Marianoite, a new member of the cuspidine group of minerals, occurs in phlogopite–calcite silicocarbonatite in the western part of the Mesoproterozoic Prairie Lake intrusive complex, in northwestern Ontario, Canada. The mineral forms flattened prismatic crystals with resorbed faces, up to 0.3 mm in length, and is associated with uranoan pyrochlore, titanite and natrolite–muscovite pseudomorphs after xenocrystic nepheline. The crystals are translucent, very pale yellow macroscopically and colorless in plane-polarized light. The mineral is biaxial negative (α 1.700, β 1.715, γ 1.725, 2Vmeas 80°, 2Vcalc 78°) and shows a weak optic-axis dispersion (r < v). Marianoite is relatively homogeneous in composition; its average empirical formula is Na1.93(Ca4.00Mn0.04)∑4.04 (Nb0.97Zr0.90Ti0.09Fe0.08Mg0.03Hf0.01)∑2.08 (Si3.97O14)O2.93F1.07. By analogy with wöhlerite, the simplified formula of marianoite should be written Na2Ca4(Nb,Zr)2(Si2O7)2(O,F)4. The structure of the new mineral species, refined by single-crystal methods to an R1 of 4.65% (for | Fo | > 4σF), is monoclinic, space group P21; the unit-cell parameters are: a 10.8459(15) Å, b 10.2260(14) Å, c 7.2727(10) Å, β 109.332(3)°, V 761.1(3) Å3 (Dcalc 3.45 g/cm3). The mineral is isostructural with wöhlerite [Na2Ca4(Zr,Nb)2(Si2O7)2(O,F)4], but shows the preponderance of Nb in the smallest octahedrally coordinated cation sites in its crystal structure. The occupancy of these sites cannot be determined accurately because of the similar X-ray scattering characteristics and ionic radii of Nb5+ and Zr4+. The mineral is named in honor of Anthony Nicola Mariano (b. 1930), in recognition of his contributions to the study of alkaline rocks and carbonatites.