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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 &lt; 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+ , yielding 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.

The crystal chemistry of whitlockite and merrillite and the dehydrogenation of whitlockite to merrillite