THE ATOMIC ARRANGEMENT OF LUNAR MERRILLITE

            The atomic arrangement of lunar merrillite, ideally (Mg, Fe²⁺, Mn²⁺)₂ [Ca_18-x (Y,REE)_x]₁₈ (Na_2-x+z) (P_14-zSi_z)₁₄ O₅₆, has been refined to R = 0.0452 using X-ray diffraction data recorded on a CCD detector; previous attempts at structure solution using a point detector were not successful because of the poorly-crystallized nature of the lunar material. The atomic arrangement of lunar merrillite has a structural unit of [(Mg,Fe)(PO₄)₆]₂^16- that forms a “bracelet-and-pinwheel” unit that is common in hexagonal-closest-packed layers. The individual structural units are not polymerized, and exist in layers at z = 1/6, 1/3, 1/2, 2/3, and 5/6. In lunar merrillite, the [(Mg,Fe)(PO₄)₆] ₂^16- structural units are linked by a [(Ca,REE)₁₈Na₂(PO₄)₂]³²⁺ interstitial complex, formed of Ca1O₈, Ca2O₈, Ca3O₈, NaO₆, and P1O₄ polyhedra.

            There has long been speculation regarding the relationship between merrillite and terrestrial whitlockite, and the solution of the Fra Mauro merrillite atomic arrangement allows distinction of the lunar and terrestrial phases. Lunar merrillite and terrestrial whitlockite are largely similar atomic arrangements, but differ due to the presence or absence of hydrogen. In whitlockite, H is an essential element, and used to balance charge. H is incorporated into the whitlockite atomic arrangement by disordering one of the phosphate tetrahedra, and forming a PO₃(OH) group. Lunar merrillite is devoid of hydrogen, and thus no disordered tetrahedral groups exist. Charge balance for substituent Y and REE (for Ca) and Si (for P) in merrillite is maintained by vacancies in a partially-occupied Na site, with charge balance yielding (Y,REE) + Na – Si = 2 in merrillite. The structure solution demonstrates the effectiveness of the CCD detector in unraveling previously-intractable diffraction data, and urges that the lunar material be reexamined using this new technology.