Tusionite, Mn (super 2+) Sn (super 4+) (BO 3 ) 2 , has been found at two new localities: Thomas Mountain, Riverside County, California, and Recice, Moravia, Czech Republic. At both localities, tusionite occurs in granitic pegmatites of the elbaite subtype, together with tourmaline, hambergite, danburite, hellandite and boromuscovite. Tusionite occurs as small tabular crystals in miarolitic cavities, and as thin flakes and rosettes in massive pegmatite; in the latter occurrence, it is commonly replaced by fine-grained cassiterite. The crystal structure, a 4.781(1), c 15.381(7) Aa, V 304.5(2) Aa 3 , R3, Z = 3, has been refined to an R index of 2.4% for 204 observed reflections measured with MoKalpha X-radiation. Tusionite is isostructural with dolomite, CaMg(CO 3 ) 2 . Electroneutrality constraints show Sn to be tetravalent and Mn to be divalent, and the observed mean bond-lengths are in accord with this: "Sn-O" = 2.055, "Mn-O" = 2.224 Aa. The variation in "M-O" as a function of cation radius is significantly nonlinear for the calcite-type structures, the values for M = Zn, Fe (super 2+) , Mn (super 2+) being approximately 0.01 Aa less than predicted by linear interpolation between magnesite and calcite. For the dolomite-type structures, variations in "A-O" (A representing Ca, Mn) and "B-O" (B representing Mg, Fe (super 2+) , Mn) as a function of cation radius are linear, but the two octahedra show very different behavior. The "B-O" distance shows a response similar to that of the calcite-type structures, except that it is approximately 0.025 Aa shorter for a given cation-radius; the "A-O" distance is approximately 0.025 Aa longer than the corresponding distance in calcite, but decreases in size with decreasing cation-radius more rapidly than in the calcite-type structures.