The crystal structure of meurigite

Abstract

The crystal structure of meurigite, ideally [K(H 2 O) 2.5 ][Fe 8 3+ (PO 4 ) 6 (OH) 7 (H 2 O) 4 ], monoclinic, C2/c, a = 29.018(5), b = 5.1892(6), c = 19.695(3) Å, β = 106.987(1)°, Z = 4, from the Santa Rita mine, New Mexico, has been solved and refi ned to R 1 = 4.69%, wR 2 = 12.6% using 3325 unique [F o > 4σ(F o )] refl ections collected using a Bruker 6000 SMART CCD diffractometer and synchrotron radiation of wavelength 0.41328 Å. The structure of meurigite is a framework consisting of face-sharing octahedral Fe 2 3+ O 9 dimers, which are linked by sharing corners with corner-sharing dimers and isolated Fe 3+ O 6 octahedra to form thick slabs of octahedra parallel to the a-c plane. PO 4 tetrahedra further link octahedra within the slabs and also link slabs to one another perpendicular to the a-c plane. Relatively large channels through the framework along the b axis contain disordered K atoms and H 2 O molecules, which take part in two overlapping arrays. Partial vacancies in the Fe and P sites may account for discrepancies between the empirical and ideal chemical formulas. Packing considerations suggest that the empirical formula should be based on the total number of large ions (K + Na + O = 38.5 per formula unit), which for the chemical analysis provided in the original description yields [(K 0.91 Na 0.03 ) Σ0.94 (H 2 O) 2.56 ] Σ3.50 [(Fe 3+ 7.52 Al 0.17 Cu 0.03 ) Σ7.72 (PO 4 ) 5.48 (CO 3 ) 0.21 (OH) 7.20 (H 2 O) 5.23]. The meurigite structure is related to those of other fi brous ferric phosphates with 5 Å fi ber axes and shows a particularly close relationship with the structure of dufrénite. Crystal chemical evidence suggests that, even if meurigite and phosphofi brite are isostructural, phosphofi brite may qualify as a distinct species based upon its low K content (<0.5 apfu based on a recalculation of the original chemical analysis).

Meurigite-Na, a new species, and the relationship between phosphofibrite and meurigite

“…Kampf et al (2007) found partially occupied K and O(H 2 O) sites in the channels. The K atoms (K1 and K2) form bonds to O atoms in the surrounding framework as well as to H 2 O molecules (OW3 and OW4) in the channels.…”

“…The small peak in the infrared spectrum between 1400 and 1470 cm -1 assignable to CO 2 in meurigite-K is not observed in the meurigite-Na spectrum, indicating little or no CO 2 in meurigite-Na. Water was calculated, based upon the structure of meurigite-K (Kampf et al 2007). The results of four electron microprobe analyses are Wavenumbers (cm -1 ) Absorbance Wavenumbers (cm -1 ) Intensity bending modes, respectively.…”

“…The structure of meurigite (Kampf et al 2007) is a framework consisting of face-sharing octahedral Fe 3+ 2 O 9 dimers, which are linked by sharing corners with corner-sharing dimers and isolated Fe 3+ O 6 octahedra to form thick slabs of octahedra parallel to the a-b plane. PO 4 tetrahedra further link octahedra within the slabs and also link slabs to one another perpendicular to the a-b plane.…”

“…Furthermore, it would lead to an excess positive charge in the formula. The solution suggested by Kampf et al (2007) is to assume that H 2 O molecules take the place (though not necessarily the exact positions) of approximately half of the K atoms in the channel network, with hydrogen bonds linking the H 2 O molecules to one another. If this is the case, the channel would contribute 1 K and 2½ H 2 O per formula unit (4 K and 10 H 2 O per unit cell).…”

“…That, coupled with the possible identity of meurigite and phosphofibrite acknowledged by Birch et al (1996) and more strongly pointed out by Kolitsch (1999), made it desirable to directly compare the Na-rich material from the Silver Coin mine with the original type meurigite and, in particular, to obtain Fourier transform infrared spectra of the two under equivalent conditions. Quite serendipitously, a small piece from the meurigite type specimen provided by the Museum of Victoria (Melbourne, Australia), also yielded a crystal fragment of sufficient quality for the determination of its crystal structure using a synchrotron source (Kampf et al 2007).…”