A NEW POLYMORPH OF WAGNERITE, MG2(PO4)(F,OH)

Phosphates with the composition (Mg,Fe,Mn)₂(PO₄)(F,OH) include two groups (Strunz): magniotriplite- zwieselite-triplite (b ~ 6.45 Å) and wagnerite-wolfeite-triploidite (b ~ 12.5-13.2 Å). A new polymorph has been found in the Larsemann Hills, East Antarctica with apatite, plagioclase, biotite, and quartz in a granulite-facies paragneiss that crystallized at T ~750 – 860°C, P ~ 6-7 kbar. Electron microprobe analysis (OH calculated for OH + F=1) gave (Mg_1.88Fe_0.10Ti_0.02)(P_0.99O₄)(F_0.61OH_0.39), and single-crystal study, a=9.6448(19)Å, b=31.659(6) Å , c=11.914(2) Å; b=108.26 (3) º, V=3454.6(12) ų for Z=40, space group Ia. The crystal structure has been solved by direct methods and refined to R₁=0.0413 for the independent 4221 reflections using MoKa radiation. PO₄ tetrahedra are joined to MgO₄(F,OH)₂ octahedra and MgO₄(F,OH) trigonal bipyramids by sharing vertices, but MgO₄(F,OH)₂ octahedra and MgO₄(F,OH) trigonal bipyramids are joined to one another by sharing edges and vertices, in such a way as to build up a three-dimensional framework consisting of chains roughly parallel to the crystallographic axes. Compared to wagnerite there is a higher proportion of shared edges between trigonal bipyramids and octahedra relative to the total shared edges between these polyhedra: 40% vs. 25%. It differs from wagnerite in its b dimension (greater by a factor of 2.5) and lower symmetry. The difference between the triplite and triploidite groups has been attributed to ordering of F/OH, but the 2.5-fold increase in the b cell dimension for the new polymorph relative to wagnerite is related to linkage of the polyhedra. Variations between cell volume and F content in wagnerite with (Fe+Mn)/(Fe+Mn+Mg) < 0.1 suggest that the new polymorph could be ~1.4 % denser than wagnerite for a given F content. This polymorph appears to be characteristic of relatively high-temperature environments.