Zoharite, (Ba,K)6 (Fe,Cu,Ni)25S27, and Gmalimite, K6□Fe2+24S27—New Djerfisherite Group Minerals from Gehlenite-Wollastonite Paralava, Hatrurim Complex, Israel

Zoharite (IMA 2017-049), (Ba,K)6 (Fe,Cu,Ni)25S27, and gmalimite (IMA 2019-007), ideally K6&#9633:Fe2+24S27, are two new sulfides of the djerfisherite group. They were discovered in an unusual gehlenite&ndash:wollastonite paralava with pyrrhotite nodules located in the Hatrurim pyrometamorphic complex, Negev Desert, Israel. Zoharite and gmalimite build grained aggregates confined to the peripheric parts of pyrrhotite nodules, where they associate with pentlandite, chalcopyrite, chalcocite, digenite, covellite, millerite, heazlewoodite, pyrite and rudashevskyite. The occurrence and associated minerals indicate that zoharite and gmalimite were formed at temperatures below 800 &deg:C, when sulfides formed on external zones of the nodules have been reacting with residual silicate melt (paralava) locally enriched in Ba and K. Macroscopically, both minerals are bronze in color and have a dark-gray streak and metallic luster. They are brittle and have a conchoidal fracture. In reflected light, both minerals are optically isotropic and exhibit gray color with an olive tinge. The reflectance values for zoharite and gmalimite, respectively, at the standard COM wavelengths are: 22.2% and 21.5% at 470 nm, 25.1% and 24.6% at 546 nm, 26.3% and 25.9% at 589 nm, as well as 27.7% and 26.3% at 650 nm. The average hardness for zoharite and for gmalimite is approximately 3.5 of the Mohs hardness. Both minerals are isostructural with owensite, (Ba,Pb)6(Cu,Fe,Ni)25S27. They crystallize in cubic space group Pm3&macr:m with the unit-cell parameters a = 10.3137(1) &Aring: for zoharite and a = 10.3486(1) &Aring: for gmalimite. The calculated densities are 4.49 g&middot:cm&minus:3 for the zoharite and 3.79 g&middot:cm&minus:3 for the gmalimite. The primary structural units of these minerals are M8S14 clusters, composed of MS4 tetrahedra surrounding a central MS6 octahedron. The M site is occupied by transition metals such as Fe, Cu, and Ni. These clusters are further connected via the edges of the MS4 tetrahedra, forming a close-packed cubic framework. The channels within this framework are filled by anion-centered polyhedra: SBa9 in zoharite and SK9 in gmalimite, respectively. In the M8S14 clusters, the M atoms are positioned so closely that their d orbitals can overlap, allowing the formation of metal&ndash:metal bonds. As a result, the transition metals in these clusters often adopt electron configurations that reflect additional electron density from their local bonding environment, similar to what is observed in pentlandite. Due to the presence of shared electrons in these metal&ndash:metal bonds, assigning fixed oxidation states&mdash:such as Fe2+/Fe3+ or Cu+/Cu2+&mdash:becomes challenging. Moreover, modeling the distribution of mixed-valence cations (Fe2+/3+, Cu+/2+, and Ni2+) across the two distinct M sites&mdash:one located in the MS6 octahedron and the other in the MS4 tetrahedra&mdash:often results in ambiguous outcomes. Consequently, it is difficult to define an idealized end-member formula for these minerals.