The atomic arrangement of merrillite from the Fra Mauro Formation, Apollo 14 lunar mission: The first structure of merrillite from the Moon

Abstract

The atomic arrangement of lunar merrillite has been refined to R = 0.0452 in R3c 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 merrillite has a structural unit of [(Mg,Fe)(PO4)6]216- 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]216- structural units are linked by a [(Ca,REE)18Na2(PO4)2]32+ interstitial complex, formed of Ca1O8, Ca2O8, Ca3O8, NaO6, and P1O4 polyhedra.