Year: 2025
Type: article
Abstract: In this study, natural apatite containing fine-grained magnetite inclusions with well-developed shape-preferred orientation is investigated. These inclusions are aligned in trails parallel to the hexagonal prism facets and they are elongated along the c -axis of apatite. Electron probe microanalysis (EPMA) reveals a chemically homogeneous apatite, while energy-dispersive X-ray spectroscopy (EDS) and Raman spectroscopy identify these inclusions as magnetite. Carbonate-bearing inclusions, including calcite and gas-liquid inclusions with CO 2 , N 2 , and dissolved carbonate ions, are also observed. Calcite inclusions probably indicate early carbonatitic magma during apatite growth, whereas gas-liquid inclusions suggest late hydrothermal alteration. Optical microscopy reveals three shape-preferred orientation (SPO) groups of lamellar magnetite inclusions, each elongated parallel to <0001> Ap and to one of the three <11 2 bar $\bar 2$ 2 ¯ 0> Ap directions. Electron back-scatter diffraction (EBSD) analysis defines a consistent axial relationship of <0001> Ap //<110> Mag across all inclusions. Additionally, inclusions following the three SPO directions exhibit distinct but symmetrically related planar relationships, reflecting two specific crystallographic orientation relationships (CORs). COR-1: one of the {11 2 bar $\bar 2$ 2 ¯ 0} Ap planes parallel to one of the {411} Mag planes, and one of the {10 1 bar $\bar 1$ 1 ¯ 0} Ap planes parallel to one of the {211} Mag planes. This COR is expressed in direction 1 and 3 as two equivalent variants. COR-2: one of the {11 2 bar $\bar 2$ 2 ¯ 0} Ap planes parallel to one of the {211} Mag planes, and one of the {10 1 bar $\bar 1$ 1 ¯ 0} Ap planes parallel to one of the {411} Mag planes. These orientation patterns suggest variant selection during 2D interface-controlled nucleation, probably governed by the symmetry and surface geometry of the apatite host. The crystallographic relationships and the surface features of apatite suggest a secondary origin for magnetite, in which Fe was introduced through etch pits on the (0001) facet and nucleated along dislocations in crystal channels or pits. The hexagonal morphology of magnetite inclusions reflects the crystallographic control by apatite symmetry. We suggest that a fluid-induced dissolution-recrystallization process produces the hex-like morphologies of magnetite inclusions and facilitates their incorporation into the host. The necking-down of fluid inclusions further support this process.
Source: Mineralogical Magazine
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W4414448105
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