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

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 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.

[1]  JorrN M. Hucnns.  Lyonsite , Cu 3 + Fel + ( VOo ) ?-, a new fumarolic sublimate from Izalco volcano , El Salvador : Descriptive mineralogy and crystal structure , 2007 .

[2]  John M. Hughes,et al.  Crystal chemistry of lunar merrillite and comparison to other meteoritic and planetary suites of whitlockite and merrillite , 2006 .

[3]  John M. Hughes,et al.  The atomic arrangement of merrillite from the Fra Mauro Formation, Apollo 14 lunar mission: The first structure of merrillite from the Moon , 2006 .

[4]  A. Hofmann,et al.  Discovery of whitlockite in mantle xenoliths: Inferences for water- and halogen-poor fluids and trace element residence in the terrestrial upper mantle. , 2006 .

[5]  S. Stefanovich,et al.  Chemical and Structural Properties of a Whitlockite-like Phosphate, Ca9FeD(PO4)7 , 2002 .

[6]  S. Stefanovich,et al.  Structural Changes and Phase Transitions in Whitlockite-Like Phosphates , 2002 .

[7]  M. Gazzano,et al.  Rietveld structure refinement of synthetic magnesium substituted β-tricalcium phosphate , 1996 .

[8]  B. Jolliff,et al.  Partitioning in REE-saturating minerals - Theory, experiment, and modelling of whitlockite, apatite, and evolution of lunar residual magmas , 1993 .

[9]  John M. Hughes,et al.  Lyonsite, Cu 3 (super 2+) Fe 4 (super 3+) (VO 4 ) 6 (super 3-) , a new fumarolic sublimate from Izalco Volcano, El Salvador; descriptive mineralogy and crystal structure , 1987 .

[10]  F. Hawthorne.  Graphical enumeration of polyhedral clusters , 1983 .

[11]  W. E. Brown,et al.  Crystallographic studies of the role of Mg as a stabilizing impurity in β-Ca3(PO4)2. II. Refinement of Mg-containing β-Ca3(PO4)2 , 1977 .

[12]  E. Dowty.  Structure and composition of the Ca3(PO4)2 minerals , 1977 .

[13]  C. Calvo,et al.  The Crystal Structure of Whitlockite from the Palermo Quarry , 1975 .

[14]  W. E. Brown,et al.  Crystallographic studies of the role of Mg as a stabilizing impurity in β-Ca3(PO4)2. The crystal structure of pure β-Ca3(PO4)2 , 1974 .

[15]  C. Calvo,et al.  Structural Relationship of Whitlockite and βCa 3 (PO 4 ) 2 , 1972 .

[16]  P. Noerdlinger.  Radiation Pressure on a Test Particle in General Relativity , 1972, Nature.

[17]  Brian Moore,et al.  Bracelets and Pinwheels: A Topological-Geometrical Approach to the Calcium Orthosilicate and Alkali Sulfate Structures , 2022 .