Abstract

A new mineral species, bobdownsite, the F-dominant analogue of whitlockite, ideally Ca9Mg(PO4)6(PO3F), has been found in Lower Cretaceous bedded ironstones and shales exposed on a high ridge on the west side of Big Fish River, Yukon, Canada. The associated minerals include siderite, lazulite, an arrojadite-group mineral, kulanite, gormanite, quartz, and collinsite. Bobdownsite from the Yukon is tabular, colorless, and transparent, with a white streak and vitreous luster. It is brittle, with a Mohs hardness of ~5; no cleavage, parting, or macroscopic twinning is observed. The fracture is uneven and subconchoidal. The measured and calculated densities are 3.14 and 3.16 g/cm3, respectively. Bobdownsite is insoluble in water, acetone, or hydrochloric acid. Optically, it is uniaxial (−), ω = 1.625(2), ɛ = 1.622(2). The electron-microprobe analysis yielded (Ca8.76Na0.24)∑9.00 (Mg0.72Fe3+0.13Al0.11Fe2+0.04)∑1.00(P1.00O4)6(P1.00O3F1.07) as the empirical formula. Bobdownsite was examined with single-crystal X-ray diffraction; it is trigonal with space group R3c and unit-cell parameters a 10.3224(3), c 37.070(2) Å, V 3420.7(6) Å3. The structure was refined to an R1 factor of 0.031. Bobdownsite is isotypic with whitlockite, whose structure and relationships with other phosphate compounds have been studied extensively. Its structure is characterized by the [Mg(PO4)6]16− ligand, or the so-called “Mg pinwheel”. The isolated pinwheels are held together by intralayer Ca cations to form layers parallel to (001), which are linked together by interlayer Ca cations along [001]. The Raman spectra of bobdownsite strongly resemble those of whitlockite and merrillite. Bobdownsite represents the first naturally formed phosphate known to contain a P–F bond. It has subsequently been found in the Tip Top mine, Custer County, South Dakota, USA. On the basis of our study, we conclude that the “fluor whitlockite” found in the Martian meteorite SaU 094B also is bobdownsite.

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