Crystal Structure Controls on Oriented Primary Magnetite Micro-Inclusions in Plagioclase From Oceanic Gabbro

Abstract

Oriented needle, lath and plate shaped magnetite micro-inclusions in rock forming plagioclase from mafic intrusive rocks were investigated using correlated optical microscopy and scanning transmission electron microscopy. The magnetite micro-inclusions were analysed on cuts parallel and perpendicular to the inclusion-elongation directions. The crystal structures of the two phases are in direct contact along the interfaces. The shape, shape orientation and crystallographic orientation relationships between the magnetite micro-inclusions and the plagioclase host appear to be controlled by the tendency of the system to optimise lattice match along the interfaces. The elongation direction of the inclusions ensures good match between prominent oxygen layers in the magnetite and plagioclase crystal structures across the interfaces bounding the inclusions parallel to their elongation direction. In cross section, additional modes of lattice match, such as the commensurate impingement of magnetite and plagioclase lattice planes along the interfaces, the parallel alignment of the interfaces to low-index lattice planes of magnetite or plagioclase, or the parallel alignment to low index lattice planes of both phases are observed, which appear to control the selection of interface facets as well as the shape and crystallographic orientation relationships between magnetite micro-inclusions and plagioclase host. The systematics of the inclusion cross sectional shapes and crystallographic orientation relationships indicate recrystallization of magnetite with potential implications for natural remanent magnetization of magnetite bearing plagioclase grains.

A transmission electron microscopy study of the crystallographic characteristics of magnetite needles in plagioclase

“…The present polyhedral model for magnetite nucleation at the highly TO 4 -coordinated open sites within the four-member rings of the plagioclase lattice seems more straightforward than the 'nucleation orientation' scenario of Ageeva et al (2020Ageeva et al ( , 2022 and Bian et al (2021Bian et al ( , 2023b. Such a nucleation scenario assumed the formation of a FeO 6 octahedron sharing two corners with the neighboring TO 4 at the center of the sixmember rings of the plagioclase structure [see Fig.…”

“…2 of Bian et al (2023b) and the stereograms in Figs. 6(e) and 6( f ) of this study, it is obvious that the two magnetites of Bian et al (2023b) correspond to the � 312*c and � 312*a-needles without and with {011} m // (150) p , respectively. Note that the essence in classic solid-state nucleation theory is that the precipitate should possess the 'optimal orientation' with the minimum interfacial energy with respect to the host, in order to reduce the nucleation energy barrier (Porter & Easterling, 1981;Putnis, 1992).…”

“…In addition, this nucleation scenario implies that the magnetite crystals at the 'nucleation orientation' would later be subjected to energetically difficult rotation/ tilting through large angles to reach the 'main orientation' under the stringent three-dimensional lattice constraints of the plagioclase host. In Lastly, a few words regarding the recent TEM study of Bian et al (2023b) are warranted. Bian et al (2023b) presented two pl( � 312)n-mt needles in the 'main orientation' and the 'nucleation orientation'.…”

“…In Lastly, a few words regarding the recent TEM study of Bian et al (2023b) are warranted. Bian et al (2023b) presented two pl( � 312)n-mt needles in the 'main orientation' and the 'nucleation orientation'. By comparing the stereograms in Fig.…”

“…By comparing the stereograms in Fig. 2 of Bian et al (2023b) and the stereograms in Figs. 6(e) and 6( f ) of this study, it is obvious that the two magnetites of Bian et al (2023b) correspond to the � 312*c and � 312*a-needles without and with {011} m // (150) p , respectively.…”