Scorticoite, Mn ²⁺ ₆ (Sb ⁵⁺ □)Σ2(SiO ₄ ) ₂ O

Scorticoite (IMA 2018 -159), ideally Mn 6(Sb5+□)(SiO4)2O3(OH)3, is a new member of the welinite group discovered in the Mn ore deposit of Scortico -Ravazzone, Apuan Alps, Tuscany, Italy. It occurs as prismatic crystals, up to 0.1 mm in size, reddish -black in color, with a reddish -brown streak. Luster is vitreous. It is brittle, with a conchoidal fracture. Electron microprobe analysis gave (in wt%): WO 3 9.02, Nb 2O5 0.06, Sb 2O5 10.02, SiO 2 16.05, VO2 0.02, Al2O3 0.09, MgO 0.60, MnO 54.55, Fe 2O3 2.63, FeO 2.34, H 2O 3.50, total 98.88. The empirical chemical formula, based on 2 Si atoms per formula unit (apfu) and assuming an amount of OH to give 14 anions pfu, is (Mn5.76Fe2+ 0.24)Σ6.00(Sb5+ 0.46W6+ 0.29Fe3+ 0.25Mg0.11Al0.01□0.88)Σ2.00(SiO4)2O3.09(OH)2.91. Unit -cell parameters are a = 8.2052(13), c = 4.7737(10) Å, V = 278.33(11) Å3, Z = 1. The crystal structure was solved in the trigonal space group P3 to R1 = 0.0239 for 1540 unique reflections with F > 4σF and 78 least-square parameters. Scorticoite is isotypic with other welinite -group minerals. It occurs in a rhodochrosite vein, in association with sarkinite, friedelite, sonolite, and baryte . In the same occurrence, welinite was identified as tabular prismatic orange crystals up to 0.5 mm in https://doi.org/10.1180/mgm.2026.10194 Published online by Cambridge University Press 2 association with rhodochrosite and sonolite. Electron microprobe analysis of this mineral gave (in wt%): MoO 3 0.30, WO 3 19.45, As 2O5 0.67, Sb 2O5 0.14, SiO 2 14.95, VO 2 0.08, Al2O3 0.14, MgO 0.32, CaO 0.04, MnO 5 4.03, Mn2O3 2.32, Fe2O3 0.12, F 0.67, H2O 3.01, O = −F −0.28, total 9 5.96. The empirical formula, based on (Si + As) = 2 apfu, is (Mn2+ 5.99Ca0.01)Σ6.00 (W6+ 0.66Mo6+ 0.02Sb5+ 0.01V4+ 0.01Mn3+ 0.23Al0.02Fe3+ 0.01Mg0.06□0.98)Σ2.00 [(Si0.975As0.025)O4]2 O3.09 [(OH)2.63F0.28]Σ2.91. Unit-cell parameters are a = 8.2032(3), c = 4.8170(3) Å, V = 280.72(3) Å3, Z = 1. Several issues related to the space-group symmetry of welinite (P3 or P63) are discussed, as well as the relations between scorticoite and örebroite.