Mineralogy and Petrology最新文献

A second study of ferriallanite-(Ce) from Nya Bastnäs, Sweden, extends current data by using electron probe micro-analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis and brings new insights about its crystal chemistry obtained by Raman spectroscopy. The study presents the first Raman spectra for ferriallanite-(Ce) member of the allanite group (not considering the rather low-quality spectra published in preceding papers). The material does not show significant radiation damage, which is rare as allanite-group minerals often have undergone metamictisation due to significant amounts of incorporated radionuclides (U, Th). Some interior regions show pronounced zoning that correlates with variations in Raman-band positions. In spite of its significant REE content, the material is virtually non-luminescent. New additional data for allanite-(Ce) from Oßling, Germany and Domanínek, Czech Republic are also presented, which were used for comparison.

The microporous crystal structure of zemannite, Mg(H₂O)₆[Zn²⁺Fe³⁺(TeO₃)₃]₂·nH₂O, n ≤ 3, was re-investigated based on single-crystal X-ray diffraction data measured at 298 ± 0.5 K, 200 ± 1 K and 100 ± 3 K. So far, zemannite was described in space group P6₃ exhibiting a pronounced pseudosymmetry (P6₃/m). All refinements confirm the [Zn²⁺Fe³⁺(TeO₃)₃]¹⁻ framework topology with the extra-framework constituents (Mg atoms and H₂O molecules) being located within the channels along [001]. Measurements on a sample from the type locality revealed the unexpected occurrence of 00l reflections with l = 2n + 1, which clearly violate the 6₃ screw-axis symmetry. The minor but significant intensities of the low-order 00l reflections are assigned to the small differences in the scattering power between the Fe and Zn atoms; thus, the Zn and Fe cations are partly ordered between crystallographically distinct sites within the framework. In addition, the low symmetry allows a full order of the extra-framework atoms for the first time. A series of comparative refinement models were performed in the space groups P6₃/m, P6₃, P(overline{6}), and P3. A fully ordered arrangement of the extra-framework guest atoms confirms the earlier postulated theoretical structure model with a hexahydrated Mg²⁺ ion besides additional interstitial H₂O molecules. The final refinements in space group P3 yield R1 ≤ 0.025 for the entire data sets measured at the distinct temperatures (2θ_max = 101.4°, MoKα radiation). The polarity of the arrangement in the channels is restricted to individual domains of equal twin fractions related by a mirror plane parallel to (0001).

Three ammonium-iron-sulfites (AIS) from a burning coal dump in an abandoned open coal pit at Pécs-Vasas (Mecsek Mountains, South Hungary) were identified: (NH₄)₉Fe³⁺(SO₃)₆ (AIS-1), (NH₄)₂Fe²⁺(SO₃)₂ (AIS-2), and (NH₄)₂Fe³⁺(OH)(SO₃)₂·H₂O (AIS-3). They were formed by the interaction of decomposing iron sulfides and ammonia released from organic matter. AIS-1 and AIS-2 are metastable; they break down in a few weeks (AIS-1) respectively years (AIS-2). AIS-1 forms red, stubby columnar to thick tabular crystals up to 0.2 mm in length. AIS-2 appears as brown tabular to short prismatic crystals up to 0.1 mm, often they create columnar intergrowths. AIS-3 is more stable. It was approved as a new mineral species (mineral name kollerite, IMA-CNMNC 2018–131). Sprays of natural kollerite up to 1.5 mm are composed of yellow, long-prismatic or lath-like crystals up to 0.1 mm in length. AIS-1 is characterized by powder-X-ray diffraction only. The crystal structures of AIS-2 [synthetic material, R(overline {3})m, a = 5.3879(8), c = 19.980(4) Å] and kollerite [Cmcm, a = 17.803(15), b = 7.395(5), c = 7.096(5) Å] were investigated by single-crystal X-ray diffraction. AIS-2 is topologically equivalent to bütschliite. Isolated Fe²⁺O₆ polyhedra are corner-connected to sulfite anions. 2D nets with composition [Fe²⁺(SO₃)₂]²⁻ are parallel to (0001). Kollerite crystallizes in a new structure type. The FeO₆ octahedra are corner linked to buckled [Fe³⁺(OH)(SO₃)₂]²⁻ chains. In both cases, ammonium cations are intercalated. Connection is verified by hydrogen bonds only; all H atom positions are located experimentally.

Abstract

Kobellite is a Pb-Bi-Sb sulfosalt with minor amounts of (Cu, Fe) and with the crystal structure composed of two types of rods, one of which has unusual lateral extensions (‘lobes’), which depart from the usual lozenge-shaped rod cross-section in sulfosalts. Several Pb-Bi-Sb and Pb-Sb-rich sulfosalts form a small group built on similar principles. Some of them are related by homology (e.g., izoklakeite), and differ by the perpendicular dimensions of rods (length and multiplicity of atomic layers in a rod; e.g., sterryite), and especially by different combinations of archetypes and archetype portions which participate in the rods (PbS archetype and the two orientations of SnS archetype). The present article summarizes and discusses the published data on the group. Homeotypism makes the group interesting and potentially a fertile source of further structural varieties.

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.