Synthetic gedrite: a stable phase in the system MgO-Al2O3-SiO2-H2O (MASH) at 800 degrees C and 10 kbar water pressure, and the influence of FeNaCa impurities

Seeded, solid-media piston-cylinder runs of unusually long duration up to 3 1 days indicate growth or persistence of synthetic gedrite of the compositi on square Mg6Al[AlSi7O22](OH)(2)(=6.1.7), prepared from the purest chemical s available, at 10 kbar water pressure and 800 degrees C. Conversely, break down was observed at 11 kbar and 850 degrees C to aluminous enstatite, Al2S iO5, and a melt of the composition MgO . Al2O3. 8SiO(2). Thus, pure gedrite free of iron, sodium, and calcium is likely to have only a small PT stabil ity field in the MASH system, estimated as 10 +/- 1 kbar, 800 +/- 20 degree s C, even though metastable growth of gedrite can be observed over a larger PT range. A second starting material with the anhydrous composition 5MgO . 2Al(2)O(3). 6SiO(2) also yielded gedrite of the composition 6.1.7, togethe r with more aluminous phases such as kyanite, corundum or sapphirine, thus suggesting that the end-member gedrite defined as square Mg5Al2[Al2Si6O22]( OH)(2)( = 5:2:6) by the IMA Commission on New Minerals and Mineral Names pr obably does not exist. With the use of this second starting material, which contains FeNaCa impurities, growth of 6:1:7-gedrite was observed over a st ill wider PT-range. Seeded runs indicate that the true stability field of s uch slightly impure 6:1:7-gedrites may also be larger than that of the pure MASH phase and extend at least to 15 kbar, 800 degrees C. There is, thus, a remarkable stabilization effect on the orthoamphibole structure by impuri ties amounting only to a total of less than one weight percent of oxides in the starting material. The gedrites synthesized are structurally well orde red amphiboles nearly free of chain multiplicity faults, as revealed by HRT EM. The X-ray diffraction work on the gedrites synthesized yielded the smal lest cell volume yet reported for this phase. The small stability field of the purl MASH gedrite is intersected by the upper pressure stability limit of hydrous cordierite for excess-H2O conditions, thus leading to complicate d phase relations for both gedrite and cordierite involving the additional phases aluminous enstatite, talc, quartz, Al2SiO5, melt and perhaps boron-f ree kornerupine.