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Metamorphic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Protoliths vary greatly in composition, and some compositions develop foliations more than others do because of their mineral constituents. If clays or micas, for example, are in a rock, they can align to produce a foliated fabric. But if only quartz or feldspar are present, such as in granite, the mineral grains are too equant and alignment will not occur. More than any other kind of rock, clay-rich rocks, generally called pelites by petrologists, tend to develop foliations of several kinds. During metamorphism, a shale protolith, for example, may sequentially evolve to become rocks called slate, phyllite, schist, or gneiss while being metamorphosed at increasing temperature and pressure.
Geochemical characteristics and structural setting of lithium–caesium–tantalum pegmatites of the Dorchap Dyke Swarm, northeast Victoria, Australia
Published in Australian Journal of Earth Sciences, 2023
B. R. Hines, D. Turnbull, L. Ashworth, S. McKnight
The bulk mineralogy of the recrystallised Omeo Metamorphic Complex assemblage is dominated by quartz, biotite and muscovite, with minor feldspars, sillimanite and trace garnet. Poikoblastic cordierite is common across the Dorchap Range and surrounding region (Figure 4d), and thin-sections give indication of retrogressed cordierite. This prograde assemblage is aluminous and may be indicative of a clastic sedimentary protolith of pelite–psammopelite metamorphosed to sillimanite grade (e.g. possible upper amphibolite facies). Petrographic study undertaken by Ashley (2020) shows total recrystallisation to a foliated and thinly banded assemblage dominated by fine- to medium-grained biotite-rich bands intercalated (on a scale of <1 mm) with those richer in quartz and feldspar (K-feldspar and plagioclase) and muscovite (Ashley, 2020). Also intercalated are thin bands (<0.2 mm wide) of fine-grained fibrolitic sillimanite (Ashley, 2020). There are a few small occurrences of garnet, perhaps representing largely retrogressed remnants of formerly larger porphyroblasts and traces of tourmaline and zircon. Strong foliation is defined by preferred orientation of micas. Pervasive moderate retrograde metamorphism has affected the prograde assemblage. Feldspars and sillimanite show variable replacement by sericite, and cordierite is replaced by chlorite and/or sericite (Ashley, 2020). Similarly, most garnet is replaced by these phases, and biotite locally has been replaced by chlorite, sericite and trace rutile.
Lithostratigraphy of Paleozoic metasediments in southern Fiordland, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2023
Richard Jongens, Ian M. Turnbull, Andrew H. Allibone
Metasediments where pelite is a distinctive lithology, commonly associated with psammite, are mapped as a pelitic lithological association (after Powell 2006). The association is limited in extent, occurring as north trending bands no greater than 1 km wide in the central part of the Merrie Range, in the Lake Roe-upper Hauroko Burn, and in the western Townley Range (Figure 2). The pelitic lithological association consists of pelitic to semi-pelitic schist, commonly migmatitic, characteristically interlayered with up to equal proportions of quartzose psammitic schist. Minor interlayering of calc-silicate rocks is observed but amphibolitic rocks are unknown (Powell 2006). Pelites typically consist of quartz, plagioclase, sillimanite, biotite, ilmenite, graphite ± cordierite ± K-feldspar (Powell 2006). Contacts are either gradationally concordant with Russet Formation to the east, or intrusive against mid-Paleozoic plutons.
Study of the volumetric behavior of a residual tropical clay
Published in European Journal of Environmental and Civil Engineering, 2022
Bilal Yoka, Mathilde Morvan, Pierre Breul
This study concerns two residual tropical soils (ARG1 and ARG2) belonging to the same geological formation. These soils are known as pelitic soils. This type of soil comes from a clayey sedimentary rock with fine grains, called ’pelite’. Pelite comprises siliclastic elements consisting mainly of silts (particles size between 4 and 62 μm) and clays (particles size smaller than 4 μm ) (Haldar & Tišljar, 2014). The studied materials are residual tropical clay of weathering grade VI, according to the classification of residual soils given by Little (1969). The studied materials have a fairly high percentages of clays (greater than 50%).