Bobjonesite, V4+ O (SO4) (H2O 3, is a new mineral species from Temple Mountain, Emery County, Utah, U.S.A. It occurs as blue-green crusts and efflorescences in fractures in a fossil (Triassic) tree: individual crystals are ??1 mm and are intimately intergrown. Bobjonesite hydrates very easily, and is unstable in all but the driest atmosphere. Its structure was determined on a crystal of bobjonesite: however, the physical properties, optical properties and X-ray powder-diffraction pattern were recorded on the synthetic equivalent, and an electron-microprobe analysis was not possible. Bobjonesite has a pale blue streak, a vitreous luster and no observable fluorescence under ultraviolet light. It has no cleavage or parting. The Mohs hardness is ???1, and the calculated density is 2.28 g/cm3. Bobjonesite is biaxial positive, with ?? 1.555(2 , ?? 1.561(1), ?? 1.574(2), 2V(obs.) = 72(1)??, 2V(calc.) = 69??; it is non-pleochroic, X = b, Y ??? 19?? (in ?? obtuse). Bobjonesite is monoclinic, space group P21/n, cell dimensions from single-crystal data: a 7.3940(5), b 7.4111(3), c 12.0597(9) A??, ?? 106.55(1)??, V 633.5(1) A??3, Z=4. The strongest seven lines in the X-ray powder-diffraction pattern [d in A??(I)(hkl)] are as follows: 5.795(100)(002), 3.498(90)(112), 3.881(48)(1??03), 5.408(37) (101), 4.571(20)(012), 6.962(11 (1??01) and 6.254(11)(011). The chemical formula was derived from crystal-structure analysis; the end-member formula is V O (SO4) (H2O)3. The crystal structure of bobjonesite was refined to an R index of 3.6% for 1105 observed (Fo> 5??F) reflections measured with an automated four-circle X-ray diffractometer using MoK?? X-radiation. There is one V site occupied by V4+ and surrounded by three O atoms and three (H2O) groups in an octahedral arrangement, with one short vanadyl bond (1.577 A??), four similar equatorial bonds (<2.022 A??), and one longer V-O bond (2.278 A??) trans to the vanadyl bond. The structure consists of isolated [V4+2 O2 (H2O)6 (SO4)2] clusters linked by hydrogen bonds.