Crystallographic ordering of aluminium in laueite at Hagendorf-Süd

Abstract

Crystals of laueite, Mn 2+ Fe 3+ 2 (PO 4 ) 2 (OH) 2 ·8H 2 O, from the Cornelia mine open cut, Hagendorf Sü d, Bavaria, are zoned due to aluminium incorporation at the iron sites, with analysed Al 2 O 3 contents varying up to 11 wt.%. Synchrotron X-ray data were collected on two crystals with different Al contents and the structures refined. The laueite structure contains two independent Fe 3+ -containing sites; M2 and M3, which alternate in 7 Å corner-connected octahedral chains. The coordination polyhedra are different for the two sites, M2O 4 (OH) 2 and M3O 2 (OH) 2 (H 2 O) 2 respectively. The structure refinements show that Al preferentially orders into site M3. Refined site occupancies for M2 and M3 for the two crystals are: for crystal L-1, M2 = 0.70(1) Fe + 0.30(1) Al, M3 = 0.54(1) Fe + 0.46(1) Al and for crystal L-2, M2 = 0.67(1) Fe + 0.33(1) Al, M3 = 0.48(1) Fe + 0.52(1) Al. For crystal L-2, the octahedral chains have dominant Fe in M2, alternating with dominant Al in M3 along the chain, an ordering phenomenon not previously reported for laueite-related minerals.

Nomenclature for the laueite supergroup

“…Indeed, two synthetic phases with heteropolyhedral sheets that are topologically identical to those in laueite, but with M2 and M3 sites containing different cations, have been reported by Wang et al, (2000): (enH 2 )NbFeOF (PO 4 ) 2 (H 2 O) 2 [en = ethylenediamine] with Nb 5+ in site M2 and Fe 3 + in site M3 and (enH 2 )Ti(Fe,Cr)(F,O)(H 0.3 PO 4 ) 2 (H 2 O) 2 with Ti 4+ in site M2 and Fe 3+ /Cr 3+ in site M3. We have also reported recently on intergrowths between laueite and mangangordonite at Hagendorf Süd, in which a single-crystal refinement on one of the compositions gave dominant Fe in site M2 and dominant Al in site M3 and would correspond to a new mineral species (Grey et al, 2015).…”

“…These groups are then subdivided into subgroups on the basis of the dominant cation, Al 3+ or Fe 3+ , in the octahedral sites M2/M3. This is analogous to the jahnsite group in which the minerals are subdivided into whiteites and jahnsites based upon the dominance of Al 3+ or Fe 3+ , respectively, in the M3 site (Moore and Ito, 1978 (Grey et al, 2015).…”

Kummerite, Mn2+Fe3+Al(PO4)2(OH)2·8H2O, a new laueite-group mineral from the Hagendorf Süd pegmatite, Bavaria, with ordering of Al and Fe3+

“…Scanning electron microscope images of kummerite grains commonly display compositional zoning due to Al/Fe variations, as seen in Fig. 2 of Grey et al (2015). The cores of the grains have compositions close to that for laueite and the rims are enriched in Al.…”

“…We have previously reported crystal structure refinements for two Al-bearing laueite crystals (Grey et al, 2015). The higher-Al containing crystal, L2, gave refined site occupancies for the trivalent M2 and M3 sites of 0.67(1)Fe + 0.33Al and 0.48(1)Fe + 0.52Al respectively.…”

“…In an on-going study on secondary phosphate minerals from the Hagendorf-Süd granitic pegmatite (Birch et al, 2011;Grey et al, 2010Grey et al, , 2012 we have recently reported on the preferential ordering of Al in the M3 site in laueite crystals that have undergone mineral replacement reactions (Grey et al, 2015). Crystal structure refinements for two crystals, L1 and L2, showed that the crystal L2, with the higher Al content, had dominant Al in the M3 site and dominant Fe 3+ in the M2 site, thus qualifying it as a potential new mineral species.…”