Crystallographic and shape orientations of magnetite micro-inclusions in plagioclase
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(2020) 175:95
ORIGINAL PAPER
Crystallographic and shape orientations of magnetite micro‑inclusions in plagioclase Olga Ageeva1,2 · Ge Bian1 · Gerlinde Habler1 · Alexey Pertsev2 · Rainer Abart1 Received: 15 June 2020 / Accepted: 5 September 2020 © The Author(s) 2020
Abstract Plagioclase hosted, oriented magnetite micro-inclusions are a frequently observed phenomenon in magmatic and metamorphic rocks. Understanding the orientation relationships between these inclusions and the plagioclase host is highly relevant for interpreting paleomagnetic measurements. The systematics of the shape and crystallographic orientation relationships between needle- and lath-shaped magnetite micro-inclusions and their plagioclase host from oceanic gabbro were investigated using optical microscopy including universal stage measurements, scanning electron microscopy, and crystal orientation analysis by electron backscatter diffraction. The magnetite inclusions show preferred shape orientations following six welldefined directions and with specific crystallographic orientation relationships to the plagioclase host. These relationships are rationalized based on angular and dimensional similarities between the crystal structures of magnetite and plagioclase, which favor the parallel alignment of oxygen layers with similar lattice spacing in both phases. The parallel alignment of oxygen layers in plagioclase and magnetite can be traced back to the oriented nucleation of magnetite, which occurs by the accommodation of F eO6 octahedra in six-membered rings of S iO4 and A lO4 tetrahedra of the plagioclase structure. The orientation systematics of the magnetite micro-inclusions is related to four orientation variants for placing the F eO6 octahedra into the plagioclase structure. Keywords Plagioclase · Magnetite inclusions · Shape orientation relationships · Crystallographic orientation relationships · Crystal structures · Oxygen layers
Introduction Oriented micro-inclusions of Fe–Ti oxide minerals such as magnetite, ulvospinel, hematite, and ilmenite hosted in rockforming silicate minerals are frequently observed in igneous and metamorphic rocks (Poldervaart and Gilkey 1954; Divljan 1960; Neumann and Christie 1962; Armbrustmacher and Banks 1974; Sobolev 1990; Usui et al. 2006; Wenk et al, 2011; Biedermann, et al. 2016; Ageeva et al. 2016, 2017). Communicated by Othmar Müntener. * Olga Ageeva [email protected] 1
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Wien, Austria
Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS), Staromonetnyi 35, Moscow 119017, Russia
2
The Fe–Ti oxide micro-inclusions most likely precipitate within the silicate minerals that originally crystallized with high Fe and Ti contents from evolved melt (Natland et al. 1991) and became supersaturated with respect to Fe–Ti oxide minerals during cooling (Okamura et al. 1976) and/ or under changing oxygen fugacity (Doukhan et al. 1990; Ashworth a
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