Mineralogy and Petrology最新文献

This study presents a detailed flow-by-flow petrographic modal description and major element data integrated into the stratigraphic framework for the central part of the southeastern Ethiopian plateau (the Chole Section). The stratigraphic data are placed in the context of the regional stratigraphic framework and correlated with existing geochronology of the province, allowing us to estimate the timing and duration of volcanic activity in the area. Based on petrographic variations and physical volcanological features, the Chole section is divided into: (1) lower basalt; (2) middle basalt; and (3) upper basalt. The phase assemblages and major element data show that the lower and upper basalts underwent possible fractionation involving olivine, plagioclase, and augite. In contrast, the middle basalt equilibrium phase and major element data exhibits low-degree fractionation involving olivine, clinopyroxene, and orthopyroxene at greater depths. The stratigraphic position and the petrographic data suggests, the lower and middle basalt successions likely correlate with the Oligocene to Early Miocene basaltic succession (28–15.5 Ma) of the southeastern Ethiopian volcanic province. The presence of several volcanic hiatuses and the decrease in flow thickness in the middle basalt suggest a waning of magmatic activity during this period. The upper basalt of the Chole section correlates with the upper Miocene basalt of the southeastern volcanic province (15.5–11 Ma). From this, we determine that volcanic activity in the southeastern Ethiopian volcanic province is well constrained within the broader tectono-magmatic framework of the East African volcanic province.

The Early Cretaceous gold deposits in the North China Craton are known as Jiaodong-type gold deposits. Here, we summarize a quantity of reported (including this study) pyrite trace element, δ³⁴S, and Pb isotope data from the Jiaodong, Xiaoqinling, and Central Taihangshan gold fields to comprehensively reveal the characteristics of the ore-forming fluids in Jiaodong-type gold deposits. The results show that the Au concentrations in the pyrites from the Jiaodong-type gold deposits are typically less than 8.12 ppm and do not correlate with As but have significant positive correlations with Bi and Te, implying that Bi and Te may govern Au enrichment in pyrite. The overall As concentrations in the pyrites are relatively low, and only the Jiaodong gold field is significantly higher, which may be caused by fluid flowing through As-rich metamorphic sedimentary rocks. In Jiaodong-type gold deposits, Au is dominantly found as visible gold, followed by invisible gold. Invisible gold generally occurs as a solid solution (Au⁺), and only the proportion of nanoparticles (Au⁰) in the Xiaoqinling gold field is slightly higher (20%). Sulfidation and fluid immiscibility or boiling were the key mechanisms leading to visible gold precipitation. The Co/Ni ratio, δ³⁴S, and Pb isotopes indicate that the ore-forming fluids in the Jiaodong-type gold deposits exhibit remarkable magmatic features, and the ore-forming materials are primarily derived from a mixture of lower crust and mantle sections. Among them, the ore-forming materials from the lower crust of the Central Taihangshan gold field are slightly higher.

As part of a systematic investigation into the ternary system CaO-Fe₂O₃-SiO₂, we discovered a phase with a general chemical composition of A₁₄O₂₀ (where A represents Ca, Fe, and Si) and previously unknown symmetry. Synthesis experiments were conducted at 1200 °C in air with an oxide ratio of CaO:Fe₂O₃:SiO₂ = 3:5:1 in the starting mixture. In addition to β-Ca₂SiO₄, and hematite (Fe₂O₃), powder X-ray diffraction analysis indicated the presence of compounds related to the aluminum-free counterpart of a silico-ferrite of calcium and aluminum (SFCA), one of the major bonding phases in iron ore sinter. Single crystals of this so-called SFC phase, large enough for diffraction experiments, were found in the sintered pellet fragments under a digital microscope. The compound with composition Ca_2.68Fe_10.32Si_1.00O₂₀ crystallizes in space group I2/c and has the following basic crystallographic data: a = 10.4643(10) Å, b = 15.2740(11) Å, c = 5.3050(5) Å, β = 110.017(9)°, V = 796.69(13) ų and Z = 2. The final composition, as determined by crystal structure refinement, differs slightly from the weight-specified starting mixture of Ca₃Fe₁₀SiO₂₀ and suggests the presence of both Fe(III) and small amounts of Fe(II) in the sample. The crystal structure is related to that of the triclinic polytype-1A of SFC, but exhibits a higher degree of disorder due to the partial occupation of additional octahedrally and tetrahedrally coordinated sites. This results in a smaller unit cell and an increased space-group symmetry. The described phase can be regarded as a basic or family structure, from which the two simplest polytypes (1A and hypothetical 2M) can be derived through ordering processes of cations among different possible vacancies. The chemical similarity to one of the phases of the SFCA-family suggests that such disordered compounds could also occur in industrial sinters.

A single crystal of guérinite from the Plaka Mine, Lavrion, Greece, was examined by X-ray diffraction. The mineral crystallizes in the monoclinic space group P2₁/n with unit cell parameters a = 17.628 (2), b = 6.728 (1), c = 23.384 (3) Å, β = 90.68 (1)°, V = 2773.4 (6) ų. The originally reported mineral formula Ca₅(HAsO₄)₂(AsO₄)₂∙9H₂O, Z = 5 should be revised to Ca₆(HAsO₄)₃(AsO₄)₂∙10.5H₂O, Z = 4. Accordingly, guérinite is no longer considered a polymorph to ferrarisite. The structure is characterized by complex layers consisting of corner- and edge sharing of CaO_n (n = 7, 8) polyhedra and arsenate groups. The connection between these units is achieved exclusively via hydrogen bonding.

Dawsonite, a hydrated carbonate, is a key mineral studied for Carbon Capture and Storage (CCS) initiatives. It forms in high pCO₂ environments, enabling gas storage in a solid state within geological reservoirs, thereby helping mitigate greenhouse gas emissions. The Rio Bonito Formation has gained attention as a potential CO₂ reservoir due to its favorable characteristics such as porosity, permeability, depth, thickness, organic matter content, and the presence of an effective sealing layer (Palermo Formation), particularly in the central region of the Paraná Basin. This study reveals the natural occurrence of dawsonite within the Rio Bonito Formation in the southern part of the Paraná Basin, in Rio Grande do Sul State, Brazil. Dawsonite was identified in quartz sandstones through petrographic analysis, indicating its formation during mesodiagenesis, where it crystallized within moldic pores. The presence of dawsonite was further confirmed through scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX) and X-ray diffraction (XRD) techniques. This discovery marks the first documented occurrence of dawsonite within the Rio Bonito Formation. It suggests that under similar conditions, other sections of the Rio Bonito Formation may also include dawsonite, thereby expanding the potential for onshore CCS in the Paraná Basin.

Karlleuite, ideally Ca₂MnO₄, is a newly approved accessory mineral found in the xenolith sample within the basaltic lava from the Caspar quarry, Bellerberg volcano, Eifel, Germany. It usually occurs as thin tabular/plate crystals, which range from 40 to 80 μm in diameter, and is associated with other members of the perovskite supergroup such as srebrodolskite, brownmillerite, sharyginite, perovskite, and lakargiite distributed within rock-forming minerals represented by reinhardbraunsite, fluorellestadite, fluorapatite, larnite, gehlenite, and several hydrated Ca aluminosilicates. Karlleuite crystals are brown with sub-metallic lustre, a light brown streak, and a good cleavage along (001). It is non-fluorescent, brittle and has an uneven fracture, a Mohs hardness of 3.5 and calculated density D_x = 3.79 g/cm³. The empirical formula of the holotype karlleuite calculated based on O = 4 atoms per formula is (Ca_1.97Ce³⁺_0.06)_2.03(Mn^4 + _0.39Ti_0.36Fe³⁺_0.19Al_0.09)_1.03O₄, which shows that it is a multicomponent phase characterised by various substituents at the octahedral site. Karlleuite is tetragonal I4/mmm (no. 139), with a = 3.7683(2) Å, c = 11.9893(8) Å, V = 170.254(17) ų, and Z = 2. The calculated strongest lines in the X-ray powder diffraction pattern are [d in Å (I) hkl]: 5.995 (43), 2.742 (100), 2.665 (91), 2.023 (25), 1.998 (28), 1.884 (61), 1.553 (38), 1.371 (24). The new mineral is the first natural phase which exhibits a first order of Ruddlesden-Popper type structure, which indicates a modular nature and consists of Ca(Mn, Ti, Fe, Al)O₃ perovskite layers, packed between CaO rock-salt layers arranged along the c-axis. Raman spectroscopy supports the interpretation of the chemical and structural data. Mineral association, structural data, as well as the study of the synthetic Ca-Mn-O system suggest that karlleuite could form under high-temperature conditions, above 1000˚C.

Thorite, as a principally thorium (Th)-bearing mineral, is an important indicator for Th mineralization. However, its occurrence and enrichment processes are still discussed and debated. Here, a unique occurrence of thorite, discovered in Nubian granodiorite rather than in highly evolved granites from southwestern Egypt, is reported. This report presents data derived from optical and backscattered electron (BSE) microscopy and energy-dispersive X- ray spectrometry (EDS) analyses conducted on the thorite and its host rock. The Nubian granodiorite thorites are viewed as secondary, not primary products. Two distinct types of secondary thorites are identified that are referred to as type A thorite and type B thorite herein. Type A thorite occurs as small grains that are enclaved in a fine-grained matrix of altered oligoclase and ferrohornblende, and clinochlore. Thorite grains, up to 100 μm in size are characterized by corona-type structures comprising of clinochlore and hematite with some barite. Their sources are most likely hydrothermal solutions occurring during an alteration stage and having relatively high conditions of sulfate activity. Type B thorite, on the other hand, forms crystallites in altered domains of magmatic allanite-(Ce), ranging in size from ~ 0.1 to ~ 10 μm. Formation of Type B thorite is a direct result of fluid-driven alteration processes, since it requires the in situ-redistribution of elements, particularly thorium, silicon, and uranium. Thorite types A and B are composed mainly of thorium uranium silicate, with variable minor amounts of Y, Al, Ce, Nd, Fe, Ca, Na, Mg, P, and Cl. Thorite compositions are within the range reported for uranothorites from other occurrences.

The origin of magmatic microgranular enclaves has been investigated in the Mesoproterozoic granitoid Krasnopol intrusion (1.5 Ga), part of the AMCG (anorthosite–mangerite–charnockite–granite) Mazury Complex in the East European Craton (NE Poland). The granitoids are ferroan and metaluminous, and display the typical characteristics of A-type granites, with high contents of Zr, Nb, Ga and rare earth elements (REEs). The enclaves are metaluminous and have a broad compositional range with two groups distinguished: silica-poor (45–50 wt% SiO₂) and silica-rich (54 to 59 wt% SiO₂), the latter overlapping in composition with the granitoid samples. The silica-poor enclaves are enriched in REEs compared to the silica-rich type, while the silica-rich enclaves exhibit trace-element patterns similar to those of the granitoids. Initial whole rock ε_Nd values range between -3.8 and -4.0 for the granitoids and give a slightly wider range from -2.6 to -3.8 for the enclaves. The ⁸⁷Sr/⁸⁶Sr initial values vary from 0.7084 to 0.7138 for the granitoids and between 0.7052 and 0.7075 for the enclaves and indicate that the granitoids and enclaves are not isotopically identical. These may suggest that the two magmatic systems represented by the granitoid host rock and the enclaves, were probably derived from different sources, but with sufficient interaction, which led to a progressive change in the composition of the enclaves towards intermediate composition. We suggest that the mafic melts of the enclaves were generated at the base of the thickened crust through partial melting of the lower crustal source, with a significant contribution from mantle material. The increase in temperature resulted in anatexis of the lower crust and the formation of the granitoid parental magma.

The ca. 480 Ma Albești granite (Southern Carpathians, Romania) is characterized by the presence of color zoned blue quartz grains, and is part of the rather extensive European Cambro-Ordovician blue quartz landscape. The color is heat sensitive, fading at temperatures as low as 300℃, inconsistent with the thermally stable, light scattering, nanometric rutile/ilmenite inclusions cited in literature. Extensive X- and Q-band electron paramagnetic resonance (EPR) investigations were carried out, searching for distinctive features of the Albești quartz that are directly or indirectly involved in the generation of the blue coloration. The analyzed quartz grains were extracted from three granite samples of varying coloration and anisotropy, and the quartz from each rock sample was further separated into colored and colorless fractions. The paramagnetic E’ and [AlO₄]⁰ centers, as well as Mn²⁺ ions localized in traces of amorphous associated minerals at grain boundaries or fissure planes, were observed in all quartz samples. Broad EPR lines associated with the presence of magnetic clusters were observed in the spectra of the white quartz sample and the corresponding colorless one. Isochronal annealing up to 500℃ induced the correlated recombination of the E’ and [AlO₄]⁰ centers, the strong decrease of the Mn²⁺ spectrum and the formation of a minority iron oxide phase at the grain boundaries and/or fissure planes. The EPR signature was similar for the colored and the corresponding colorless quartz samples, before and after annealing, showing that the heat sensitive coloration of the Albești quartz does not directly involve the presence of paramagnetic defects and/or minority magnetic phases.