Saneroite: chemical and structural variations of manganese pyroxenoids with hydrogen bonding in the silicate chain
Nagashima, Mariko; Armbruster, Thomas
European Journal of Mineralogy Volume 22 Number 3 (2010), p. 393 - 402
published: Jun 1, 2010
ArtNo. ESP147052203006, Price: 29.00 €
Saneroite, Na1−1.5Mn5[Si5O14(OH)](Si,V,As)O3(OH) (Z = 2, space group P1), 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 Lβ and Mn Lα 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.