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Lexicon of lithostratigraphic units for the Sudan
Published in J.R. Vail, Lexicon of Geological Terms for the Sudan, 2022
Intruded into granitic gneisses near the Chad Republic border is a large syenite boss mapped in reconnaissance by Lotfi (1963b). The rock is medium- to coarse-grained, porphyritic in part, of microcline, microcline perthite, aegirine, sodic amphibole, accessory quartz and apatite.
Minerals
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
Figure 61 shows a microcline perthite. The vein-like areas are of albite (NaAlSi3O8) and the cross-hatched parts are microcline (KAlSi3O8). This was probably formed as a solid solution and subsequently the two minerals separated to form this perthitic intergrowth.
Mineralogical composition and ceramic properties of some Egyptian alkali feldspars
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
M.A. Mandour, M.Z. Mostafa, M.M. Abdel Karim, A.M. Bishadi, G.M. Attia
Microscopically they are composed mainly of K-feldspar represented by orthoclase perthite and mierocline perthite of the string, patchy and flame types. The enclosed fine albite blebs in the perthitic potassium feldspars are usually oriented and regularly distributed within the host orthoclase. Mierocline perthite occurs as subhedral elongated crystals exhibiting its characteristic cross-hatch twinning. Micrographic quartz is common in these perthitized potassium feldspars (Waldron 1992). The mixed albite-potassium feldspar pegmatites are essentially composed of albitic plagioclase, potassium feldspar in addition to an appreciable amount of quartz, biotite and the accessory iron oxides, allanite, zircon and apatite.
The You Yangs batholith in Southeastern Australia, the sources of its magmas and inferences for local crustal architecture
Published in Australian Journal of Earth Sciences, 2019
The batholith appears to represent a composite intrusive event, involving more than one magma pulse and, within the regional park, is quite uniform in its mineralogical composition (Baker, 1935). The rocks are typically porphyritic, with large (1–7.5 cm long) euhedral phenocrysts of microcline perthite, locally aligned in what are interpreted here as magmatic flow foliations. The modes of the rock samples published by Baker (1935, table 1) show that the You Yangs granitic rocks are mostly K-feldspar-rich monzogranites. The main minerals present are anhedral quartz, sodic plagioclase subhedra and microcline (as euhedral phenocrysts as well as interstitial and commonly poikilitic anhedra), with platy to interstitial brown biotite (Figure 9a), brownish to pale green hornblende (interstitial or as prisms) and accessory clinopyroxene (mainly as relict cores within hornblende crystals and inclusions in feldspars but with a few small isolated euhedra as well), apatite, ilmenite, zircon, titanite and allanite (Supplementary paper A1, Figure A1.13). The quartz shows wavy extinction and subgrain development (Figure 9a). A small corroded prism of twinned clinopyroxene, with a typical overgrowth of hornblende, is illustrated in Figure 9b.
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 Tobermory Monzogranite (W0807) exhibits some evidence of strain, like the Abroi Granodiorite sample W0514. Quartz exhibits undulose extinction, sutured, and incipient sutured grain boundaries and recrystallisation at various stages of development (Figure 4b). Although strain is evident, the foliation is poorly developed, as biotite grains remain randomly oriented. Some biotite grains exhibit undulose extinction and warping of the basal cleavage. Biotite grains are subhedral, but have straight grain boundaries with embayments. Pyrite is a common accessory phase. Pleochroic haloes within biotite grains evidence the presence of radioactive inclusions. K-feldspar grains are subhedral with rare incipient perthite, and myrmekite was not observed. Rarely, K-feldspar has tartan twinning and concentric zonation in some grains. Two enclaves, with smaller grainsize than the host and similar mineralogy, are also observed (Figure 2f).
Triassic deposits of Chukotka, Wrangel Island and Mendeleev Rise, Arctic Sea: sedimentology and geodynamic implications
Published in GFF, 2020
M.I. Tuchkova, S.P. Shokalsky, O.V. Petrov, S.D. Sokolov, S.A. Sergeev, A.V. Moiseev
For each thin section, 100–200 grains were counted and the grain type was classified according to the following scheme: 1. Monocrystalline quartz (Qm); 2. Polycrystalline quartz (Qp); 3. Rock fragments with micro- and crypto-crystalline quartz aggregates were counted as quartzo-lithic (chert-like) grains; 4. Potassium feldspar including perthite, myrmekite, orthoclase, microcline (K); 5. Plagioclases (P); 6. Total feldspar (Ft), including grains of potassium feldspar, or plagioclase feldspar, etc.; 7. Volcanic rock fragments (Lv), including clasts of quartz +feldspar and quartz + feldspar + mica; 8. Fragments of sedimentary rocks as such as siltstone, sandstone and coal (Ls); 10. Metamorphic rocks fragments (Lm), including metasedimentary fragments.