노진환 Jin Hwan Noh , 이석훈 Seok Hoon Lee

DOI: Vol.15(No.3) 147-159, 2002

Weathered bentonites occcur as surficial alterations of some domestic bentonite deposits in the Tertiary formations. with the thickness of less than about 50 cm, along naturally-formed weathering surface with slopping in gentle. 7 A°-halloysite was found together with montmorillonite in the weathered bentonite. Compared to normal bentonite, the weathered one is generally more clay-rich and contains little amounts of original rock-forming minerals and residues. In the electron microscopy, fine-scale occurrence of the clay minerals tends to be somewhat discrete and segregated rather than closely associated. A curled margin of montmorillonite lamella is deformed to become obtuse in the weathered bentonite. Halloysite occurs as acicular to tubular crystals with the length of less than 2 μm and the width of about 0.3μm, which commonly forms bundle-shaped aggregates. Electron microscopic observations on the fine-scale occurrence and texture of the weathered bentonites indicate that the clay mineral transition from montmorillonite to halloysite has undergone without accompanying any intermediate phases of both clay minerals such as a mixed-layered type (M/H). The alteration reaction between these two clay minerals probably took place in the form of dissolution and precipitation mechanism in oxidation condition. An intense chemical leaching of SiO_2, Na, K and Ca might occur during the alteration reaction, forming a lot of dissolution cavity and residual concentration of Al_2O_3 and Fe, relatively. As the result of the chemical change, a favorable condition for halloysite formation seemed to be provided.

김영호 Young Ho Kim

DOI: Vol.15(No.3) 161-169, 2002

Several phase transition studies have been previously reported on ilmenite (FeTiO_3) at high pressure and temperature conditions. However, those results are not consistent with one another, and phase boundary between ilmenite and perovskite phases determined only from the quenching method may be not so reliable at all. Therefore, in-situ high pressure-temperature (hP-T) X-ray diffraction measurements were performed up to 19 GPa and 700℃ in a large volume press apparatus using synchrotron radiation. Experimental results show that perovskite phase is stable at pressures above 16 GPa, and transforms back to LiNbO_3 phase near 15 GPa at room temperature, and that the perovskite-ilmenite transition is back and forth near 15 GPa at 500℃. LiNbO_3 phase transforms to ilmenite at 13 GPa and 300℃ and at 10.8 GPa and 400℃, respectively. These data indicate that LiNbO_3 phase may have a stability region in the hP-T phase diagram and that the perovskite-ilmenite phase boundary would be quite different from that previously reported.

손병국 Byeong Kook Son , 고상모 Sang Mo Koh , 김학주 Hag Ju Kim

DOI: Vol.15(No.3) 171-181, 2002

Glauconization was investigated by morphological and chemical analysis of glaucony from the Pohang Tertiary marine sediments. The glaucony, which is present sparsely in turbidite sandstones, shows two distinct stages in morphology and chemistry. Crystallization of glaucony starts preferentially from the interior of pellet-like grains, then to the outer region of the grains with increasing K and Fe, and decreasing Al. Furthermore, smectite-like glaucony is evolved into illite-like glaucony through interstratified glauconite-smectite with increases in K and Al, and decrease in Fe.

정기영 Gi Young Jeong , 김혜빈 Hye Bin Kim

DOI: Vol.15(No.3) 183-194, 2002

Biotite and its weathering products in the weathering profile of Andong granite were examined using X-ray diffraction, chemical analysis, and electron microscopy, Major weathering product of biotite was oxidized biotite, which is decomposed into kaolinite in the upper part. Discrete vermiculite or hydrobiotite was not detected although minor vermiculite (5%) was randomly interstratified with oxidized biotite. Excess positive charge induced by iron oxidation was balanced by release of Fe (16%) and Mg (12%) from octahedral site and K (13%) from interlayer site. After slight chemical and structural modification induced by iron oxidation, oxidized biotite persists through the weathering profiles with partial decomposition in the upper part of the profile. Formation environments and dissolution experiments of oxidized biotite highly resistant to weathering are required to understand the elemental behavior in the surface environments on the biotite-bearing bedrocks.

여상진 Sang Jin Yeo , 이범한 Bum Han Lee , 성소영 So Young Sung , 김수진 Soo Jin Kim

DOI: Vol.15(No.3) 195-204, 2002

Mineralogy and the exposed surface area are two of the most important factors controlling dissolution and weathering rates of soils. The mixture of inorganic and organic materials of various size distributions and structures that constitute soils makes the calculation of weathering rates difficult. The surface area of soil minerals plays an important role in most of programs for calculating the weathering rates and critical loads. The Brunauer-Emmett-Teller (BET) measurement is recommended for the measurement of specific surface area. However, BET values measured without organic matter removal are in fact those for all the N_2-adsorbed surface areas, including the surfaces covered and aggregated with organisms. Surfaces occupied by organisms are assumed to be more reactive to weathering by organic activities. Therefore, the BET surface area difference before and after organic removal depicts the area occupied by organisms. The present study shows that the BET values after organic matter removal using H_2O_2 are larger than those without removal by 1.68∼4.87㎡/g. This implies that BET measurement without organic removal excludes the reactive area occupied by organisms and that the area occupied by organisms in soils is much larger than expected. It is suggested that specific surface area measurement for calculating weathering rates of mineral soils should be made before and after organic matter removal. The results of a column experiment are presented to demonstrate the potential retarding influence that this organic matter may have on mineral dissolution and weathering.

김창성 Chang Seong Kim , 최선규 Seon Gyu Choi , 최상훈 Sang Hoon Choi , 이인우 In Woo Lee

DOI: Vol.15(No.3) 205-220, 2002

The gold-silver deposits in the Gasado district were formed in the sheeted and stockwork quartz veins which fill the fault fractures in volcanic rocks. K-Ar dating of alteration sericite(about 70 Ma) indicates a Late Cretaceous age for ore mineralization. These veins are composed of quartz, adularia, carbonate, and minor of pyrite, sphalerite, chalcopyrite, galena, Ag-sulfosalts (argentite, pearceits, Ag-As-Sb-S system), and electrum. These veins are characterized by chalcedonic, comb, crustiform and feathery textures. Based on the hydrothermally altered mineral assemblages, regional alteration zoning associated with mineralization in the Gasado district is defined as four zones; advanced argillic (kaolin mineral-alunite-quartz), argillic (kaolin mineral-quartz), phyllic (quartz-sericite-pyrite) and propylitic (chlorite-carbonate-quartz-feldspar-pyroxene)zone. Phyllic and propylitic zones is distributed over the study area. However, advanced argillic zone is restricted to the shallow surface of the Lighthouse vein. Compositions of electrum ranges from 14.6 to 53.7 atomic % Au, and the depositional condition for mineralization are estimated in terms of both temperature and sulfur fugacity: T=245°∼285℃, logf_s2=10^-10∼10^-12. Fluid inclusion and stable isotope data show that the auriferous fluids were mixed with cool and dilute (158°∼253℃ and 0.9∼3.4 equiv. wt. % NaCl) meteoric water (δ^-18O_water=10.1∼8.0‰, δD=-68∼-64‰). These results harmonize with the hot-spring type of the low-sulfidation epithermal deposit model, and atrongly suggest that Au-Ag mineralization in the Gasado district was formed in low-sulfidation alteration type environment at near paleo-surface.

이석훈 Seok Hoon Lee , 김수진 Soo Jin Kim

DOI: Vol.15(No.3) 221-230, 2002

Weathering of biotite shows a biotite-vermiculite-kaolinite sequence at the early stage, but presents biotite-kaolinite sequence without a significant intermediate phase (vermiculite) at the late stage from the weathering profile of the granitic gneiss. Secondary 1:1 phyllosilicates are kaolinite and halloysite which show different weathering textures originated by a different formation mechanism. Kaolinitization began from the edges of biotite and propagated toward the interior of grain along a multilayered front. 10 A°layers of biotite are interleaving with 7 A°layers of kaolinite and c-axis of two phases is consistent. Kaolinite pseudomorph of biotite is isovolumetric, compared to the biotite boundary and includes many band-like porosities parallel to the cleavage. Platy kaolinite formed by 1:1 layer for layer replacement f biotite. Halloysitization proceeded outward from the grain edges which were foliated as fine flakes and bent at the right angle for cleavage. Halloysites were extensively fanning out and greatly increased the volume of grain. This indicated that halloysite tubes were formed by epitaxial overgrowth on the surface of biotite with import of Si and Al from the external solution by dissolution of plagioclase. These halloysites have abnormally high Fe content (∼11%).

노열 Yul Roh , 문희수 Hi Soo Moon , 송윤구 Yun Goo Song

DOI: Vol.15(No.3) 231-240, 2002

Microbial metal reduction influences the biogeochemical cycles of carbon and metals as well as plays an important role in the bioremediation of metals, radionuclides, and organic contaminants. The use of bacteria to facilitate the production of magnetite nanoparticles and the formation of carbonated minerals may provide new biotechnological processes for material synthesis and carbon sequestration. Metal-reducing bacteria were isolated from a variety of extreme environments, such as deep terrestrial subsurface, deep marine sediments, water near hydrothermal vents, and alkaline ponds. Metal-reducing bacteria isolated from diverse extreme environments were able to reduce Fe(Ⅲ), Mn(Ⅳ), Cr(Ⅵ), Co(Ⅲ), and U(Ⅵ) using short chain fatty acids and/or hydrogen as the electron donors. These bacteria exhibited diverse mineral precipitation capabilities including the formation of magnetite (Fe_3O_4), siderite (FeCO_3), calcite (CaCO_3), rhodochrosite (MnCO_3), vivianite [Fe_3(PO_4)_2·8H_2O], and uraninite (UO_2). Geochemical and environmental factors such as atmospheres, chemical milieu, and species of bacteria affected the extent of Fe(Ⅲ)-reduction as well as the mineralogy and morphology of the crystalline iron mineral phases. Thermophilic bacteria use amorphous Fe(Ⅲ)-oxyhydroxide plus metals (Co, Cr, Ni) as an electron acceptor and organic carbon as an electron donor to synthesize metal-substituted magnetite. Metal reducing bacteria were capable of CO_2 conversion into sparingly soluble carbonate minerals, such as siderite and calcite using amorphous Fe(Ⅲ)-oxyhydroxide or metal-rich fly ash. These results indicate that microbial Fe(Ⅲ)-reduction may not only play important roles in iron and carbon biogecochemistry in natural environments, but also be potentially useful for the synthesis of submicron-sized ferromagnetic materials.