Andalusite occurs as an accessory mineral in many types of per aluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability cunes for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with, felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities worldwide. Our purpose is to determine the origin of andalusite, including the T-P-X controls on andalusite formation, using eight textural and chemical criteria: sizecompa tibility with grain sizes of igneous m inera ls in the same rock; shape-ranging from euhedral to anhedral, with, no simple correla tion with, origin; state of aggregation-single grains or clusters of grains; association with, muscovite-with, or without, rims of mono crystalline or polycrystalline muscovite; inclusions-rare mineral inclusions and melt inclusions; chemical composition-andalusite with, little significant chemical variation, except in iron content (0-08-1-71 wt. °/o FeO); compositional zoning-concentric, sec tor, patchy, oscillatory zoning cryptically reflect growth, conditions; compositions of coexisting phases-biotites with. high, siderophy llite-eastonite contents (AT ~2-68 ± 0-07 atoms per formula unit), muscovites with 0-57-4-01 wt % P'eO and 0-02-2-85 wt % TiOg, and apatites with. 3-53 ± 0-18 wt % F. Coexisting muscovite-biotite pairs have a wide range of F contents, and FSt = 1-612FAIs + 0-015. Most coexisting minerals have compositions consistent with, equilibration at. magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic-(a) prograde metamorphic (in ther mally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic-(a.) peritectic (water-undersaturated, TJ) associated with, leucosomes in migma tites, (b) peritectic (water-undersaturated, T^J, as reaction rims on garnet, or cordierite, (c) cotectic (water-undersaturated, T j direct, crystallization from a silicate melt, and (d) pegmatitic (watersaturated, T^J, associated with, aplite-pegmatite contacts or peg matitic portion alone; Type 3 Metasomatic-(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement, of feldspar and/or biotite, intergrowths with, quartz. Tie great, majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess AfOy,, water concentration and fluid evolution, high. Be-B-LiP , high. F, high. Fe-Mn-Ti, and kinetic considerations), the two most, important, factors appear to be excess Af03 and the effect, of releasing water (either to strip alkalis from the melt, or to reduce alumina solubility in the melt). Of particular importance is...
