Suenoite, □Mn₂Mg₅Si₈O₂₂(OH)₂, a new member of the amphibole supergroup from the Scortico–Ravazzone ore deposit (Apuan Alps, Tuscany, Italy)

Abstract. Suenoite (IMA 2019-075), ideally □Mn2Mg5Si8O22(OH)2, is a new member of the amphibole supergroup discovered in the Mn ore deposit of Scortico–Ravazzone, Apuan Alps, Tuscany, Italy. It occurs as colourless tabular striated crystals, up to 0.1 mm in length, associated with spessartine and baryte. The streak is white, and the lustre is vitreous. Mohs hardness is estimated between 5.5 and 6. Cleavage is perfect on {210}. The calculated density is 3.283 g cm−3. Suenoite is optically biaxial (+), with α=1.655(5), β=1.660(5), and γ=1.670(5) (in white light). 2Vmeas is 75(10)°, and 2Vcalc is 70.9°. The orientation is X=a, Y=b, and Z=c. Pleochroism was not observed, as suenoite is colourless. The empirical chemical formula of suenoite is A(□0.91Ca0.07Na0.02)Σ1.00 B(Mn1.642+Fe0.362+)Σ2.00 C(Mg3.56Fe0.912+Mn0.612+Zn0.02)Σ5.10 T(Si7.86Al0.06)Σ7.92 O22 W[(OH)1.92F0.08]Σ2.00, and it is based on electron microprobe analyses, infrared spectroscopy, and Mössbauer spectroscopy. The unit-cell parameters of suenoite are a=18.7508(12), b=18.1396(12), c=5.3173(3) Å, and V=1808.6(2) Å3, with space group Pnma. The crystal structure was refined to R1=0.0490 for 2236 unique reflections with F>4σF and 194 refined parameters. The origin of suenoite is probably related to the recrystallisation of the Scortico–Ravazzone Mn ore deposit during the Tertiary tectono-metamorphic events, under greenschist-facies conditions, affecting the rocks belonging to the Alpi Apuane metamorphic complex.