Refined unit-cell dimensions determined for a suite of perthitic alkali feldspars from a nearsurface plutonic environment were used to construct standard aluminum occupancy plots. From comparison of natural and dry-heated specimens it is apparent that, on heating, aluminum moves from the T1o site into the T1m site faster than into the combined T2o and T2m sites. This suggests a two-step disordering path which is nearly the reverse of the ordering path(s) inferred for igneous and metamorphic Introduction

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