ROUMAITE, (Ca,Na,?)3(Ca,REE,Na)4(Nb,Ti)[Si2O7]2(OH)F3, FROM ROUMA ISLAND, LOS ARCHIPELAGO, GUINEA: A NEW MINERAL SPECIES RELATED TO DOVYRENITE

Roumaite, (Ca,Na,square)(3)(Ca,REE,Na)(4)(Nb,Ti)[Si2O7](2)(OH)F-3, a new mineral species, occurs as a late-stage product in cavities of peralkaline nepheline syenites on Rouma Island, in the Los Archipelago, in Guinea; associated minerals are aegirine, albite, analcime, arfvedsonite, catapleiite, a eudialyte-group member, microcline, sodalite, sphalerite, steacyite and villiaumite. Roumaite is monoclinic, space group Cc, with cell parameters a 7.473(2), b 11.294(2), c 18.778(4) angstrom, beta 101.60(2)degrees, V 1552.5(6) angstrom(3), Z = 4. The modular structure of roumaite consists of stacks of tobermorite-like layers, "octahedral" layers and disilicate groups; it is closely related to dovyrenite.