Paola Bonazzi

Hydropyrochlore, ideally (H₂O,)₂Nb₂(O,OH)₆(H₂O), is a cubic mineral (space group Fd3m, a = 10.56-10.59 Å, Z = 8) belonging to the pyrochlore supergroup (general formula: A_2−mB₂X_6−wY_1−n). The K-rich variety of this species is unique to the Lueshe syenitic-carbonatitic deposit (D.R. Congo), where it occurs as the alteration product of primary (Ca,Na)₂Nb₂O₆F pyrochlores. The structure of this mineral is made of a B₂X₆ (B = Nb, Ti; X = O, OH) framework that generates tunnels along the [110] direction, where the interstitial sites are partially occupied by water molecules and minor amounts of different cations. These features form the basis for the ion-exchange properties of hydropyrochlore, making it a promising candidate as a mineral sink for heavy metals (e.g., Tl⁺) dispersed in aqueous matrices, with interesting environmental implications. Tl⁺ incorporation was induced through imbibition experiments in a diluted Clerici solution using single crystals of hydropyrochlore from Lueshe (D.R. Congo); the modifications induced by Tl⁺ incorporation were then evaluated through single-crystal X-ray diffraction (SCXRD), electron microprobe analyses (EMPA) and Fourier transform infrared (FT-IR) spectroscopy. After Tl⁺ imbibition, a dramatic increase of the A-site electron density (n.e⁻ from ~4 to ~60) confirms the entry of a substantial amount of Tl⁺ at this site (up to about 70%), leading to a lengthening of the A-X distances and the consequent expansion of the unit cell. A decrease of the site scattering at the Y' site (from ~9 to ~4 e⁻) also occurs, suggesting a loss of aqueous species. Although the predominance of neutral H₂O molecules at the interstitial sites of hydropyrochlore from Lueshe is widely accepted by the mineralogical community, our crystal-chemical and FT-IR data provide evidence that the dominant species might be the hydronium ion, with significant implications on the nomenclature of the pyrochlore supergroup. Understanding the crystallographic aspects of hydropyrochlore as a potential waste form for monovalent thallium immobilization not only addresses a pressing environmental concern, but also contributes to the broader field of waste management.

Dienerite, ideally Ni₃As, was discovered in 1919 near Radstadt (Salzburg, Austria) and its description and chemical characterization date back to the 1920s. The paucity of reliable experimental data, as well as the absence of any other documented occurrences of such a mineral in over 80 years, led to the supposition of a typographic error in the transcription of the original chemical analysis, suggesting the mineral might in fact be nickelskutterudite [(Ni,Co,Fe)As₃]. As a consequence, the mineral was discredited and deleted in the post-2006 IMA list of valid mineral species. Nonetheless, several minerals having a metal/As ratio close to 3:1 and a description fitting that of dienerite were reported after its discreditation. Here we report the discovery of minute inclusions in a sample of josephinite from Josephine Creek (Oregon, USA) exhibiting high optical and electron reflectance. Structural and chemical investigations unequivocally showed that a mineral having cubic structure [a = 9.6206(9) Å, sp. gr. I 3d; R1 = 0.0353] and ideal chemical formula Ni₃As does exist, suggesting that dienerite could in fact be a valid species. The proposal to revalidate dienerite has been approved by the Commission on New Minerals, Nomenclature and Classification (IMA-Proposal 19-E). The neotype is deposited in the mineralogical collections of the Natural History Museum, University of Florence, Italy, under catalogue number 3364/I.

Kesebolite-(Ce), ideal formula CeCa₂Mn(AsO₄)[SiO₃]₃, is a new mineral (IMA No. 2019-097) recovered from mine dumps at the Kesebol Mn-(Fe-Cu) deposit in Västra Götaland, Sweden. It occurs with rhodonite, baryte, quartz, calcite, talc, andradite, rhodochrosite, K-feldspar, hematite, gasparite-(Ce), chernovite-(Y) and ferriakasakaite-(Ce). It forms mostly euhedral crystals, with lengthwise striation. The mineral is dark grayish-brown to brown, translucent, with light brown streak. It is optically biaxial (+), with weak pleochroism, and n_calc = 1.74. H = 5–6 and VHN₁₀₀ = 825. Fair cleavage is observed on {100}. The calculated density is 3.998(5) g·cm⁻³. Kesebolite-(Ce) is monoclinic, P2₁/c, with unit-cell parameters from X-ray single-crystal diffraction data: a = 6.7382(3), b = 13.0368(6), c = 12.0958(6) Å, β = 98.578(2)°, and V = 1050.66(9) ų, with Z = 4. Strongest Bragg peaks in the X-ray powder pattern are: [I(%), d(Å) (hkl)] 100, 3.114 (20-2); 92, 2.924 (140); 84, 3.138 (041); 72, 2.908 (014); 57, 3.228 (210); 48, 2.856 (042); 48, 3.002 (132). The unique crystal structure was solved and refined to R1 = 4.6%. It consists of 6-periodic single silicate chains along (001); these are interconnected to infinite (010) strings of alternating, corner-sharing MnO₆ and AsO₄ polyhedra, altogether forming a trellis-like framework parallel to (100).

The new mineral species, eliopoulosite, V₇S₈, was discovered in the abandoned chromium mine of Agios Stefanos of the Othrys ophiolite, located in central Greece. The investigated samples consist of massive chromitite hosted in a strongly altered mantle tectonite, and are associated with nickelphosphide, awaruite, tsikourasite, and grammatikopoulosite. Eliopoulosite is brittle and has a metallic luster. In plane-reflected polarized light, it is grayish-brown and shows no internal reflections, bireflectance, and pleochroism. It is weakly anisotropic, with colors varying from light to dark greenish. Reflectance values of mineral in air (R_o, R_e’ in %) are: 34.8–35.7 at 470 nm, 38–39 at 546 nm, 40–41.3 at 589 nm, and 42.5–44.2 at 650 nm. Electron-microprobe analyses yielded a mean composition (wt.%) of: S 41.78, V 54.11, Ni 1.71, Fe 1.1, Co 0.67, and Mo 0.66, totali 100.03. On the basis of Σ_atoms = 15 apfu and taking into account the structural data, the empirical formula of eliopoulosite is (V_6.55Ni_0.19Fe_0.12Co_0.07Mo_0.04)_{Σ = 6.97}S_8.03. The simplified formula is (V, Ni, Fe)₇S₈ and the ideal formula is V₇S₈, which corresponds to V 58.16%, S 41.84%, total 100 wt.%. The density, based on the empirical formula and unit-cell volume refined form single-crystal structure XRD data, is 4.545 g·cm⁻³. The mineral is trigonal, space group P3₂21, with a = 6.689(3) Å, c = 17.403(6) Å, V = 674.4(5) ų, Z = 3, and exhibits a twelve-fold superstructure (2a × 2a × 3c) of the NiAs-type subcell with V-atoms octahedrally coordinated by S atoms. The distribution of vacancies is discussed in relation to other pyrrhotite-like compounds. The mineral name is for Dr. Demetrios Eliopoulos (1947–2019), a geoscientist at the Institute of Geology and Mineral Exploration (IGME) of Greece and his widow, Prof. Maria Eliopoulos (nee Economou, 1947), University of Athens, Greece, for their contributions to the knowledge of ore deposits of Greece and to the mineralogical, petrographic, and geochemical studies of ophiolites, including the Othrys complex. The mineral and its name have been approved by the Commission of New Minerals, Nomenclature, and Classification of the International Mineralogical Association (No. 2019-96).

Two new monoclinic (P2₁/m) epidote supergroup minerals manganiakasakaite-(La) and ferriakasakaite-(Ce) were found in the small Mn ore deposit of Monte Maniglia, Bellino, Varaita Valley, Cuneo Province, Piedmont, Italy. Manganiakasakaite-(La) occurs as subhedral grains embedded in pyroxmangite. Its empirical formula is ^A(1)(Ca_0.62Mn²⁺_0.38) ^A(2)(La_0.52Nd_0.08Pr_0.07Ce_0.07Y_0.01Ca_0.25) ^M(1)(Mn³⁺_0.52Fe³⁺_0.28Al_0.18V³⁺_0.01) ^M(2)Al_1.00 ^M(3)(Mn²⁺_0.60Mn³⁺_0.27Mg_0.13) ^T(1−3)(Si_2.99Al_0.01) O₁₂ (OH), corresponding to the end-member formula CaLaMn³⁺AlMn²⁺(Si₂O₇)(SiO₄)O(OH). Unit-cell parameters are a = 8.9057(10), b = 5.7294(6), c = 10.1134(11) Å, β = 113.713(5)°, V = 472.46(9) ų, Z = 2. The crystal structure of manganiakasakaite-(La) was refined to a final R₁ = 0.0262 for 2119 reflections with F_o > 4σ(F_o) and 125 refined parameters. Ferriakasakaite-(Ce) occurs as small homogeneous domains within strongly inhomogeneous prismatic crystals, where other epidote supergroup minerals coexist [manganiandrosite-(Ce), “androsite-(Ce)”, and epidote]. Associated minerals are calcite and hematite. Its empirical formula is ^A(1)(Ca_0.64Mn²⁺_0.36) ^A(2)(Ce_0.37La_0.17Nd_0.06Pr_0.03Ca_0.35□_0.02) ^M(1)(Fe³⁺_0.61Al_0.39) ^M(2)Al_1.00 ^M(3)(Mn²⁺_0.64Mn³⁺_0.33Fe³⁺_0.02Mg_0.01) ^T(1−3)Si_3.01 O₁₂ (OH), the end-member formula being CaCeFe³⁺AlMn²⁺(Si₂O₇)(SiO₄)O(OH). Unit-cell parameters are a = 8.9033(3), b = 5.7066(2), c = 10.1363(3) Å, β = 114.222(2)°, V = 469.66(3) ų, Z = 2. The crystal structure of ferriakasakaite-(Ce) was refined to a final R₁ = 0.0196 for 1960 unique reflections with F_o > 4σ(F_o) and 124 refined parameters.