Mineralogy

New data on the crystal structure of metavivianite, Fe²⁺Fe³⁺₂(PO₄)₂(OH)₂·6H₂O

Aksenov S.M.¹, Rastsvetaeva R.K. ¹, Chukanov N.V.²

1Institute of Crystallography ras, Moscow, Russia

2Institute of Problems of Chemical Physics ras, Chernogolovka, Russia

rast@ns.crys.ras.ru

Metavivianite was first described with the formula Fe₃(PO₄)₂·8H₂O and was considered as a dimorph of vivianite, isostructural with symplesite [1]. However this description is incomplete and corresponds to a heterogeneous material. In addition, no separate analyses were reported for Fe²⁺ and Fe³⁺, as well as for H₂O. In the cited work, the crystal structure of metavivianite was not investigated; the analogy with symplesite was supposed basing on the similarity of X-ray powder-diffraction data. Subsequent investigations [2, 3] have shown that metavivianite contains Fe³⁺ and is not dimorphous with vivianite. However the crystal structure of metavivianite was solved with a high R factor of 13.3%, and the formula Fe²⁺_3-xFe³⁺_x(PO₄)₂(OH)_x·(8–x)H₂O suggested for metavivianite in the cited works does not reflect crystal-chemical features of this mineral, in particular, the distribution of Fe²⁺ and Fe³⁺ between two independent sites.

In this work, crystal structure investigation of metavivianite has been carried on a homogeneous sample from the Boa Vista pegmatite, Brazil. The chemical composition is (electron microprobe combined with Mössbauer data, mean of 5 analyses, wt.%): MgO 0.70, MnO 0.92, FeO 17.98, Fe₂O₃ 26.60, P₂O₅ 28.62, H₂O (by gas chromatography) 26.5; total 101.32. The mineral is triclinic, a = 4.629(1), b = 7.989(1), c = 9.321(2) Å,  = 108.59(2),  = 97.34(1),  = 95.96(1), V = 320.18(11) ų, Z = 1. Single-crystal X-ray data were collected using diffractometer Xcalibur Oxford Diffraction with CCD-detector. The crystal structure of metavivianite was solved by “charge flipping” method and refined in space group P with corresponding R-index 6.0% for 1350 |F_obs| > 3σ_F. The structure consists of heteropolyhedral layers along diagonal bc. Each layer contains pairs of edge-sharing octahedra with the composition [Fe³⁺₂O₄(H₂O,OH)₄] and isolated octahedra [Fe²⁺O₂(H₂O)₄] combined by [PO₄]-tetrahedra (Fig.). The layers are linked by hydrogen bonds formed by water molecules and OH groups.

Fig. The crystal structure of metavivianite, the cb projection.

Site	O1	O2	O3	O4	W1	W2	W3	W4	Vi
Fe1			0.40(x2)→ 0.40↓			0.29(x2)→ 0.29↓		0.36(x2)→ 0.36↓	2.1
Fe2	0.43	0.50		(0.41+0.37)↓→	0.51		0.41		2.63
P	1.21	1.23	1.28	1.19					4.91
Vi*	1.64+ (0.42)	1.73+ (0.18)	1.68+ (0.22)	1.97	0.51+ (0.2)	0.29+ (0.18)	0.41+ (0.18)	0.36+ (0.2)

Table. Bond valence calculations for metavivianite

*Bond valences for anions are supplemented by contributions (in brackets) from hydrogen atoms of water molecules.

The distribution of cations between octahedral sites was made taking into account the ratio Fe²⁺ : Fe³⁺ = 43:57 (obtained from the Mössbauer spectrum), the values of interatomic distances and results of bond valence calculations (Table). Although the H-atoms were not localized, we took into account the distances between O atoms of water molecules that are equal to ~2.75 Ǻ. The values of bond valence corresponding to these distances are ~ 0.2 v.u. With the account of these contributions we assume that W1 and W3 can partly be replaced by OH groups.

The refined crystal-chemical formula of metavivianite (Z=1) is [(Fe²⁺_0.93Mn_0.07)(H₂O)₄] [(Fe³⁺_0.81Fe²⁺_0.11Mg_0.08)₂(H₂O,OH)₄] [PO₄]₂. Bond valence calculation for Fe³⁺ in Fe2 position gives low sum, that corresponds to the presence of bivalent elements (Table). The simplified crystal-chemical formula of metavivianite is Fe²⁺(Fe³⁺,Fe²⁺)₂(PO₄)₂(OH,H₂O)₂·6H₂O. Bond valence calculations for metavivianite show that Fe²⁺ and Fe³⁺ are predominant components in the sites Fe1 and Fe2 respectively in opposite to the sample investigated by Dormann et al. (1982), in which both sites are predominantly occupied by Fe³⁺ Consequently, the idealized (end-member) formula of this mineral is Fe²⁺Fe³⁺₂(PO₄)₂(OH)₂·6H₂O. In other words, metavivianite is dimorphous with ferrostrunzite. As distinct from ferrostrunzite, metavivianite is a transformational mineral species formed as a result of natural oxidation of vivianite when the content of Fe³⁺ exceeds 1.4 pfu.

This work was financially supported by Russian Foundation for Basic Research (grant No. 10-05-00092-a). The authors thank R. Scholz for the sample of metavivianite.

References:

1. Ritz, C., Essene, E.J. and Peacor, D.R. (1974) Metavivianite, Fe₃(PO₄)₂·8H₂O, a new mineral. American Mineralogist, 59, P. 896 – 899.

2. Dormann, J., Gaspérin, M. and Poullen, J.-F. (1982) Étude structurale de la séquence d'oxydation de la vivianite Fe₃(PO₄)₂·8(H₂O). Bulletin de

Minéralogie, 105, P.147 – 160 (in French).

3. Rodgers, K.A. (1986) Metavivianite and kerchenite: a review. Mineralogical Magazine, 50, P. 687 – 691.