• journal_title：The Canadian Mineralogist
• Contributor：Anton R. Chakhmouradian ; Roger H. Mitchell ; Peter C. Burns ; Yulia Mikhailova ; Ekaterina P. Reguir
• Publisher：Mineralogical Association of Canada
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.46.4.1023
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：46
• issue：4
• firstpage：1023
• section：Articles

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, 2V_meas 80°, 2V_calc 78°) and shows a weak optic-axis dispersion (r < v). Marianoite is relatively homogeneous in composition; its average empirical formula is Na_1.93(Ca_4.00Mn_0.04)_∑4.04 (Nb_0.97Zr_0.90Ti_0.09Fe_0.08Mg_0.03Hf_0.01)_∑2.08 (Si_3.97O₁₄)O_2.93F_1.07. 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 | F_o | > 4σ_F), 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) ų (D_calc 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.