Gi Young Jeong

Early Permian peak metamorphism recorded in U–Pb system of black slates from the Ogcheon metamorphic belt, South Korea, and its tectonic implication

Abstract We report Pb–Pb whole rock and uraninite CHemical Th–U–total Pb Isochron MEthod (CHIME) ages of carbonaceous black slates from the Ogcheon metamorphic belt, South Korea. The Pb isotopic data of whole rock samples yield 207 Pb/ 206 Pb ages of 283±33 and 291±13 Ma for two outcrops from the northeastern part of the belt. The uraninite CHIME age is estimated at 283±26 and 281±27 Ma for the northeastern and the middle part of the belt, respectively. All of the above ages are identical within error ranges, and represent the timing of peak metamorphism after the late Precambrian intraplate rifting. On the other hand, the 207 Pb/ 206 Pb whole rock age for the southwestern part of the belt is estimated at 194±27 Ma, probably representing the timing of contact thermal metamorphism associated with the intrusion of Jurassic granitic plutons. Rb–Sr isotopic data of the black slates do not define any meaningful isochron. The early Permian metamorphic age of this study does not support any tectonic scheme in favor of major tectonometamorphism at either the Silurian–Devonian or the Triassic time. Instead, it corroborates the probability that the two zones in the Ogcheon belt, the Ogcheon metamorphic belt and the Taebaegsan zone, were separated from each other before the development of major structural framework in the former. Our data do not support an idea that the Ogcheon belt corresponds to the continuation of the Triassic collision belt in east central China.

Long-range transport of giant particles in Asian dust identified by physical, mineralogical, and meteorological analysis

The paper by Jeong et al. discusses a special event, an incursion of dus over Korea that contained a high concentration of large dust particles. Giant aeros l p rticl s are a rather neglected topic in atmospheric science, even though there is plenty of evidence that they contribute significantly to the mass and volume of atmospheric mineral dust. The introduction gives a concise story why smaller particles have been more exciting to study for atmospheric chemists, but also gives enough reasons to study the larger ones. In this study, particles from the 2012 dust event are analyzed in detail and the results compared to those from two earlier dust events.

Formation of vermicular kaolinite from halloysite aggregates in the weathering of plagioclase

Halloysite and kaolinite coexist in anorthosite weathering profiles in the Sancheong district, Korea. X-ray diffraction (XRD) analysis on a transect of partially weathered anorthosites reveals an increasing amount of kaolinite development with weathering age. Microtextures were examined by scanning electron microscopy (SEM) of thin sections and raw samples in an attempt to resolve the genetic relation between halloysite and kaolinite. In the earliest stage of weathering, halloysite ellipsoids and short tubes form on the plagioclase surfaces. The successive formation of new grains from the early grains results in globular aggregates where halloysite grains are partially interconnected. With the further progress of weathering, the halloysite grains coalesce from the inner part of the aggregate outward, and convert into stacked kaolinite plates. Continuous addition of halloysite grains on existing plates and their subsequent conversion into plates form long vermicular kaolinite. Solid-state transformation is suggested as a major conversion mechanism. Concurrently with its successive transformation into kaolinite, halloysite also continuously precipitates, giving rise to overgrowths on vermicular kaolinite. Halloysite forms as a kinetically favored metastable precursor and is subsequently transformed into thermodynamically stable kaolinite. Halloysite and kaolinite coexist temporarily in the weathering profile far from equilibrium.

Boxwork fabric of halloysite-rich kaolin formed by weathering of anorthosite in the Sancheong area, Korea

Boxwork fabric in which numerous thin or thick halloysite walls are interconnected into a microscopically porous cellular pattern is widely developed in the halloysite-rich kaolin formed by weathering of anorthosite in Sancheong, Korea. Studies using optical microscopy, scanning electron microscopy, and transmission electron microscopy have been carried out in order to elucidate the detailed features and origin of the boxwork.In the early stage of weathering, halloysite spheres formed in etch pits on the walls of microstructural discontinuities in the slightly weathered rock. With further weathering, the halloysite spheres grew to discs or flattened globules, which in turn coalesced to form large planar halloysite plates amid narrow fissures. The halloysite plates were detached by dissolution of the plagioclase in groundwater. Continued growth of the halloysite tubes in the plates resulted in the wrinkling of the plates. Finally, the plagioclase was completely dissolved by groundwater, leaving the boxwork of wrinkled halloysite walls and large pores. The relatively high rigidity of the boxwork is due to the compact agglomeration of halloysite tubes within the wrinkled halloysite walls.Cation balance calculation shows that Al was significantly mobilized during the formation of the boxwork in the weathering environment. The well-developed microfissures, the high dissolution rate of the calcic plagioclase, and the rapid flow of groundwater in a mountainous topography with relatively steep (20°) slope have been the factors controlling the formation of the porous boxwork in the halloysite-rich kaolin of the Sancheong area.

THE DEPENDENCE OF LOCALIZED CRYSTALLIZATION OF HALLOYSITE AND KAOLINITE ON PRIMARY MINERALS IN THE WEATHERING PROFILE OF GRANITE

The formation of kaolin-group minerals in the weathering profile of granite, under the humid, temperate climate as found in Korea, was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron microprobe analysis (EMA). The granite was gradually weathered to saprolite. K-rich feldspar was not weathered in the profile, but plagioclase partially weathered to halloysite septa (i.e., wall-like masses). At the bottom of the profile, biotite had weathered to regularly interstratified biotite-vermiculite (B-V), and subsequently to kaolinite, with a considerable increase in grain volume. In the upper part of the profile, loose aggregates of transported clays, including halloysite and kaolinite, coated the preformed halloysite septa in the weathered plagioclase. Halloysite had precipitated as a metastable phase in the microfissures of partially weathered plagioclase. Kaolinite had precipitated heavily in the weathered biotite, where surfaces supply abundant templates facilitating the nucleation of kaolinite. The localized crystallization of halloysite and kaolinite, depending on the distribution of primary minerals, strongly influenced the kaolin mineralogy of the granite weathering profile.

Secondary mineralogy and microtextures of weathered sulfides and manganoan carbonates in mine waste-rock dumps, with implications for heavy-metal fixation

Abstract The secondary mineralogy and microtextures of weathered waste-rock dumps derived from the mining of galena-sphalerite ore in quartz veins containing manganoan carbonates were examined using back-scattered electron imaging, X-ray diffraction, and chemical analysis. Sphalerite, pyrrhotite, and arsenopyrite were coated or replaced by iron oxyhydroxides in the earliest stage of the weathering, and were then replaced by sulfur. Galena shows a thin alteration rim of anglesite. Oxidation of pyrite has resulted in porous boxworks of Fe oxyhydroxides. The relative resistance to oxidation, from most resistant to least resistant, was observed to be pyrite ≈ galena > arsenopyrite ≈ sphalerite > pyrrhotite. Rhodochrosite dissolved to form hydrohetaerolite pseudomorphs, and manganoan calcite has an outer alteration rim of hydrohetaerolite and an inner zone of smithsonite. Rock and mineral fragments were cemented by fine aggregates of plumbojarosite, Fe oxyhydroxides/sulfates, and manganates. Microchemical analysis and sequential extractions showed a close association of As with Fe oxyhydroxides/sulfates, of Pb, Cu, Zn, and As with plumbojarosite, and of Pb and Zn with manganates. Despite their lower acid-neutralizing capacity, manganoan carbonates played an important role in the fixation of Pb and Zn by the formation of manganates.

Mineralogy, chemistry, and formation of oxidized biotite in the weathering profile of granitic rocks

Abstract Biotite was oxidized in deep saprolitic weathering profiles developed on granitic rocks in a humid temperate climate in the Youngju-Andong area of South Korea. The mineralogy and chemistry of these oxidized biotites were characterized by chemical analysis, electron microscopy, X-ray diffraction, thermal analysis, and radiogenic Ar analysis. The results showed that a decrease in the bo-dimension, loss of radiogenic Ar, and formation of vermiculite are fairly well correlated with the degree of oxidation of ferrous iron. The chemical composition of oxidized biotite was modified by a non-stoichiometric removal of interlayer and octahedral cations to compensate for the charge imbalance induced by oxidation of Fe. The pervasive loss of cations and radiogenic Ar suggests their diffusion through oxidizing biotite in a non-expanded state. Iron oxidation and cation loss caused a decrease in the bo-dimension with the formation of discontinuities that acted as conduits for the weathering solutions, resulting in partial vermiculitization (<10%). The Fe oxidation was nearly completed in the lower part of the profile, concomitant with mineralogical and chemical modification to oxidized biotite that persists throughout the profile without further notable modification. Cation release from biotite is governed in early stage by the formation of oxidized biotite, and later by its decomposition.

Vermicular kaolinite epitactic on primary phyllosilicates in the weathering profiles of anorthosite

The microtextural changes in the kaolinization of primary phyllosilicates including biotite, sericite, clinochlore and muscovite were investigated by scanning electron microscopy (SEM) and microchemical analysis of thin sections of weathered anorthosite. Kaolinization began at grain edges and propagated toward the interior. Grains were highly fanned out from the edges and exfoliated into several flakes along the basal cleavages, producing lenticular voids. Finally, long vermicular kaolinite pseudomorphs were formed after primary phyllosilicates. Statistical analysis showed a ninefold increase in volume during the kaolinization of biotite, suggesting that most AI in the kaolinite was imported from ambient weathering solution. Weathering primary phyllosilicates supplied templates suitable for the thick epitactic overgrowth of kaolinite to form long vermicular pseudomorphs. AI was sufficiently available due to the intense weathering of soluble anorthosite. Although present in small amounts, primary phyllosilicates gave high volumetric and mineralogical contributions to the weathering profiles by facilitating kaolinite precipitation.

Black carbon pollution of speleothems by fine urban aerosols in tourist caves

Abstract Speleothems in the karst caves of South Korea, which receive many visitors, are losing their aesthetic appeal due to black coloring. Mineralogical, textural, and chemical analyses were conducted on the speleothems to discover the cause of the discoloration. An abrupt color change from the natural color seen in the inner zones to the black color of the outer zones suggests that pollution commenced just after the opening of caves to visitors, and has continued since then. The main mineral compositions of both the outer black and the inner layers are the same, but the concentration of non-carbonate carbon is much higher in the black layers than in the inner layers. Electron microscopy showed that chain-like agglomerates (ca. 0.2-1.1 μm diameter) of sub-micrometer carbon spheres (ca. 0.02-0.05 μm diameter) are absent from the inner layer but present in the black layer, as well as in the cave aerosol. On the basis of their sub-micrometer size, agglomeration pattern, and composition, the carbon spheres and their agglomerates are considered to originate mostly from automobile exhaust. They are presumed to have been carried into the caves by visitors from urban environments and then deposited on the surface of growing speleothems. Protection of speleothems from discoloration requires control of these fine anthropogenic aerosols.

Relations of As concentrations among groundwater, soil, and bedrock in Chungnam, Korea: Implications for As mobilization in groundwater according to the As-hosting mineral change

Arsenic (As) concentrations and As-bearing minerals in bedrock and soil, and their relations with groundwater concentrations were investigated in a small agricultural area of Korea. The As concentration of the bedrock shows a wide variation (<0.5-3990 mg/kg) and is well correlated with that in the contacting groundwaters (23-178 μg/L). Soils, the weathering product of bedrock, show the lower and more dispersed As concentrations (8.8-387 mg/kg) than the bedrock. But the soil As concentrations are very high relative to those reported from other areas. The As concentrations in the shallow groundwaters are comparatively low (<20 μg/L) and are independent of the soil concentration. Arsenopyrite is the major As-bearing mineral in the bedrock and its oxidation controls the As levels in deep groundwater. In contrast, As mostly resides in soil as Fe-(hydr)oxide-bound forms. Due to low pH and oxidizing redox condition, the release of As from Fe-(hydr)oxides is largely suppressed, and the shallow groundwater shows low As concentrations generally satisfying the drinking water limit. However, it is suggested that the disturbance of soil geochemical conditions by land use changes would cause a serious As contamination of the shallow groundwaters.