Magnesiochangesite‐(Ce), (Ca 8 Ce)□Mg(PO 4 ) 7 , a New Lunar Mineral From the Pakepake 005 Meteorite

ABSTRACT Magnesiochangesite‐(Ce), ideally (Ca 8 Ce)□Mg(PO 4 ) 7 , is a new lunar mineral of the merrillite subgroup from the fragmental breccia meteorite Pakepake 005. It forms colourless, transparent, subhedral to euhedral crystals, 5–25 µm across, commonly along anorthite margins and associated with fluorapatite, forsterite, pigeonite, augite, and ilmenite. Chemical and structural analyses establish Mg dominance at the M site, Ce dominance among the rare‐earth elements and vacancy dominance at the X site, identifying it as the Mg analogue of changesite‐(Ce). It is trigonal, space group R 3 c , with a = 10.3813(4), c = 37.2778(10) Å, V = 3479.2(3) Å 3 , and Z = 6. The crystal structure was determined by three‐dimensional electron diffraction, with kinematic refinement converging to R 1 = 0.1596 for 5073 unique reflections with I > 2σ( I ). The Raman spectrum contains a dominant PO 4 symmetric‐stretching band at 972 cm −1 , with a weak shoulder at 958 cm −1 . Under identical CL imaging conditions, magnesiochangesite‐(Ce) is markedly brighter than Fe‐dominant changesite‐(Y). Its associations are consistent with crystallization from P–REE‐enriched residual melt. Mg–Fe buffering by magnesian silicates and REE partitioning among coexisting phosphates may have contributed to Mg dominance and the observed Ce/Y, respectively. Its luminescence provides a natural reference for related materials.