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 Ca18Mg2(PO4)12[PO3(OH)]2 and merrillite, ideally Ca18Na2Mg2(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]216− 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 [Ca19(PO4)2]32+, and in whitlockite, the terrestrial phase in which hydrogen is accommodated, the interstitial unit has the formula [Ca18(PO3[OH])2]32+, yielding the charge-balancing relationship [H(whit) ↔ Ca0.5(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.