Arsenotučekite, Ni18Sb3AsS16, a new mineral from the Tsanglichromitites, Othrys ophiolite, Greece

Arsenotučekite, Ni18Sb3AsS16, is a new mineral discovered in the abandoned chromium mine of Tsangli, located in the easternportion of the Othrys ophiolite complex, central Greece. Tsangliis one of the largest chromite deposit at which chromite was minedsince 1870. The Tsangli chromitite occurs as lenticular and irregular bodies. The studied chromitites are hosted in a stronglyserpentinized mantleperidotite. Arsenotučekite forms anhedral to subhedral grains that vary in size between 5μmupto100μm,and occurs as single phase grains or is associated with pentlandite, breithauptite, gersdorffite and chlorite. It is brittle and has ametallic luster. In plane-polarized light, it is creamy-yellow, the bireflectance is barely perceptible and the pleochroism is weak. Incrossed polarized reflected light, the anisotropic rotation tints vary from pale blue to brown. Internal reflections were not observed.Reflectance values of arsenotučekite in air (Ro,Re′in %) are: 41.8–46.4 at 470 nm, 47.2–50.6 at 546 nm, 49.4–52.3 at 589 nm, and51.3–53.2 at 650 nm. The empirical formula of arsenotučekite, based on 38 atoms per formula unit, and according to the structuralresults, is (Ni16.19Co1.01Fe0.83)Σ18.03Sb3(As0.67Sb0.32)Σ0.99S15.98. The mass density is 6.477 g·cm−3. The simplified chemical formulais (Ni,Co,Fe)18Sb3(As,Sb)S16. The mineral is tetragonal and belongs to space groupI4/mmm,witha= 9.7856(3) Å,c= 10.7582(6)Å,V=1030.2(6) Å3andZ= 2. The structure is layered (stacking along thec-axis) and is dominated by three different Ni-coordination polyhedral, one octahedral and two cubic. The arsenotučekite structure can be considered as a superstructure oftučekite resulting from the ordering of Sb and As. The name of the new mineral species indicates the As-dominant of tučekite.Arsenotučekite occurs as rims partly replacing pentlandite and irregularly developed grains.Furthermore, it is locally associatedwith chlorite. These observations suggest that it was likely precipitated at relatively low temperatures during: 1) the late hydrother-mal stages of the ore-forming process by reaction of Sb- and As-bearing solutions with magmatic sulfides such as pentlandite, or 2)during the serpentinization of the host peridotite. The mineral and its name have been approved by the Commission of NewMinerals, Nomenclature, and Classification of the International MineralogicalAssociation (number 2019–135)