Cavansite, Ca(VO) (Si4O10)·4H2O, a new mineral found in cavities and veinlets in basalt and vugs in tuff at Goble and at Owyhee Dam in Oregon, occurs as radiating greenish-blue prismatic crystals associated with calcite, analcime, thomsonite, heulandite, stilbite, and apophyllite. The crystals are orthorhombic, space group Pcmn (D2h16), and have a unit cell with a = 9.778(3), b = 13.678(4), c = 9.601(2) Å, containing 4 formula units. They are optically biaxial positive and strongly pleochroic. The composition was determined by X-ray fluorescence analysis and crystal structure analysis. Pentagonite, a dimorph of cavansite, was found at Owyhee Dam in prismatic crystals twinned to form fivelings with star-shaped cross sections. It is also orthorhombic, space group Ccm21 (C2+12), and has a unit cell with a = 10.298(4), b = 13.999(7) and c = 8.891(2) Å, containing 4 formula units. These cell dimensions, and those for cavansite, are typical and tend to vary over a small range, presumably because of varying zeolitic water content. The crystals are optically very similar to cavansite, but are biaxial negative. Both cavansite and pentagonite have silicate layer structures in which the layers are held together by VO2t groups and Ca2+ ions, but they differ in the way the SiO4 tetrahedra link to form the layers. The crystal chemical study fully accounts for the morphological and physical properties of these unusual minerals.