Rüdlingerite, Mn²⁺₂V⁵⁺As⁵⁺O₇·2H₂O, a New Species Isostructural with Fianelite

• Main Authors: Philippe Roth, Nicolas Meisser, Fabrizio Nestola, Radek Škoda, Fernando Cámara, Ferdinando Bosi, Marco E. Ciriotti, Ulf Hålenius, Cédric Schnyder, Roberto Bracco
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Minerals
• Subjects: rüdlingerite; new mineral species; fianelite; vanadium; arsenic; crystal structure
• Online Access: https://www.mdpi.com/2075-163X/10/11/960

The new mineral species rüdlingerite, ideally Mn²⁺₂V⁵⁺As⁵⁺O₇·2H₂O, occurs in the Fianel mine, in Val Ferrera, Grisons, Switzerland, a small Alpine metamorphic Mn deposit. It is associated with ansermetite and Fe oxyhydroxide in thin fractures in Triassic dolomitic marbles. Rüdlingerite was also found in specimens recovered from the dump of the Valletta mine, Canosio, Cuneo, Piedmont, Italy, where it occurs together with massive braccoite and several other As- and V-rich phases in richly mineralized veins crossing the quartz-hematite ore. The new mineral displays at both localities yellow to orange, flattened elongated prismatic, euhedral crystals measuring up to 300 μm in length. Electron-microprobe analysis of rüdlingerite from Fianel gave (in wt%): MnO 36.84, FeO 0.06, As₂O₅, 25.32, V₂O₅ 28.05, SiO₂ 0.13, H₂O_calc 9.51, total 99.91. On the basis of 9 O anions per formula unit, the chemical formula of rüdlingerite is Mn_1.97(V⁵⁺_1.17 As_0.83Si_0.01)_Σ2.01O₇·2H₂O. The main diffraction lines are [<i>d_obs</i> in Å (<i>I_obs</i>) <i>hkl</i>]: 3.048 (100) 022, 5.34 (80) 120, 2.730 (60) 231, 2.206 (60) 16-1, 7.28 (50) 020, 2.344 (50) 250, 6.88 (40) 110, and 2.452 (40) 320. Study of the crystal structure showcases a monoclinic unit cell, space group <i>P</i>2₁<i>/n</i>, with <i>a</i> = 7.8289(2) Å, <i>b</i> = 14.5673(4) Å, <i>c</i> = 6.7011(2) Å, <i>β</i> = 93.773(2)°, <i>V</i> = 762.58(4) ų, <i>Z</i> = 4. The crystal structure has been solved and refined to <i>R</i>₁ = 0.041 on the basis of 3784 reflections with <i>F</i>_o > 4σ(<i>F</i>). It shows Mn²⁺ hosted in chains of octahedra that are subparallel to [-101] and bound together by pairs of tetrahedra hosted by V⁵⁺ and As⁵⁺, building up a framework. Additional linkage is provided by hydrogen-bonding through H₂O coordinating Mn²⁺ at the octahedra. One tetrahedrally coordinated site is dominated by V⁵⁺, <i>^T</i>⁽¹⁾(V_0.88As_0.12), corresponding to an observed site scattering of 24.20 electrons per site (eps), whereas the second site is strongly dominated by As⁵⁺,<i>^T</i>⁽²⁾(As_0.74V_0.26), with, accordingly, a higher observed site scattering of 30.40 eps. The new mineral has been approved by the IMA-CNMNC and named for Gottfried Rüdlinger (born 1919), a pioneer in the 1960–1980s, in the search and study of the small minerals from the Alpine manganese mineral deposits of Grisons.