Minerals

The presence of “horsetail” inclusions—hair-like inclusions diverging in bundles from the central part of the grain towards its periphery—is considered a sign of the Ural origin of demantoid, although such inclusions are not unique to the Urals. Demantoid grains are usually rounded and have a spherulitic or sectorial structure (spherocrystals). Well-formed crystals are rare, but they contain the same inclusions, which emerge at the surface as hollow channels with a polygonal cross-section. The question arises: are these growth features or the result of dissolution? Demantoid grains and crystals were studied by optical methods, scanning electron microscopy (SEM) and EBSD mapping. It was established that when a cluster of demantoid crystal nuclei arose, their growth proceeded under conditions of geometrical selection and was blocked by adjacent crystals. Grains located at the periphery of the aggregate continued to grow in the direction of their sector. In nodules from serpentine–carbonate veins, the grains are separated by serpentine, whereas in oriented intergrowths of crystal nuclei with euhedral crystals, serpentine interlayers are absent. In demantoid crystals, the cross-sections of the hollow channels have the shape of a parallelogram or rhombus, a square and an equilateral triangle, which possibly allows them to be assigned to negative crystals. Demantoid crystals have a sectorial structure (spherocrystals) with numerous newly nucleated spherocrystals expressed by hyperboloids of revolution with induction surfaces and by hollow-channel inclusions. The cause of the sectorial growth of demantoid may be decompression conditions.

This study targets mineral exploration, specifically the precise identification of basement structures that may have influenced the depositional framework of the economically and industrially important phosphate-bearing sequence. Airborne magnetic data from the Wadi Gasus area, Eastern Desert, Egypt, were analyzed to reveal the subsurface structures governing phosphate distribution and formation, using the Tilt Angle method, Source Parameter Imaging (SPI), and Global Particle Swarm Optimization (GPSO). The reduced-to-pole (RTP) magnetic map shows anomalies ranging from −283 to 707 nT. The Tilt Angle method indicates source depths of 395 to 950 m (average about 672 m), while SPI yields depths of 98 to 1276 m (average about 687 m). GPSO was applied to four profiles perpendicular to the strike of high-amplitude RTP anomalies, estimating source depths of 68.92 to 185.65 m, amplitude coefficients (A_f) of 627.94 to 5764.71 nT, shape factors (S_f) of 0.97 to 1.3, and horizontal origins (x_o) of 1505.41 to 2316.87 m. The computed depths are consistent with established geological findings. GPSO’s distinct advantage is that it determines not only source depth but also the complete set of source parameters, including location, shape, and amplitude, offering a more holistic understanding of subsurface geology than traditional depth-focused methods. These findings provide indirect structural constraints on the basement framework beneath the phosphate-bearing sedimentary cover and complement direct geological and geochemical exploration in similar settings.

Coal gangue is a complex mineral assemblage in which weathering processes regulate mineral dissolution, alkalinity evolution, and element mobility. Understanding these processes is important for evaluating the environmental behavior of gangue-derived materials exposed to seasonal weathering conditions. This study investigated the leaching behavior of alkaline coal gangue from Xuzhuang Coal Mine in Pei County, Xuzhou City, Jiangsu Province, China, under laboratory-simulated rainfall, wet-dry cycling, and freeze–thaw cycling. Initial solution pH strongly affected leaching behavior: more acidic conditions increased the release of Cr, Mn, Fe, Cu, and Zn, whereas higher initial pH resulted in greater cumulative alkalinity release. Under initial solution pH values of 5.0, 6.0, and 6.5, leachate pH decreased from 9.67–9.70 after the first leaching cycle to 8.28–8.45 after the fifteenth cycle, indicating persistent alkaline buffering during repeated leaching. Applied water volume affected alkalinity and elemental release differently: cumulative alkalinity release was highest at 400 mL per cycle, whereas the release of individual elements varied with water volume. Wet-dry cycling produced limited changes at early stages but, after prolonged treatment, increased Cr, Mn, Fe, and Cu release, while Zn showed rapid initial release followed by a decline. Freeze–thaw cycling also altered element mobility, with the largest changes generally observed at intermediate stages of cycling. Under freeze–thaw conditions, Cr release reached 652.57 mg kg⁻¹ after 3 cycles, suggesting enhanced Cr mobility during early-stage freeze–thaw weathering. The observed trends are consistent with combined mineral dissolution and physical weathering, although direct mineralogical and microstructural evidence after cycling is still required. These results identify rainfall acidity and repeated weathering as key environmental factors that should be considered when evaluating alkaline coal gangue as a reclamation substrate in temperate mining areas.