Saneroite: chemical and structural variations of manganese pyroxenoids with hydrogen bonding in the silicate chain

Saneroite, Na_1–1.5Mn₅[Si₅O₁₄(OH)](Si,V,As)O₃(OH) (Z = 2, space group P1̅), has only been reported from two localities: (1) Val Graveglia, Italy, and (2) Fianel, Switzerland. Samples from both localities were studied using electron-microprobe analysis (EMPA), semi-quantitative analysis of the Mn²⁺/Mn³⁺ ratio based on the ratio of the X-ray intensities of the Mn L_β and Mn L_α lines, and single-crystal X-ray diffraction methods. The structure has two Na sites (Na1 and Na2), which are 8-coordinated by O. Na1 is fully occupied whereas Na2 is partially occupied. Five crystallographically independent octahedral sites are occupied by Mn (mainly Mn²⁺ with 10–20 % Mn³⁺), with additional small amounts of Ca and V³⁺. An undulating pyroxenoid-like chain is built by five SiO₄ tetrahedra. A sixth tetrahedron, forming a branch of the chain, is occupied by V⁵⁺, Si⁴⁺ and As⁵⁺. Structure refinements converged to R₁ values of 2.47–2.89 %. In agreement with bond-valence analyses, OH groups are found at the O7 and O19 positions. The relationships between donor and acceptor oxygen atoms and their hydrogen bonds in saneroite are O19–H19···O16 and O7–H7···O11. The O19···O16 and O7···O11 distances are 2.868(2)–2.876(2) and 2.475(2)–2.477(2) Å, respectively. The latter hydrogen bond is extremely short and strong. Corresponding strong hydrogen-bonds were also found in scheuchzerite and several hydrous pyroxenoids with 3- and 5-periodic chains.