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Metamorphic rocks
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
This is a fine grained worm-like (vermiform) intergrowth of two or more minerals resulting from a reaction between unstable minerals. When the intergrowth is of quartz in plagioclase the term myrmekite is used.
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 Abroi Granodiorite (W0514; Figure 2a) is a K-feldspar–quartz–plagioclase–biotite–opaque ± garnet–muscovite granodiorite with accessory zircon, monazite and apatite. This sample, collected some 5 km west of the Wongwibinda Fault, is the least deformed sample of Abroi Granodiorite examined, but still exhibits evidence of minor deformation (Figure 3a). A foliation is lacking, as biotite grains are randomly oriented; some grains exhibit undulose extinction and warping of the basal cleavage. Biotite grains are subhedral, but have ragged grain boundaries. The biotite grains also have pleochroic haloes within the grain that evidence the presence of radioactive inclusions. Quartz exhibits undulose extinction, recrystallisation, sutured and incipient sutured grain boundaries, at various stages of development. K-feldspar grains are weakly porphyritic. Euhedral to subhedral microcline is finer-grained and exhibits well-developed tartan twinning. Myrmekite (quartz and sodic plagioclase) partially replaces some K-feldspar grains. Oscillatory concentric zoning is observed in some plagioclase feldspar grains. Plagioclase exhibits bent twin boundaries.
Late Paleoproterozoic deposition and Mesoproterozoic metamorphism of detrital material in the southernmost Baltic Sea region (Gdańsk IG1 borehole): monazite versus zircon and chemical versus isotopic age record
Published in GFF, 2023
Dominik Gurba, Anna Grabarczyk-Gurba, Ewa Krzemińska
The composition of alkali feldspar ranges from Or84 to Or91 with Ab content up to 14.0 mol% (Appendix 1A). The concentration of Ba is low (0.33–0.43 wt%). Alkali feldspar contains microperthitic exsolutions and rarely produces myrmekite between plagioclase. Sericitized plagioclase has An content between 31 and 35 mol% and is solely of andesine composition. Or content in andesine does not exceed 3.5 mol% (Appendix 1B). Albite (An = 0.5–1.0 mol%, Ab = 98.4–99.4 mol%) is present only in association with chlorite in partly altered alkali feldspar (Fig. 3C).