Paper No. 8
Presentation Time: 3:55 PM
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 (Mg1.88Fe0.10Ti0.02)(P0.99O4)(F0.61OH0.39), 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) Å3 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.