Publication Date:
2015-11-21
Description:
Merrillite, ideally Ca 9 NaMg(PO 4 ) 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 9.00 Na 0.98 (Mg 0.95 Fe 0.06 ) 1.01 (P 1.00 O 4 ) 7 , from the Suizhou meteorite, a shock-metamorphosed L6-chondrite. Suizhou merrillite is trigonal with space group R 3 c and unit-cell parameters a = 10.3444(3), c = 37.0182(11) Å, and V = 3430.5(2) Å 3 . Its crystal structure, refined to R 1 = 0.032, is characterized by a structural unit consisting of a [(Mg,Fe)(PO 4 ) 6 ] 16– complex anion that forms a "bracelet-and-pinwheel" arrangement. Such structural units are linked by interstitial complexes with a formula of [Ca 9 Na(PO 4 )] 16+ , which differs from that of [Ca 9 (PO 3 [OH])] 16+ , [Ca 9 (PO 3 F)] 16+ , [Ca 9 (Ca 0.5 0.5 )(PO 4 )] 16+ , or [(Ca 9–x REE) x (Na 1–x x )(PO 4 )] 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.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
Permalink