Postite, Mg(H2O)6Al2(OH)2(H2O)8(V10O28)·13H2O, is a new mineral species from the Vanadium Queen mine, La Sal Creek Canyon, and the Blue Cap mine, Lyon Canyon Creek, San Juan County, Utah, U.S.A. Postite occurs as very thin, needle-like prisms with pyramidal terminations; crystals commonly occur in parallel bundles and grow in divergent and “jackstraw” masses. Individual crystals are up to 1 mm long and 50 μm in diameter, and are golden-yellow with a yellow streak. The mineral is transparent, with a subadamantine luster; it does not fluoresce in short- or long-wave ultraviolet radiation. Postite has a Mohs hardness of approximately 2 and brittle tenacity. The mineral has one good cleavage on {001} and at least two perfect cleavages parallel to [001], possibly {100} and {010}. The fracture is splintery. The density calculated from the empirical formula using the single-crystal cell data is 2.226 g/cm3. Postite is biaxial (+) with a 2trong>ctrong>, trong>btrong>, trong>atrong>. Dispersion was not observed and pleochroism was not perceptible. Electron probe microanalysis and the crystal structure solution gave the empirical formula (Mg0.97Na0.06Ca0.04Sr0.01K0.01)∑1.09Al1.94 [(OH)1.92(H2O)0.08]‡”2.00 (V10O28)·27H2O. The simplified structural formula of postite is Mg(H2O)6Al2(OH)2(H2O)8(V10O28) ·13H2O. Postite is orthorhombic,