Cavansite and pentagonite, dimorphs of Ca(VO) (Si4O10)· 4H2O found in Malheur County, Oregon, are both orthorhombic and represent novel, layer-silicate structure types. Zigzag pyroxene-like (SiO3)n chains, joined laterally into sheets parallel to the a-c plane, are present in both with tetrahedral apices pointed alternately plus and minus along the b axes. The lateral linkage in cavansite results in a network of 4-fold and 8-fold rings, but in pentagonite the network is entirely made up of 6-fold rings. The vanadyl groups VO2+ and CO2+ ions lie in mirror planes between the silicate layers and are coordinated alternately to pairs of tetrahedral apices along the chains on opposite sides of the mirror planes. Vanadium is in square-pyramid coordination, and Ca is in 7-fold coordination in both structures. The H2O molecules are poorly resolved, have high apparent thermal parameters, and are probably zeolitic. The special features of the structures favor twinning in pentagonite (commonly found in fiveling groups) but prohibit twinning in cavansite (in which twinning has not been observed). Refinement of the crystal structures (to R = 0.109 for cavansite, and R = 0.081 for pentagonite) gives bond lengths with estimated errors of ± 0.02 Å.