Rocks containing three Al2SiO5 polymorphs - andalusite (And), kyanite (Ky), and sillimanite (Sil) - represent disequilibrium resulting from polymorph instability driven by changing pressure-temperature (P-T) conditions during related or unrelated tectonometamorphic events. There are six possible crystallization sequences. The two most common sequences of seventeen localities surveyed are: (1) Ky -> Sil -> And (six localities), and (2) And -> Ky -> Sil (six localities), including one locality for which two different sequences have been proposed. The two most common crystallization sequences have very distinct P-T conditions and paths and differ in whether low-P/moderate- to high-T (And zone) metamorphism preceded or postdated Ky-Sil zone conditions. Ky -> Sil -> And sequences may form by a continuum of tectonometamorphic events that drive isothermal decompression from Ky zone conditions through the Sil zone to shallow (And zone) crustal levels, such as during extension +/- magmatism that ultimately results in thin, hot crust. In sequences with early andalusite (And -> Ky -> Sil), formation of andalusite is typically unrelated to the later Barrovian metamorphism that formed Ky-Sil at moderate P-T conditions. Two other sequences have been reported but are much rarer: (3) Ky -> And -> Sil (three localities), and (4) And -> Sil -> Ky (two, possibly three localities). In these, Ky-zone metamorphism occurred as a distinct tectonometamorphic episode before or after the one that formed And-Sil. In cases in which Al2SiO5 polymorphs occur in veins or shear zones, the effects of deformation and fluids, including the possibility of non-lithostatic pressure effects, must be considered. Three-Al2SiO5 polymorph-bearing rocks from Lesjaverk, Norway, have not previously been described in the literature, and are interpreted here as forming by the sequence And -> Ky -> Sil. Metamorphic conditions determined using Zr-in-rutile thermometry indicate peak T = 600-640 degrees C at moderate P near the Ky-Sil boundary, similar to the peak conditions reported for other localities with the same crystallization sequence. These results are consistent with the finding that And -> Ky -> Sil rocks record lower P-T conditions than the other reported sequences.
