Mineralogy and Petrology最新文献

High-aluminous, fluoro-titanites (~ 6.8–11.5 wt% Al₂O₃, up to ~ 3.8 wt% F) from a suite of calc-silicate granulites in the Chotanagpur Granite Gneiss Complex, East Indian shield, were examined to investigate the controls on Al incorporation in titanite. The studied high aluminous titanites have the third highest Al content (X_Al= up to ~ 0.46), reported from low to medium-pressure rocks till date. These titanites develop in three different associations (association 1, 2 and 3) along with the F-bearing hydrous minerals like amphibole or vesuvianite. These three associations occur as veins and patches close to the pegmatitic veins that intruded the granulite facies calc-silicate rocks. The titanite in the host calc-silicate rock (association 4), away from the pegmatite veins, preserves an anhydrous assemblage: garnet-clinopyroxene-plagioclase and low Al titanite (Al₂O₃ = 3.4–3.8 wt%, F ~ 0.8 wt%). Integrating field features, petrography and textural modeling, it is suggested that infiltration of F-bearing aqueous fluid, presumably derived from the pegmatites, into the host calc-silicate rock was responsible for the partial destabilization of the anhydrous assemblage 4, and formation of the Al-F rich titanite bearing assemblages 1–3. The published information and close proximity of the association 1–4 outcrops suggest that the infiltration-driven growth of Al-F-rich titanite occurred virtually under isothermal-isobaric conditions (5.5–6.5 kbar and 650–750 °C). The titanite in associations 1–3 show a positive correlation between Al₂O₃/TiO₂ and F/OH indicating the substitution Ti^+ 4 +O²⁻ = Al^+ 3+ (F + OH)⁻. Based on the findings of the present study, combined with the published information on titanite chemistry, it is argued that the f_F2 present in the system plays a dominant role, if not the most important, in regulating the extent of Al substitution in titanites, in addition to pressure, temperature or coexisting mineral assemblage.

The long-known presence of a sharp OH absorption band in the tetragonal fluoride mineral sellaite, MgF₂, inspired us to conduct a detailed study of the OH incorporation modes into this IR-transparent (where IR stands for Infrared) material as well as to search for hydrogen traces in two other IR-translucent halides—villiaumite (NaF) and fluorite (CaF₂). Among these three phases, sellaite is the only one to incorporate ‘intrinsic’ OH groups, most commonly as O–H∙∙∙F defects oriented nearly perpendicular to the c-axis along the shortest edge of the constituent MgF₆ polyhedra, in analogy with the isostructural mineral rutile, TiO₂. Another defect type, seen only scarcely in untreated natural material, develops when subjecting sellaite to temperatures above 900 °C. It involves an O–H∙∙∙O cluster along the 2.802 Å edge of the original MgF₆ dipyramid, as fluorine atoms are progressively expelled from the structure, being replaced by O^2- anions. This is corroborated by the appearance of spectral absorption features typical for brucite (Mg(OH)₂) and ultimately periclase (MgO), the presence of which could be proven via powder diffraction of the heat-treated material. Except for a ‘dubious’ peak most probably caused by included phases, neither villiaumite (NaF) nor fluorite (CaF₂) showed any presence of ‘intrinsic’ OH defects. They do however decompose along a similar route into the respective oxide and hydroxide phases at high temperature. This thermal decomposition of the studied halide phases is accompanied by the emission of gaseous (HF)_n species at temperatures well below their established melting point - a subject which seems to be quite overlooked.

Cawood et al. (this issue) critize our hypothesis of a pre-Klondikean weathering/oxidation event having affected the Aggeneys-Gamsberg ore district. Instead, they reinforce the long-held view that the sulfide deposits of the ore district with its pronounced metal zonation, its unusually high mineralogical variability and numerous geochemical anomalies are the product of amphibolite- to granulite-facies metamorphic overprint of originally syn-sedimentary exhalative deposits. Here we gladly use the opportunity to counter all issues raised and explain further our evidence of oxidation and subsequent re-sulfidation of the original synsedimentary deposits.

Höhn et al. (2021) proposed that the giant Gamsberg Zn deposit, South Africa, initially formed as a sedimentary exhalative (SEDEX) deposit during the Mesoproterozoic and was subsequently oxidized near surface. The oxidized ore was then supposedly sulfidized by sulfur-rich metamorphic fluids during and after upper amphibolite facies metamorphism. We view this model as untenable for various reasons and suggest that the Gamsberg deposit and others in the Aggeneys-Gamsberg district (Swartberg, Broken Hill-Deeps, Big Syncline) are metamorphosed clastic SEDEX deposits rather than having formed by synmetamorphic sulfidation processes.

During a systematic study on formation conditions of new compounds with zemannite-type structures, crystals of ten new oxidotellurate(IV) phases were grown under hydrothermal conditions, partially by employing a drastic reduction of the water content. The crystal structures of the obtained phases were determined by single-crystal X-ray diffraction. Na₂[Ni₂(TeO₃)₃]·2.5H₂O, K₂[Ni₂(TeO₃)₃]·H₂O, K₂[Zn₂(TeO₃)₃]·2H₂O, Rb_1.25[Co₂(TeO₃)₃]·1.5H₂O and Rb_1.24[Mn₂(TeO₃)₃]·2H₂O exhibit a unit-cell with hexagonal symmetry (Z = 2, a ≈ 9.3 Å, c ≈ 7.7 Å), in which most of the other compounds with a zemannite-type structure are known to crystallize. Relative to this unit-cell, K₂[Cu₂(TeO₃)₃]·2H₂O exhibits a twofold superstructure, K₂[Co₂(TeO₃)₃]·2.5H₂O a twofold superstructure with an additional incommensurate modulation, Na₂[Cu₂(TeO₃)₃]·1.5H₂O a threefold superstructure, and Rb_1.5[Mn₂(TeO₃)₃]·1.25H₂O and Cs[Mn₂(TeO₃)₃]·H₂O a fourfold superstructure. Disorder of the alkali metal cations and crystal water molecules in the channels as well as variable water contents complicate modelling and structure refinement.

Katsarosite, ideally Zn(C₂O₄)·2H₂O, named for Īraklīs Katsaros, is a new mineral found at the Esperanza Mine in the Kaminiza area of the Lavrion Mining District, Greece. Katsarosite usually occurs directly on sphalerite or embedded in jarosite and/or hydrozincite, often intimately intergrown with gypsum and overgrown by goslarite and/or epsomite. Crystal aggregates are mostly fine granular to earthy, with individual crystals being usually rounded with an average diameter of 30 µm, sometimes prismatic along [001] or platy, exhibiting the indistinct forms {100}, {001}, {110}, and {101}. Katsarosite is malleable with a Mohs hardness of 1½ – 2 and exhibits a perfect cleavage on {110}; the fracture is uneven in all other directions. The colour depends on the iron (Fe²⁺) content, ranging from pure white in almost Fe-free samples to yellow in Fe-rich specimens. It has a resinous luster and a white streak; no luminescence has been observed under either short- or long-wave ultraviolet radiation. Katsarosite is optically biaxial (+). Refractive indices measured at a wavelength of 589 nm are n_α = 1.488(2), n_β = 1.550(2), n_γ = 1.684(2), with 2V_obs = 71(3)°. Chemical analysis gave on average C₂O₃ 38.32 wt%, ZnO 38.99 wt%, FeO 1.92 wt%, and H₂O 19.04 wt% (the latter was deduced based on the crystal-structure refinement), with traces of MgO and MnO. The new mineral is readily soluble in dilute acids. Katsarosite is monoclinic, space group C2/c, with unit-cell parameters a = 11.768(3), b = 5.3882(12), c = 9.804(2) Å, β = 127.045(8)°, V = 496.2(2) ų (Z = 4). The strongest lines in the Gandolfi X-ray powder pattern [d_obs in Å, I_obs/I₁₀₀, (hkl)] are: 4.6745, 100, (200); 4.7678, 94, (20(overline{2 })); 2.9533, 51, (40(overline{2 })); 4.7030, 37, (1 (overline{1 },overline{1 })); 3.9266, 33, (002); 3.5686, 27, (111); 2.6574, 22, (1 (overline{1 },overline{3 })); 3.5992, 8, (1 (overline{1 },overline{2 })); 2.7032, 4, (020). The crystal structure was refined based on single-crystal X-ray diffraction data to R(F) = 0.08. The observed mass density of 2.50(2) g cm⁻³ compares well with the calculated value (2.508 g cm⁻³). Katsarosite belongs to the humboldtine group, whose crystal-structure type is well described for both isotypic minerals and synthetic compounds in the literature. The atomic arrangement in Zn(C₂O₄)·2H₂O is characterized by chains consisting of isolated ZnO₆ octahedra which are alternately linked along [010] via oxalate anions. These chains are interconnected through hydrogen bonds only, with Ow···O (with Ow denoting the O atom of the