G. Artioli et al., The crystal structure of Mg-8(Mg2Al2)Al8Si12(O,OH)(56) pumpellyite and itsrelevance in ultramafic systems at high pressure, AM MINERAL, 84(11-12), 1999, pp. 1906-1914
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.