Crystal structure refinements and compositional control of Mn-Mg-Ca ardennites from the Belgian Ardennes, Greece, and the Western Alps

The As5+ - V5+-bearing silicate ardennite is commonly present in highly oxidized, manganiferous metasediments that were affected by low- to high-pressure metamorphism in the T range from ca. 300 to 600-degrees-C. Electron microprobe analyses of ardennite from the Belgian Ardennes, Greece, and the Western Alps combined with the results from structure refinements on four ardennite crystals from Andros, Greece; Haute-Maurienne, French Western Alps; and Lago di Cignana, Valtournanche, Italy (two specimens of different composition), suggest the general formula VI(A1)2 (VII)(A2)2 (VI)(M1-M3)6 [(IV)T4(O,OH)4/(SiO4)2/Si3O10/(OH,O)6], where Al = Mn2+, Mg; A2 = Mn2+, Ca; Ml = Al, +/- Fe3+; M2 = Al; M3 = Al, Mg, +/- Fe3+, +/- Mn3+, +/- Cu2+, +/- Ni2, ; and T4 = As5+, V5+, P5+, Si4+. Least squares refinements of the four structures (space group Pnmm, a = 8.710(6)-8.767(3) Å, b = 5.803(4)-5.846(2) Å, c = 18.542(9)-18.613(6) Å) converged with R = 0.036-0.083 and allowed to locate the hydrogen atoms in ardennite from Haute-Maurienne. Mn3+ Al-1 substitution (up to 1.1 Mn3+ per 16 total cations) principally occurs in the M3 octahedron which is larger ([M3 - O] = 2.006 - 2.018 Å) and more distorted (two long and four short cation-oxygen distances) than the M1 (1.918 - 1.941 angstrom) and the M2 octahedron (1.897-1.906 angstrom). Within the range from zero to 0.4 Mn3+ p.f.u., obtained for the refined structures, however, the degree of distortion of the M3 octahedra is unrelated to the Mn3+. content. Fe3+ (up to 0.4 atoms p.f.u. in microprobe analyses) may enter either the M1 or, in Mn3+-free ardennite, the M3 site. Mg2+ is partitioned between the M3 octahedron (0.5 - 0.95 atoms p.f.u.) and the irregular (6 + 1)-coordinated A1 polyhedron (0 - 1.6 atoms p.f.u.). Ca has a strong preference for the larger 7-coordinated A2 site, where it substitutes for up to 77 mol% of the Mn2+ in Mn3+-rich ardennites from low-grade assemblages. Besides As5+ and/or V5+, the isolated T4 tetrahedron may incorporate significant Si4+ commonly up to 40 mol%) as well as up to 28 mol% P5+ in ultrahigh-pressure metamorphic ardennites. The [T4-O] distances range from 1.688 angstrom to 1.659 angstrom and reflect the variable extent of substitution of Si4+ and P5+ for As5+ and V5+. The chemical analyses suggest that charge balance for Al3+-Mg2+ substitution in M3 and Si4+-(As, V, P)5+ substitution in T4 is maintained by variations in the overall hydrogen content. During prograde high-pressure metamorphism, Mg (in A1 sites) and P5+ gradually increases as the result of multivariant reactions of ardennite with coexisting garnet, apatite, phengite, clinochlore/talc, quartz/coesite and piemontite. Ca, Mn3+ and Fe3+ contents in ardennite vary i) with T and P by virtue of Mn3+Al-I and Mn3+Fe-13+ exchange with braunite, hematite and piemontite and ii) as a result of different mineral assemblages in closely associated rocks.