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

The atomic arrangements of two natural samples of whitlockite. a synthetic whitlockite specimen, a synthetic whitlockite specimen heated at 500 degrees C, and a synthetic merrillite specimen (forrned through dehydrogenation of synthetic whitlockite by heating at 1050 degrees 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 Ca18Mg2(PO4)(12)[PO3(OH)](2) and merrillite, ideally C18Na2Mg2(PO4)(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)(PO4)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(PO4)(2)](32+) and in whitlockite, the terrestrial phase in which hydrogen is accommodated, the interstitial unit has the formula [Ca-18(PO3[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.