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Licensed Unlicensed Requires Authentication Published by De Gruyter June 30, 2020

The Cr-Zr-Ca armalcolite in lunar rocks is loveringite: Constraints from electron backscatter diffraction measurements

Ai-Cheng Zhang, Run-Lian Pang, Naoya Sakamoto and Hisayoshi Yurimoto
From the journal American Mineralogist


“Cr-Zr-Ca armalcolite” is a mineral originally found in Apollo samples five decades ago. However, no structural information has been obtained for this mineral. In this study, we report a new occurrence of “Cr-Zr-Ca armalcolite” and its associated mineral assemblage in an Mg-suite lithic clast (Clast-20) from the brecciated lunar meteorite Northwest Africa 8182. In this lithic clast, plagioclase (An = 88–91), pyroxene (Mg#[Mg/(Mg+Fe)] = 0.87–0.91) and olivine (Mg# = 0.86–0.87) are the major rock-forming minerals. Armalcolite and “Cr-Zr-Ca armalcolite” are observed with other minor phases including ilmenite, chromite, rutile, fluorapatite, merrillite, monazite, FeNi metal, and Fe-sulfide. Based on 38 oxygen atoms, the chemical formula of “Cr-Zr-Ca armalcolite” is (Ca0.99Na0.01)Σ1.00(Ti14.22Fe2.06Cr2.01 Mg1.20Zr0.54Al0.49Ca0.21Y0.05Mn0.04Ce0.03Si0.03La0.01Nd0.01Dy0.01)Σ20.91O38. Electron backscatter diffraction (EBSD) results reveal that the “Cr-Zr-Ca armalcolite” has a loveringite R3 structure, differing from the armalcolite Bbmm structure. The estimated hexagonal cell parameters a and c of “Cr-Zr-Ca armalcolite” are 10.55 and 20.85 Å, respectively. These structural and compositional features indicate that “Cr-Zr-Ca armalcolite” is loveringite, not belonging to the armalcolite family. Comparison with “Cr-Zr-Ca armalcolite” and loveringite of other occurrences implies that loveringite might be an important carrier of rare earth elements in lunar Mg-suite rocks. The compositional features of plagioclase and mafic silicate minerals in Clast-20 differ from those in other Mg-suite lithic clasts from Apollo samples and lunar meteorites, indicating that Clast-20 represents a new example of diverse lunar Mg-suite lithic clasts.

Acknowledgments and Funding

We thank Takashi Mikouchi, an anonymous reviewer, and associate editor Steven Simon for their helpful comments that greatly improved the quality of this manuscript. This work was financially supported by the B-type strategic Priority Program of the Chinese Academy of Sciences (grant XDB41000000), a pre-research Project on Civil Aerospace Technologies funded by CNSA (grant D020204), Natural Science Foundations of China (grant 41673068) and Jiangsu Province of China (grant BK20170017).

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Received: 2019-08-14
Accepted: 2020-01-17
Published Online: 2020-06-30
Published in Print: 2020-07-28

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