Selwynite is a new alkali beryllium zirconium phosphate hydrate from a pegmatite vein in Devonian granite at Wycheproof, in northwestern Victoria, Australia. The mineral occurs as irregular infillings that consist of intergrowths of indistinct radiating crystals and as granular aggregates filling cavities up to 8 mm across in the quartz - orthoclase - albite - muscovite - schorl-bearing pegmatite. Accompanying minerals include two other Zr-bearing phosphates, kosnarite and the new species wycheproofite, as well as wardite, eosphorite, cyrilovite, leucophosphite, rockbridgeite, a kidwellite-like mineral, saleeite and montmorillonite. Selwynite is transparent deep purplish blue with a vitreous luster, but may weather to be pale lavender and translucent. The streak is pale lavender, the fracture, semiconchoidal, the cleavage, undeveloped, and the Mohs hardness, 4. The measured density is 2.94 g.cm (super -3) . Selwynite is uniaxial positive, omega 1.624, epsilon 1.636, and shows distinct pleochroism from medium to very pale bluish lavender. Average results of chemical analyses give Na 2 O 4.77, K 2 O 6.26, Rb 2 O 0.20, BeO 1.43, CaO 0.98, SrO 0.16, BaO 0.16, MgO 0.15, MnO 0.99, FeO 0.49, Cs 2 O 0.70, Al 2 O 3 1.04, Ce 2 O 3 0.03, ZrO 2 33.76, HfO 2 1.17, P2O5 40.90, SiO 2 0.49, F 0.37, H 2 O 5.4, less O = F 0.16, total 99.29 wt.%. The simplified formula is NaK(Be,Al)Zr 2 (PO 4 ) 4 .2H 2 O, based on (O + F) = 18. Selwynite is tetragonal, with unit-cell parameters refined from powder data: a 6.570(3), c 17.142(6) Aa, V 739.9(3) Aa 3 . For Z = 2, the calculated density for the empirical formula is 3.08 g.cm (super -3) . By analogy with gainesite the space group is I4 1 /amd. The strongest lines in the X-ray powder-diffraction pattern [d obs in Aa(I obs )(hkl)] are 6.161(100)(101), 4.291(25)(004), 3.286(50)(200), 3.039(30)(105), and 2.895(20)(211). Selwynite is related to gainesite by having the larger of the two Na sites in gainesite occupied by K, and to mccrillisite (Foord et al. 1994), which has that site occupied by Cs. All three minerals are considered to be hydrated, with between 2 and 4 water molecules per unit cell. The mineral name honors the late A. R. C. Selwyn, founding Director of the Geological Survey of Victoria.