A Mineral Phase Intermediate in Composition Between Sillimanite
American Mineralogist, Volume61, pages1025-1026, 1976 A mineral phaseintermediate in compositionbetween sillimanite and mullite We,RRrNcroNE. Ce.nrenoN Department of Mineralogy and Petrology Uniuersity of Cambridge, CB2,3EW, England Abstract A memberof the sillimanite-mullitesolid solutionseries with a chemicalcomposition midwaybetween that of sillimaniteand 3 : 2 mulliteoccurs in a metamorphosedlithomarge from Rathlin Island,Northern lreland.Its diffractionpattern displays weak but sharp/-odd maxima.characteristic of sillimanite. Introduction 0.2 x 0.2 X 1.0 pm. No contrastattributable to structureor exsolutionwas apparent. Crystallographically,there is no reasonwhy solid domain solutionshould not be completebetween sillimanite ComPosition andrmullite(Burnham, 1964). The mostaluminous by XRF sillimanitereported so far, however,contains 48.6 Pure separatedmaterial was analyzed (method the mole percentSiO, and the most siliceousnatural of Norrish and Hutton, 1969)using has mullite43.0 mole percent SiO, (Aramaki, l96l;Cam- mostFe-rich stoichiometric sillimanite the writer eronand Ashworth,1972).The writer considersthat found(specimen 94191 from the BensonMine, New thesehave crystallized from a liquid formed by par- York), mullitefrom Rubh' a Chroman,Mull, Scot- land(specimen 76398), and a syntheticglass, FS45, as tial meltingof pelitic material,probably within the 'Accepted' temperaturerange 900-1100"C. Sillimanites occur- standards. valuesfor specimens94191 and ring in accidentalxenoliths or as xenocrystsin vol- 76398were obtainedby EPMA
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