Abstract Kopargaon ordinary chondrite, a monomict fragmented breccia, was recorded as a recent meteorite fall reported from Kopargaon taluk of Ahmednagar district, Maharashtra, India. LL5 and S2-3 are assigned as its petrologic type and low to moderate degree of shock stages, respectively. Here, we report the occurrence of chromite partially transformed to its high-pressure polymorph, namely xieite, in the highly pulverized silicate matrix. The presence of the mineral having a straight grain contact with olivine in the brecciated matrix makes it suitable for thermometric measurement using Fe–Mg ion exchange parameters. The calculated peak metamorphic temperature based on the Fe–Mg ion exchange between olivine and chromite ranges from 720° to 750°C with an average of 732°C. A detailed study of mineral chemistry has been done along with thermometric measurements. Also, a suitable analytical method, such as Raman spectroscopy, has been employed for the identification of any high-pressure polymorphic mineral phase. The targeted mineral phase in the brecciated matrix part of the Kopargaon meteorite shows a Raman spectrum with a strong peak at 684 cm−1, a peak at 602 cm−1, and smaller peaks at 493 and 362 cm−1. The Raman spectrum of chromite generally displays distinct peaks at 217, 280, 396, 595, and 680 cm−1. Scarcity of Shock Melt Vein (SMV) and absence of other characteristic high-pressure mineral phases mark this extra-terrestrial piece of rock as an example of a low to moderate intensity shocked meteorite, and these low to moderate intensity shock events were not enough to induce complete transformations from chromite to its high-pressure polymorph called xieite. Research highlights <UnorderedList Mark="Bullet"> <ItemContent> The Kopargaon meteorite is an equilibrated LL-type ordinary chondrite with different types of chondrules embedded in a highly pulverized matrix composed of olivine, pyroxene, plagioclase, troilite grains and Fe–Ni metal. In the matrix, our mineral of interest, i.e., chromite partially transformed to xieite are texturally associated with olivine, clinopyroxene, plagioclase, and troilite. </ItemContent> <ItemContent> The empirical formula of the mineral is calculated as (Fe0.98–1.01 Mg0.1–0.12 Mn0.01–0.02)~ 1.01 (Cr1.50–0.1.56 Al0.20–0.23 Ti0.09–0.10)~ 1.97 O4 based on 4 oxygen atoms (a.p.f.u.). The Fe/ (Fe + Mg) vs. Cr/ (Cr + Al) diagram shows a positive correlation of EPMA data of different chromite grains, which indicates incorporation of Cr would have facilitated an increase in Fe content by replacing Mg in the chromite grains. In this study, the calculated peak metamorphic temperature, based on the Fe–Mg ion exchange between olivine and chromite, ranges from 720° to 750°C with an average of 732°C. </ItemContent> <ItemContent> The mineral shows a Raman spectrum with a strong peak at 684 cm-1, a smaller peak at 602, 493, and 362 cm-1. Here, the peak at 684 cm-1 represents the characteristic peak for its chromite component, whereas the smaller but distinct and independent peak at 602 cm-1 indicates the presence of xieite. In the realm of study, detail comparison is attempted with Raman spectra of untransformed chromite to address any sort of uncertainty. </ItemContent> <ItemContent> The chromite grains of the Kopargaon meteorite were not fully but partially transformed to xieite, which is a high-pressure polymorph of chromite. The low to moderate shock intensity, which is also evident from the scarcity of Shock Melt Vein (SMV), and absence of other shock-induced high-pressure mineral phases, was not sufficient enough to induce complete transformation to its high-pressure polymorph. </ItemContent> </UnorderedList>

Xieite: A high-pressure polymorph of chromite in the Kopargaon ordinary chondrite

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发表日期：

2025年11月29日

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Susmita Mondal,
Monotrisha Dey,
Amit Mandal

摘要

Abstract

Kopargaon ordinary chondrite, a monomict fragmented breccia, was recorded as a recent meteorite fall reported from Kopargaon taluk of Ahmednagar district, Maharashtra, India. LL5 and S2-3 are assigned as its petrologic type and low to moderate degree of shock stages, respectively. Here, we report the occurrence of chromite partially transformed to its high-pressure polymorph, namely xieite, in the highly pulverized silicate matrix. The presence of the mineral having a straight grain contact with olivine in the brecciated matrix makes it suitable for thermometric measurement using Fe–Mg ion exchange parameters. The calculated peak metamorphic temperature based on the Fe–Mg ion exchange between olivine and chromite ranges from 720° to 750°C with an average of 732°C. A detailed study of mineral chemistry has been done along with thermometric measurements. Also, a suitable analytical method, such as Raman spectroscopy, has been employed for the identification of any high-pressure polymorphic mineral phase. The targeted mineral phase in the brecciated matrix part of the Kopargaon meteorite shows a Raman spectrum with a strong peak at 684 cm⁻¹, a peak at 602 cm⁻¹, and smaller peaks at 493 and 362 cm⁻¹. The Raman spectrum of chromite generally displays distinct peaks at 217, 280, 396, 595, and 680 cm⁻¹. Scarcity of Shock Melt Vein (SMV) and absence of other characteristic high-pressure mineral phases mark this extra-terrestrial piece of rock as an example of a low to moderate intensity shocked meteorite, and these low to moderate intensity shock events were not enough to induce complete transformations from chromite to its high-pressure polymorph called xieite.

Research highlights

The Kopargaon meteorite is an equilibrated LL-type ordinary chondrite with different types of chondrules embedded in a highly pulverized matrix composed of olivine, pyroxene, plagioclase, troilite grains and Fe–Ni metal. In the matrix, our mineral of interest, i.e., chromite partially transformed to xieite are texturally associated with olivine, clinopyroxene, plagioclase, and troilite.

The empirical formula of the mineral is calculated as (Fe_0.98–1.01 Mg_0.1–0.12 Mn0_.01–0.02)_{~ 1.01} (Cr_{1.50–0.1.56} Al_0.20–0.23 Ti_0.09–0.10)_{~ 1.97} O₄ based on 4 oxygen atoms (a.p.f.u.). The Fe/ (Fe + Mg) vs. Cr/ (Cr + Al) diagram shows a positive correlation of EPMA data of different chromite grains, which indicates incorporation of Cr would have facilitated an increase in Fe content by replacing Mg in the chromite grains. In this study, the calculated peak metamorphic temperature, based on the Fe–Mg ion exchange between olivine and chromite, ranges from 720° to 750°C with an average of 732°C.

The mineral shows a Raman spectrum with a strong peak at 684 cm^-1, a smaller peak at 602, 493, and 362 cm^-1. Here, the peak at 684 cm^-1 represents the characteristic peak for its chromite component, whereas the smaller but distinct and independent peak at 602 cm^-1 indicates the presence of xieite. In the realm of study, detail comparison is attempted with Raman spectra of untransformed chromite to address any sort of uncertainty.

The chromite grains of the Kopargaon meteorite were not fully but partially transformed to xieite, which is a high-pressure polymorph of chromite. The low to moderate shock intensity, which is also evident from the scarcity of Shock Melt Vein (SMV), and absence of other shock-induced high-pressure mineral phases, was not sufficient enough to induce complete transformation to its high-pressure polymorph.