American Mineralogist, Volume 93, pages 1300–1305, 2008 0003-004X/08/0809–1300$05.00/DOI: 10.2138/am.2008.2683 1300 The crystal chemistry of whitlockite and merrillite and the dehydrogenation of whitlockite to merrillite John M. hughes, 1, * Bradley l. Jolliff, 2 and John rakovan 1 1 Department of Geology, Miami University, Oxford, Ohio 45056, U.S.A. 2 Department of Earth and Planetary Sciences and the McDonnell Center for Space Sciences, Washington University, St. Louis, Missouri 63130, U.S.A. introduction Among naturally occurring phosphates of biological im- portance, the complex crystal chemistry of whitlockite and its relationship with merrillite have not been examined in detail. Whitlockite occurs in dental calculi and other abnormal calci- fications in the human body (Gopal and Calvo 1972), and also has been found in terrestrial rocks and ostensibly in meteorites. Gopal and Calvo (1972) examined the structural relationship of whitlockite to synthetic β-Ca 3 (PO 4 ) 2 using X-ray powder diffraction data measured at STP and at elevated temperatures. Their results showed that upon elevating the temperature of whitlockite to 900 °C the transformation from whitlockite to the β-Ca 3 (PO 4 ) 2 atomic arrangement was complete, although the charge-balancing mechanism was not elucidated. Dowty (1977) examined extraterrestrial “whitlockite” from the achondritic An- gra dos Reis meteorite, and found that the phase, although very similar to whitlockite, was devoid of the essential hydrogen found in whitlockite and that it adopted the β-Ca 3 (PO 4 ) 2 atomic arrange- ment. Confusion has existed regarding the relationship between terrestrial and extraterrestrial “whitlockite.” The contradiction of essential hydrogen in whitlockite and the purported existence of the phase in meteoritic and lunar rocks that are known to crystallize in environments in which hydrogen is rare or absent has not been addressed with high-quality structure descriptions of the two phases. Numerous previous studies have examined aspects of the whitlockite and merrillite structures (Belik et al. 2002a, 2002b; Morozov et al. 1997; Bigi et al. 1996; Dickens et al. 1974; Schroeder et al. 1977; Ionov et al. 2006), but this study is believed to be the first to juxtapose high-quality structure refinements of the two phases and examine the dehydrogenation reaction of whitlockite to merrillite. Hughes et al. (2006) reported the atomic arrangement of lunar merrillite (Jolliff et al. 1993), and demonstrated that the phase is similar to meteoritic merrillite and, predictably, devoid of hydrogen. Jolliff et al. (2006) summarized the chemistry of lunar and other extraterrestrial whitlockite in detail. As noted by Hughes et al. (2006), there has been controversy surrounding the distinction between merrillite and whitlockite, and no detailed comparison of the two phases based on modern analyses has been offered. In this work we report the atomic arrangements of two natural samples of whitlockite, of synthetic whitlockite, and of samples of synthetic whitlockite that were heated at 500 and 1050 °C for 24 h (the latter treatment resulting in the dehy- drogenation of whitlockite to form merrillite), and compare the crystal chemistry and crystal structures of the phases. aBstract The atomic arrangements of two natural samples of whitlockite, a synthetic whitlockite specimen, a synthetic whitlockite specimen heated at 500 °C, and a synthetic merrillite specimen (formed through dehydrogenation of synthetic whitlockite by heating at 1050 °C for 24 h) have been determined in space group R3c by X-ray diffraction methods; the high-quality structure refinements yielded R < 0.019. Whitlockite, ideally Ca 18 Mg 2 (PO 4 ) 12 [PO 3 (OH)] 2 and merrillite, ideally Ca 18 Na 2 Mg 2 (PO 4 ) 14 , are similar phases that differ by the lack of hydrogen and the concomitant addition of charge-balancing sodium (or calcium) in merrillite. The atomic arrangements of whitlockite and merrillite contain a structural unit consisting of a [(Mg,Fe)(PO 4 ) 6 ] 2 16– complex anion that forms a “bracelet-and-pinwheel” arrangement. The central octahedral cation and the six coordinating phosphate tetrahedra form a pinwheel, and in whitlockite and merrillite the pinwheels are not polymerized; the structural units are linked by interstitial complexes. In unsubstituted merrillite (assuming no Na or REE substituents for Ca), the interstitial complex has a formula of [Ca 19 (PO 4 ) 2 ] 32+ , and in whitlockite, the terrestrial phase in which hydrogen is accommodated, the interstitial unit has the formula [Ca 18 (PO 3 [OH]) 2 ] 32+ , yield- ing the charge-balancing relationship [H (whit) ↔ Ca 0.5(merr) ] 2 . Whitlockite and merrillite are perhaps the only phases that form a solid solution with terrestrial and extra-terrestrial end-members that differ by structural adjustments that result from the accommodation of hydrogen in the terrestrial phase. The results of the study also suggest that in terrestrial samples of whitlockite, a merrillite component of the solid solution is common, but that extraterrestrial samples of merrillite are devoid of any whitlockite component. Keywords: Whitlockite, merrillite, structures, dehydrogenation * Present address: Office of the Provost and Senior Vice Presi- dent, University of Vermont, 348B Waterman Building, Burling- ton, Vermont 05405, U.S.A. E-mail: jmhughes@uvm.edu