Chemical composition and crystal structure of merrillite from the Suizhou Meteorite

Merrillite, ideally Ca (sub 9) NaMg(PO (sub 4) ) (sub 7) , is an important accessory phosphate mineral in many different groups of meteorites, including martian meteorites, and a major carrier of rare earth elements (REE) in lunar rocks. By means of electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy, we present the first structure determination of merrillite with a nearly ideal chemical composition, Ca (sub 9.00) Na (sub 0.98) (Mg (sub 0.95) Fe (sub 0.06) ) (sub Sigma 1.01) (P (sub 1.00) O (sub 4) ) (sub 7) , from the Suizhou meteorite, a shock-metamorphosed L6-chondrite. Suizhou merrillite is trigonal with space group R3c and unit-cell parameters a=10.3444(3), c=37.0182(11) Aa, and V=3430.5(2) Aa3. Its crystal structure, refined to R (sub 1) =0.032, is characterized by a structural unit consisting of a [(Mg,Fe)(PO (sub 4) ) (sub 6) ] (super 16-) complex anion that forms a "bracelet-and-pinwheel" arrangement. Such structural units are linked by interstitial complexes with a formula of [Ca (sub 9) Na(PO (sub 4) )] (super 16+) , which differs from that of [Ca (sub 9) (PO (sub 3) [OH]] (super 16+) , [Ca (sub 9) (PO (sub 3) F)] (super 16+) , [Ca (sub 9) (Ca (sub 0.5) [] (sub 0.5) ) (PO (sub 4) )] (super 16+) , or [(Ca (sub 9-x) REE) (sub x) (Na (sub 1-x) [] (sub x) ) (PO (sub 4) )] (super 16+) in terrestrial whitlockite, terrestrial/extraterrestrial bobdownsite, meteoritic Ca-rich merrillite, or lunar REE-rich merrillite, respectively. The Suizhou merrillite is found to transform to tuite at high pressures, pointing to the likelihood of finding REE-bearing tuite on the Moon as a result of shock events on REE-merrillite.