NORDSTRANDITE (Al2O3??3H2O), previously reported only as a synthetic product1,2, occurs in Miocene limestone on Guam. In south Guam this limestone forms a cap several hundred feet thick, on the Mount Alifan-Mount Lamlan ridge, and overlies late Eocene and early Miocene basalt flows and volcanic conglomerates. The volcanic rocks below the limestone are deeply weathered. The nordstrandite occurs in secondary solution cavities and is concentrated mainly in the basal part of the limestone near its contact with the weathered basalt. In thin section the nordstrandite crystals are seen as flamboyant aggregates reaching lengths of as much as 0.4 mm. The optical properties of the mineral are as follows: ?? = 1.580 ?? 0.004 colourless optic sign = positive ?? = 1.580 ?? 0.004 colourless 2 V = low ?? = 1.596 ?? 0.004 colourless elongation = negative ??-?? = 0.016 extinction = inclined The hardness is about 3 and the specific gravity was determined by Edward J. Young to be 2.436. ?? 1962 Nature Publishing Group.