‘Unusual Zircons From the Leinster Pluton’ Are Sillimanite

‘Unusual zircons’ with exceptionally high elongation ratios were reported from the porphyritic muscovite Type III variety of the peraluminous Leinster Granite of SE Ireland in a 1973 Mineralogical Magazine paper by L.N. Gupta. However, observations and analyses of these ‘zircons’ reveal them to be sillimanites. Gupta had originally used an acid digestion technique, rather than rock crushing, to extract zircons for morphological—but not chemical—analysis. As a result, hitherto unrecognised sillimanite (originally presumed to be zircons) were liberated from within what is now known to be magmatically zoned muscovite. In this paragenesis, sillimanite occurs as epi/topotaxial inclusions within a subset of the Leinster Granite muscovite phenocrysts.