Me²⁺ + Si = Na + P Substitution in High-pressure Garnets: Is There Really an Na₃Al₂(PO₄)₃ Endmember ?

Fabrice Brunet (brunet@geologie.ens.fr)¹ & Ronald Miletich²

¹ Lab. de géologie, ENS, CNRS-UMR8538, Paris, France

² Lab. for crystallography, ETHZ, Zuerich, Switzerland

Phosphorus content of upper-mantle garnets amounts to around 500-1000 ppm P₂O₅ and has been proposed as a pressure indicator (Bishop et al., 1978). More recently, Schertl et al. (1991) analyzed up to 0.2 wt% P₂O₅ in ultra high-pressure pyropes from Dora Maira (western Alps). Additional EMPB analyses performed on garnets from the same locality allowed us to evidence an Mg + Si = Na + P substitution which leads to a hypothetical Na₃Al₂(PO₄)₃ endmember. Phosphorus (and sodium) zoning is observed in these garnets but the respective contribution of changing P and T, and/or fluid composition remains unclear. In order to bring constraints on the P-T dependency of the Me²⁺ _-1Si_-1NaP substitution in garnets, we have performed dry syntheses between 1 bar and 40 kbar, 550 and 1000°C in the Na₃Al₂(PO₄)₃ system in which the garnet structure has already been reported (Thilo, 1941). Three polymorphs of Na₃Al₂(PO₄)₃ were encountered, the unit-cell parameters of two of them have been determined by means of single-crystal diffraction on a Huber four-circle diffractometer. A low-pressure form crystallizes below 5 kbar (550, 650 and 780°C) and displays a tetragonal unit-cell (a = 9.311 Å, c = 18.658 Å). Isotypic compounds have not been identified yet even in related polymorphic systems with the same stoichiometry as Na₃Fe₂(PO₄)₃ or Na₃Cr₂(PO₄)₃. Each of the two latter compositions displays a polymorph isotypic with Na₃Zr₂Si₂PO₁₂ (NASICON), the Na₃Al₂(PO₄)₃ crystals obtained between 6 to 27 kbar exhibit a rhombohedral unit-cell (a = 8.484 Å, c = 21.215 Å) which is consistent with this structure. At 39 kbar, 800°C, a third form is encountered, the X-ray powder pattern of which could not be indexed in a cubic garnet unit-cell. The Na₃Al₂(PO₄)₃ garnet has not been obtained yet but natural data and the predicted unit-cell volume rather suggest a high-pressure stability for this garnet, experiments at higher pressure are in progress.

Bishop FC, Smith JV & Dawson JB, Lithos, 11, 155-173, (1978).

Schertl HP, Schreyer W & Chopin C, Contrib. Mineral. Petrol, 108, 1-21, (1991).

Thilo E, Naturwiss, 29, 239, (1941).

Structural Consequences of the Incorporation of Fe³⁺ in Zoisite: A Rietveld Refinement and FTIR Study

Axel Brunsmann (axelhpgh@mailszrz.zrz.tu-berlin)¹, Gerhard Franz (gerhard.franz@.tu-berlin.de)¹ & Mathias Gottschalk (mgott@gfz-potsdam.de)²

¹ FG Petrologie, Sekr. EB 15, TU Berlin, Str. d. 17 Juni 135, 10623 Berlin, Germany

² GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany

To study the structural consequences of iron incorporation in zoisite [general formula = Ca₂Al₂(Al,Fe³⁺)[Si₃O₁₁(O/OH)] we performed high-pressure synthesis experiments in CFASH at 2.0 GPa/750°C using an stoichiometric oxide/hydroxide mixture with 10 wt% excess SiO₂ as starting material (bulk compositions = 0%, 4%, 8%, 12% and 16% Al₂Fe, Al₂Fe% = Fe/(Fe+Al-2)*100%). Oxygen fugacity was buffered at HM, run duration was 5 days. The run products were analysed by SEM, EMP, XRD and FTIR.

Run products consist of > 95 vol% zoisite or zoisite + clinozoisite and < 5 vol% quartz. Individual zoisite crystals reach lengths of 20 µm yielding reliable microprobe data. Iron free zoisite has lattice parameters a = 16.195 (1) Å, b = 5.5501 (3) Å, c = 10.0345 (6) Å and V = 901.9 (1) ų. With increasing iron content, b and c increase linearly according to the equations b = 7.1E-4*Al₂Fe% + 5.5499 Å and c = 5.52E-4*Al₂Fe% + 10.0334 Å. Important structural consequences of iron incorporation are i) a volume increase of T(1), ii) a volume decrease of T(2), iii) a flattening of M(3) perpendicular to [010] and iv) an increase of A(2) coordination from seven- to eight-fold. The data indicate an increasing distortion of the zoisite structure and suggest, that iron incorporation is structurally limited.

FTIR powder absorption spectra of zoisite were recorded in the range 350 - 3500 cm^-1. With increasing iron content, the OH-band at 3160 cm^-1 shifts to higher wavenumbers according to the equation (nu) = 0.879*Al₂Fe% + 3153.0 cm^-1, whereas the bands at 2170 cm^-1 and in the range 800 - 350 cm^-1 shift to lower wavenumbers. From the position and shift of the OH-band, we calculated O(10) - O(4) distances to <O(10) - O(4)> = 3.08E-4*Al₂Fe% + 2.677 Å, reflecting the expansion of the zoisite structure in c.