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

Abstract “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.