The crystal structure of Mg-8(Mg2Al2)Al8Si12(O,OH)(56) pumpellyite and itsrelevance in ultramafic systems at high pressure

Multianvil experiments in the model system MgO-Al2O3-SiO2-H2O at 680 degree s C and 5.2-6.0 GPa yield a variety of Mg,Al-rich mineral assemblages conta ining pumpellyite-type phases. Run products include the assemblages pumpell yite-coesite, pumpellyite-forsterite-orehoenstaeite and pumpellyite-10 Angs trom phase. The pumpellyite-coesite sample contains pumpellyite with nomina l composition Mg-8(Mg2Al2)Al8Si12 (O, OH)(56) and about 6 wt% coesite. Powd er diffraction data obtained using synchrotron radiation, a parallel-beam D ebye geometry, a capillary sample mount, and an Image Plate were refined by Rietveld structure analysis. The structure was modeled in space group P2(1 )/m [cell a = 8.5759(4), b = 5.7295(2), c = 18.5376(9) Angstrom, beta = 97. 691(3)degrees, V = 902.66(9) Angstrom(3)] and shows extensive stacking diso rder along the [001] direction, which can be described as layers of X-octah edra shifted by (1/2,0,0) vectors. The shifted layers locally show a sursas site-type arrangement. No disorder was detected in the Y-octahedra, which a re fully occupied by Al ions, and in the W positions, commonly hosting seve nfold-coordinated Ca atoms in naturally occurring pumpellyite minerals, whe reas in the synthetic Mg-rich pumpellyite they are fully occupied by Mg ato ms in a distorted sixfold-coordination. The structure can then be described as a pumpellyite-type structure having a certain amount of sursassite doma ins. Alternative structure models involving: (1) possible presence of arden nite-domains; (2) a sursassite-type structure with pumpellyite domains; (3) cell doubling along c due to long range ordering of the layers containing the X octahedra; or (4) a statistically disordered structure in the centere d A2/m parent space group and in other t subgroups, were all tested and dis carded on; the basis of the full-profile Rietveld refinements of powder dif fraction data. The selected model indicates that the polytypic behaviour ob served in pumpellyite and related structures (sursassite, and possibly arde nnite) is also possible in the high pressure members of the group. The volu me of the reaction, MgSiO3 (enstatite) + 2 Mg2SiO4 (forsterite) + Mg-8(Mg2A l2)Al8Si12O42OH14 (pumpellyite) <-> 5 Mg3Al2Si3O12 (pyrope) + 7 H2O re-eval uated using the new crystallographic data, gave a negative dP/dT slope. The assemblage chlorite-enstatite-pyrope acts as compositional barrier for the occurrence of pumpellyite in most ultramafic compositions for H2O-undersat urated conditions. In more complex Fe,Ca-containing systems, the preferenti al partitionings of Fe,Ca in pumpellyite compared to garnet enhances its po tential petrological importance in ultramafic subducting slabs.