Oriented secondary magnetite micro-inclusions in plagioclase from oceanic gabbro

Abstract

Plagioclase hosted sub-micrometer to micrometer sized oriented needle and lath shaped magnetite micro-inclusions with their elongation direction aligned parallel to the plagioclase [001] direction were investigated using correlated optical, scanning electron and scanning transmission electron microscopy. The PL[001] magnetite microinclusions formed from older generations of differently oriented magnetite microinclusions by recrystallisation during hydrothermal alteration. Six orientation variants of PL[001] magnetite micro-inclusions occur, which share the same shape orientation but differ in their crystallographic orientation relationships to the plagioclase host. The magnetite-plagioclase interfaces are facetted. High resolution scanning transmission electron microscopy revealed that interface facets are aligned parallel to low index lattice planes corresponding to oxygen layers of either magnetite or plagioclase. In addition, linkage between prominent crystal structure elements of magnetite and plagioclase across the interfaces and accommodation mechanisms minimizing misfit between the two crystal structures were discerned. Combined evidence suggests that the shape and shape orientation as well as the crystallographic orientation relationships This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

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

“…S1; see also Table 1). The present TEM work reveals an additional 112*bmagnetite needle group, which has not previously been reported in the literature (Ageeva et al, 2020(Ageeva et al, , 2022Bian et al, 2021Bian et al, , 2023a. In fact, the crystal lattice of the 112*b-needle is correlated with that of the 112*a-needle by �60 � (�180 � ) rotation around the NA (// h111i m ), i.e.…”

“…In contrast, the coarser 150*, � 150*, � 312*, 112* or [001]-needles exclusively within the Na-enriched local domains probably have later metasomatic origins. In this regard, the coarse [001]-needles in plagioclase were suggested to be recrystallization products by Bian et al (2023a).…”