The fate of an old Mn–Fe amphibole species: description of clino-ferro-suenoite, □Mn₂Fe²⁺₅Si₈O₂₂(OH)₂

Abstract. Clino-ferro-suenoite, with the ideal formula □Mn2Fe52+Si8O22(OH)2, previously named “dannemorite” or “manganogrunerite”, is formally recognized as a monoclinic member of the amphibole supergroup (IMA-CNMNC 2024-032). It occurs in iron–manganese skarn from the Hilläng mines, Dalarna, Sweden, associated with fayalite, spessartine, ferro-actinolite, calcite, magnetite and pyrite. It formed by partial replacement of manganoan fayalite during retrograde metamorphism. Individual crystals, up to 0.5 mm in length, are subhedral with a pale greenish-yellow colour. The mineral is non-pleochroic and optically biaxial (–), with α=1.670(5), β=1.690(5) and γ=1.705(5); orientation is y||b, z⋀c=14(1)°. Cleavage parallel to {110} and {010} is perfect and good, respectively. The Mohs hardness is about 6, and the mean Vickers hardness number was established as 872. The calculated density value is 3.46(1) g cm−3. The crystal chemical formula of clino-ferro-suenoite is A□B(Mn0.952+Fe0.872+Ca0.16Na0.01)Σ1.99 C(Fe3.572+Mg1.38 Mn0.032+Zn0.01Al0.01)Σ5.00 TSi8.00O22 W[(OH)1.99F0.01]Σ2.00, based on microprobe analyses, spectroscopic data (Mössbauer, infrared, Raman) and a single-crystal X-ray diffraction study. Infrared bands originating from O–H vibration modes appear at 3608, 3620, 3638, 3653 and 3668 cm−1, with the first and last ones having weak intensities. The seven strongest reflections in the X-ray powder pattern (d values (in Å), Irel, (hkl)) are the following: 8.33, 100, (110); 3.278, 21, (240); 3.084, 72, (310); 2.784, 18, (330); 2.512, 19, (202¯); 1.526, 16, (0,12,0); 1.412, 19, (661¯). Unit-cell parameters obtained from single-crystal diffraction data (Mo Kα) are a=9.59840(10), b=18.3179(2), c=5.33450(10) Å, β=102.1630(10)° and V=916.87(2) Å3 for Z=2. Refinement of the crystal structure in the space group C2/m yielded R=2.25 % for 1379 reflections with I0>2σ(I). Mg ions show a preference for the octahedrally coordinated C sites in the order M(2)>M(1)>M(3).