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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
While muscovite is the most common white mica in metamorphic rocks, other white micas, which are optically almost identical, can be present, such as phengite and pyrophyllite. Hence using the general term ‘white mica’ can be preferable in some cases.
A proposal for the definition, nomenclature, and classification of soapstones
Published in GFF, 2018
Talc in blueschist facies rocks was probably first recognized by Smith (1906) who interpreted the origin of glaucophane–schists or gneisses as basic igneous intrusive rocks or tuffs. Abraham and Schreyer (1976) described a muscovite–phengite–talc mineral assemblage which is characteristic for low-temperature and high-pressure metamorphism in subduction zones (Chopin 1981, 1984; Schreyer 1985). Masonne & Schreyer (1989) confirmed that the paragenesis talc + phengite is stable only under high-pressure conditions. Talc has also been documented in ultrahigh-pressure conditions (eclogite facies) from supracrustal coesite-bearing (i.e., non-mafic) rocks (Liou & Zhang 1995).
Alteration and mineral zonation at the Mt Lyell copper–gold deposit, Tasmania
Published in Australian Journal of Earth Sciences, 2018
The Prince Lyell orebody is a large semi-vertical pipe that records an evolving hydrothermal fluid with alteration developed broadly symmetrically around the ore zones. At depth, the hydrothermal fluid deposited chalcopyrite, magnetite, pyrite mineralisation with associated phengite, chlorite ± biotite alteration, through to shallower levels where chalcopyrite and pyrite were deposited with associated muscovite ± chlorite alteration. A systematic variation in the white mica occurs both laterally and vertically from a core of phengite (Al-poor) to muscovite (Al-rich). This suggests that at depth, the hydrothermal fluid was of a neutral pH, reduced and hot, and as the fluid moved laterally and vertically towards the surface, it became more acidic, oxidised and cooler. The alteration at the Western Tharsis (Huston & Kamprad, 2001) and Glen Lyell deposits reflects increasingly shallow parts of the hydrothermal system with quartz–pyrophyllite ± topaz, ± zunyite ± woodhouseite assemblage with local bornite at the Western Tharsis deposit. The Glen Lyell alteration characterised by quartz–pyrophyllite–alunite represents the shallowest part of the hydrothermal system. The zonation for the type 1 chalcopyrite–pyrite pipe mineralisation is shown in Table 4. There are two types of phengite within the system: one is generated at depth from hot hydrothermal fluid, and the other represents the distal zone of the alteration system where the hydrothermal fluid has been neutralised by wall–rock interaction (Table 4). These results indicate that three main alteration types within the pipe-like orebodies are coeval, and they record the evolution of the hydrothermal fluid from deeper to shallower parts within the ore system.
Diverse provenance of the Lower Cretaceous sediments of the Eromanga Basin, South Australia: constraints on basin evolution
Published in Australian Journal of Earth Sciences, 2021
E. Baudet, C. Tiddy, D. Giles, S. Hill, G. Gordon
The Coorikiana Sandstone has a similar mineralogy to the top of the Bulldog Shale in Alkoomi 1, Mulapula 1 and Toodla 1. In Miandana 1, the Coorikiana Sandstone is composed of montmorillonite, kaolinite, quartz and siderite. In CBH2, it is composed of montmorillonite, muscovite, phengite, quartz and feldspars. Kaolinite, quartz, montmorillonite, K-feldspars, albite and siderite are the main phases in Poolowanna 1. In Oodnadatta 1, the main phases are montmorillonite, quartz, albite and labradorite.