Ab-initio crystal structure determination of a new aluminum phosphate sulfate mineral

Introduction

A new mineral with the composition Al₂(PO₄)(SO₄)(OH,F)·7H₂O was found on the dump of the Lichtenberg open cast, Ronneburg, Thuringia, Germany. In the open cast and surrounding mines uranium-bearing alum shale was mined from 1951 to 1990. The Ronneburg mining area was one of the largest uranium producers in Europe. The new mineral is an alteration product of pyrite and fluorapatite and was formed on the mine dump. It forms white aggregates of irregular intergrown, tiny acicular crystals of less than 50 µm in length, and was found only in very small amounts. 

Figure 1. White aluminum phosphate sulfate mineral from the mine dump of the Lichtenberg open cast, Ronneburg. Width of the picture is 16 mm.

Powder X-ray diffraction measurements on the Aeris

Only a small amount of material was available. The sample was therefore prepared on a silicon zero background holder. The powder diffraction measurement was done using an Aeris Research diffractometer with CuK_α radiation and PIXcel detector. The pattern in the range 5 < 2ϑ < 100° was obtained within 2h30min measurement time. Despite the low amount of sample, a high intensity scan with good intensity/background ratio was obtained.

The powder data could be indexed with a triclinic cell, space group P̅1, and the lattice parameter a = 6.129, b = 9.856, c = 11.433 Å, α = 70.284, β = 85.84, γ = 82.557° and V = 644.36 Å3. For Z=2 the calculated density is 2.09 g/cm³.

Ab-initio structure determination

The new mineral is chemically closely related to sanjuanite, Al₂(PO₄)(SO₄)(OH)·9H₂O (Colombo et al., 2011), and arangasite, Al₂(PO₄)(SO₄)F·9H₂O (Yakubovich et al., 2014), and shows also some relation in the powder diffraction data and the lattice parameters to these minerals. Based on the similarities and the preferred orientation on (001) and (010) observed in the powder diffraction data, it was assumed, that the Ronneburg mineral shows similar building blocks as found in sanjuanite and arangasite: chains of AlO6 octahedra and PO4 tetrahedra forming layers parallel to the direction of the preferred orientation. In the chains, two AlO6 octahedra share an oxygen. Sulfate is linked to the chains only by hydrogen bonding, and there is water between the layers. From XRD perspective, this is a very challenging sample as all the atoms scatter similarly (P: 15e-, P⁵⁺=10e-; Al: 13e^-, Al³⁺=10e^-; S: 16e^-, S6+=10e- and O: 8e-, O^2- = 10e^-). 

Figure 2. Powder diffraction data of the new aluminum phosphate sulfate mineral from Ronneburg, measured on the Aeris diffractometer.

The final Rietveld refinement is shown in Figure 2. All measured reflections could be described. Furthermore, the determined structural model is confirmed by the excellent fit of the obtained data resulting in a small difference curve.

The crystal structure is fairly complex with 20 independent atoms in the unit cell: 2 aluminum, 1 phosphorous, 1 sulfur and 16 oxygen (including water and fluorine) positions. 

Figure 3. The crystal structure of the Ronneburg aluminum phosphate sulfate mineral. View along the a direction. Unit cell outlined. Blue: AlO₆ octahedra, purple: PO₄ tetrahedra, yellow: SO₄ tetrahedra, red: oxygen (including from water).

The crystal structure of the Ronneburg mineral contains AlO₆ octahedra, forming groups of two octahedra by sharing an oxygen at a corner. These groups are connected to chains by PO₄ tetrahedra. The chains are running parallel the crystallographic a direction. The chains are forming a layered structure parallel (001), explaining the preferred orientation effect observed in the powder data. The SO4 tetrahedra are connected to the chains only via hydrogen bonding. The chains and SO₄ tetrahedra forming channels parallel the crystallographic a direction. One water molecule per formula unit occupies a position in the channels. The other water molecules occupy the corners of the AlO6 octahedra, which are not shared with a neighbouring octahedra or phosphate tetrahedra. To show the different structural positions of the water, the formula can also be written as Al₂(PO₄)(SO₄)(OH,F)(H₂O)6·H₂O. 

The aluminophosphate chains in the Ronneburg mineral are similar to the ones in sanjuanite and arangasite, but the exact arrangement of the AlO₆ octahedra and PO₄ tetrahedra is different in the three minerals. 

Figure 4. The aluminum phosphate chains in the Ronneburg mineral, running parall the a direction.

Conclusion 

The results demonstrate, that it is possible to determine the fairly complex crystal structure of the aluminium phosphate sulfate mineral purely from powder diffraction data, obtained from a small amount of material measured on the Aeris diffractometer.

References 

Colombo, F.; Rius, J.; Pannunzio-Miner, E.V.; Pedregosa, J.C.; Camí, G.E. and Carbonio, R.E. (2011) Sanjuanite: ab initio crystal-structure solution from laboratory powder data, complemented by FTIR spectroscopy and DT-TG analyses. Canadian Mineralogist, 49, 835-847 

Yakubovich, O.V.; Steele, I.M.; Chernyshev, V.V.; Zayakina, N.V.; Gamyanin, G.N. and Karimova, O.V. (2014) The crystal structure of arangasite, Al₂F(PO₄)(SO₄)·9H₂O determined using low-temperature synchrotron data. Mineralogical Magazine, 78, 889–903