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Thirty-two sillimanites from numerous world-wide localities and geological environments were analyzed for Al, Si, Fe, and Ti by use of the electronmicroprobe. These sillimanites occur as fibrolitic, tabular, and granular crystals in middle almandine amphibolite (600 C, plus/minus 5 kbar minimum?) to granulite facies rocks and within inclusions in dacitic and olivine basaltic lavas (about 1150 C, plus/minus 0.5 kbar pressure?). All the sillimanites analyzed were within very close limits pure Al2SiO5 indicating that the possibility of sillimanite-mullite solid solution in most natural environments need not be considered at values greater than the 3% level (the maximum error of measurement). This was even found true of corundum-bearing samples (Al2O3-buffered), which occurred in high temperature-low pressure olivine basalts. The conclusion does not, of course, eliminate order-disorder effects in sillimanite nor slow kinetics as possible reasons for the too common occurrence of kyanite-sillimanite pairs.