorted fine sandstone with red-brown opaques concentrated along the grain boundaries also occur (Figure 6(C and D)). The ‘dirty’ grain boundaries, retention of the original sedimentary textures, and absence of metamorphic minerals suggest a very low-grade metamorphism. Some strain is shown by the very strong undulose extinction of the quartz grains.
High-temperature–low-pressure metamorphism and the production of S-type granites of the Hillgrove Supersuite, southern New England Orogen, NSW, Australia
Published in Australian Journal of Earth Sciences, 2018
The Rockvale Monzogranite (W0809; Figure 2g) is the least deformed of the three plutons. Incipient strain is evident by undulose extinction in quartz grains. Potassium feldspar exhibits rare zoning, incipient tartan twinning and varying degrees of sericitic alteration particularly in the cores of some grains. Perthite is common and myrmekite was not observed in this section (Figure 4b). Biotite exhibits very minor warping, and some corrosion of the grain boundaries is common. The opaque phase is commonly pyrite.
Basaltic dykes and their xenoliths from the Gerroa–Kiama region, southern Sydney Basin, New South Wales: evidence for multiple intrusive episodes
Published in Australian Journal of Earth Sciences, 2022
S. Abu-Shamma, I. T. Graham, P. Lennox, G. Bann, A. Greig
Charnockitic xenoliths are also abundant and comprise quartz, orthopyroxene with exsolution lamellae, plagioclase, intergranular clinopyroxene and biotite with spatially associated opaques. The quartz grains display weak to moderately undulose extinction. All charnockitic xenoliths found have strongly sutured grain boundaries (Figure 7j), and some have intensely fractured grains, indicating significant later brittle deformation. A few charnockitic xenoliths also have reaction rims.