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

Clino-ferro-suenoite, with the ideal formula VACMn2Fe2+5Si8O22(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 0.5mm in length, are subhedral with a pale greenish-yellow colour. The mineral is non-pleochroic and optically biaxial (–), with alpha = 1.670(5), beta = 1.690(5) and gamma = 1.705(5); orientation is yIIb, 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 AVACB(Mn2+0.95Fe2+0.87Ca0.16Na0.01)Sigma1.99C(Fe2+3.57Mg1.38Mn2+0.03Zn0.01Al0.01)Sigma5.00 TSi8.00O22 W[(OH)1.99F0.01]Sigma2.00, 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, (20-2); 1.526, 16, (0,12,0); 1.412, 19, (66-1). Unit-cell parameters obtained from single-crystal diffraction data (Mo Kalpha) are a = 9:59840(10), b = 18:3179(2), c = 5:33450(10) Å, beta = 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 > 2sigma(I ). Mg ions show a preference for the octahedrally coordinated C sites in the order M(2) >M(1) >M(3).