Mineralogical characteristics and their implications of the Cr-Zr-Ca armalcolite from the Chang'e-5 lunar soil sample

Introduction:  The lunar soil collected by the Chang'e-5 mission is mainly 2-billion-year-old mare basalt from the local area, and also contains a small amount of components from various other sources, such as the ejecting material from impact craters excavated beneath the landing site or near the lunar highlands (Qian et al., 2021a; Li et al., 2022). These ejecta provide valuable information on the composition of the lunar crust other than the Chang'e-5 mare basalt, providing important materials for understanding the complex evolutionary history of the lunar crust.

Samples and Methods: The clasts analyzed in this study are from the samples of CE-5 mission allocated by the China National Space Administration (CNSA). The petrography was carried out on a Zeiss Supra 55 field-emission scanning electron microscope (SEM) equipped with the energy dispersive spectroscopy (EDS) detector. The major element compositions were analyzed with a JEOL JXA8230 electron probe (EPMA). Structural characterizations of minerals were carried out using an EBSD detector installed on the Zeiss Supra 55 SEM.

Results: The clast bearing Cr-Zr-Ca armalcolite is composed of plagioclase, pyroxene, and minor minerals such as chromite, ilmenite, rutile, apatite, merrillite, iron sulfides, armalcolite, baddeleyite, silica, troilite and FeNi metal. The coarse-grained orthopyroxene and plagioclase coexist without showing any reaction structure. Other metal oxides and sulfides are distributed in the interstices.

The plagioclase has very calcic compositions with An content varying between 87.5 and 91.5. Pyroxene is mainly low-calcium pyroxene, with an average mineral composition of Wo_3.5En_68.8Fs_27.7, and Mg# varies between 70.2 and 72.3. High-calcium pyroxene is present in very small amounts, with an average mineral composition of Wo_45.1En_43.7Fs_11.1, and Mg^# varies between 79.5 and 80.4, which is significantly higher than that of low-calcium pyroxene.

The “Cr-Zr-Ca armalcolite” grains contain 65.3~66.1 wt% TiO₂, 1.6~1.7 wt% MgO, 10.5~10.6 wt% FeO, 9.8~9.9 wt% Cr₂O₃, 4.7~5.0 wt% ZrO₂, and 2.8~3.1 wt% CaO, respectively. It also contains total REE up to 1.72 wt% (0.42~0.69 wt% La, 0.29~0.69 wt% Ce, 0.12~0.22 wt% Y, 0.06~0.22 wt% Dy, 0.06~0.22 wt% Er). The EBSD patterns of Cr-Za Ca armalcolite can only be indexed with the loveringite R3 structure with the mean angular deviations as low as 0.49.

Discuss: Based on 38 oxygen atoms, the chemical formula of Cr-Zr-Ca armalcolite is (Ca_0.90Na_0.03La_0.06Ce_0.06Dy_0.01Y_0.02)_Σ1.08(Ti_14.12Fe_2.51Cr_2.22Mg_0.71Zr_0.67Al_0.40Mn_0.04Si_0.02)_Σ20.69O₃₈, which may be abbreviated as AM₂₁O₃₈. The EPMA and EBSD results in this study is consistent with loveringite, which is different from the orthorhombic Bbmm structure of Fe-Mg armalcolite (aggerty, 1973a; Zhang et al., 2020). It is identified as an important carrier of rare earth elements or should have derived from a REE-rich rock.

The types of armalcolite are closely related to the types of rocks and can indicate the genesis and source of rocks (Haggerty, 1973a). Cr-Zr-Ca armalcolite is usually associated with baddeleyite, rutile, ilmenite, chromite and other oxides, showing a relatively complex oxide association. However, the petrographic structure of the rock containing these minerals has not been clearly described and is generally considered to be related to non-mare high-alumina basalts (Haggerty, 1973a). The Cr-Zr-Ca armalcolite from the brecciated lunar meteorite Northwest Africa 8182 is also regarded as loveringite. The mafic silicate minerals from meteorite NWA 8182 (Zhang et al., 2020) and Allan Hills (ALH) A81005 (Treiman and Gross, 2015) have relatively high Mg^# and are considered to be a new type of Mg-suite distinct from the Apollo samples. In our study, mineral compositions will fall between those of the fields defined by the Apollo Mg-suite in the classic diagram, where the An value of plagioclase is plotted against the Mg# of pyroxene. Combined with mineralogy and petrography, it clearly indicates that the clast is consistent with the norite in the Apollo samples. From the above, the appearance of loveringite are closely related to the types of rocks and can be used as an important indicator of Mg-suite rock with having a KREEP signature. The identification of this clast indicates that Chang'e-5 lunar soil samples contain Mg-suite rocks from the interior of PKT.

 

References: [1] Haggerty, S. E. (1973a) LPSC, 4,777. [2] Li, C.L., et al., (2022) NSR, 9. [3] Qian, Y.Q., et al., (2021) EPSL, 555. [4] Treiman, A. H. and Gross, J. (2015) AM, 100, 414-426. [5] Zhang, A. C., et al., (2022) AM, 105, 1021-1029.