Mica Minerals Research Articles

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange, northeastern Nicaragua

The Siuna Serpentinite Mélange (SSM) is a subduction-zone-related complex that contains diverse blocks of igneous and sedimentary origin, overprinted by various metamorphic conditions. The SSM is located at the southern border of the Chortís block and marks the boundary between continental and oceanic crusts in the western margin of the Caribbean Plate. The serpentinite matrix mainly consists of lizardite/chrysotile, Cr-rich spinel, and relict orthopyroxene that suggest a harzburgitic protolith and an upper mantle supra-subduction zone origin. Blocks within the southern and central regions range from Jurassic pelagic sediments to mafic/intermediate igneous rocks that are metamorphosed to various degrees, ranging from prehnite-pumpellyite/greenschist to likely blueschist facies (e.g. riebeckite-bearing metashale) conditions. In contrast, the northern section encloses almost exclusively epidote-amphibolite facies metabasite blocks, and minor mica- and chlorite-rich rocks of metasomatic origin, respectively. Some of the epidote-amphibolite blocks contain relic garnet-rich zones embedded in an amphibole-rich matrix. The garnets appear to record two generations of growth and contain mineral inclusions such as amphibole, apatite, titanite, aegirine-augite, and quartz. Thermobarometric estimates for the garnet-rich zones and epidote-amphibolite-rich matrix suggest a prograde blueschist facies at ~1.2 GPa and 400–450°C, an eclogite facies metamorphic peak at 1.5–1.7 GPa and 565–614°C, and a post-peak epidote-amphibolite facies metamorphism. These pressure and temperature estimates indicate a classical clockwise PT path that has been observed in many palaeo-subduction zone environments worldwide. Phengite Ar–Ar dating of mica-rich rock yields 140 Ma and suggests an Early Cretaceous exhumation along the southern edge of the continental Chortís block.

Alkaline diagenesis and its effects on reservoir porosity: A case study of Upper Triassic Chang 7 Member tight sandstone in Ordos Basin, NW China

Petrologic features, diagenetic processes and origins of different pore types of Upper Triassic Chang 7 (7th member of Yanchang Formation) tight sandstones in the Ordos Basin were analyzed based on cast thin sections, scanning electron microscope (SEM), and X-ray diffraction analysis (XRD). The diagenetic processes caused by alkaline pore fluid and associated pores were studied in detail and the relationship between porosity evolution and hydrocarbon accumulation was analyzed. The Chang 7 tight sandstone commonly experienced alkaline environment and associated diagenesis, including quartz dissolution or replaced by mica, clay and carbonate minerals, carbonate cementation, abundant authigenic illite and chlorite with minor kaolinte and smectite, and albitization. Several types of associated pores were formed in the alkaline environment, including quartz dissolution pore, intercrystal pore of clay minerals, interlamination pore of hydrated mica and pores associated with albitization. The alkaline diagenesis mainly occurred in two periods during the burial process: B stage of early diagenesis and A2 stage of mesogenetic diagenesis. The abundant intercrystal micropores of clay minerals, which mainly occurred during the B stage of early diagenesis, seriously reduced the permeability of sandstone. As a result, the acid dissolution in the later stage was not intense enough due to the low permeability, so it is inferred that the sandstone had already been compacted very tight before the major hydrocarbon charging.

Sorption processes of radiocesium in soil and bedrock

Abstract Three recent studies on cesium sorption in soil and bedrock are reviewed. 137Cs, originating from fallouts of nuclear weapons tests and the Chernobyl accident was found to decrease in an exponential manner in forest soil, and the highest fraction in soil profiles was found in the organic layer. Also, the mineral layer below the organic layer contained a large fraction of cesium inventory but at depths below 20 cm only a very small fraction was observed. In the bedrock of Olkiluoto, where the final repository for spent nuclear fuel from the Finnish nuclear power plants will be constructed, mica mineral biotite plays the most important role in cesium sorption. The selectivity of biotite decreases in the order Cs > K > Na > Ca and the overall selectivity coefficient for Cs/Ca exchange was approximately five and seven orders of magnitude higher than those for Cs/Na and Cs/K exchange reactions, respectively. Ion exchange isotherms for Cs/Na and Cs/K exchange were modelled by assuming three different ion exchange sites: frayed edge sites (FES), basal plane sites and intermediate sites. The selectivity coefficients derived for these sites were successfully used to predict cesium sorption in a mica gneiss rock. Sorption of cesium in mineral soil layers from the Olkiluoto overburden were studied using three different approaches: model batch experiments, an in-situ method and calculations. All three approaches gave the same trend but the distribution coefficient values varied in range of one order of magnitude.

Studies of Geogenic Groundwater Contamination in Southwestern Anatolia, Turkey

The present study provides an overview of the geogenic contamination in Southwestern Anatolia based on its origin, occurrence and impacts. The Isparta, Çeltikçi, Tefenni, Çöl and Sandıklı groundwater basins, whose geogenic contaminations were determined, are considered in this study. Quaternary alluvium and Pliocene units are the most important aquifers. Groundwater is widely used for drinking, domestic and irrigation purposes in these basins. The increase of Al content in groundwater is originates from water-rock interaction as related to feldspar, kaolin and mica minerals within volcanic rocks. Weathering and dissolution of arsenic minerals, water–rock interactions, and geothermal processes cause groundwater to be enriched in As contents in the study area. The increase of Fe and Mn contents in groundwater are caused by magmatic rocks and sandstone levels of the Elmalı formation, which have iron and manganese ores due to water–rock interaction. The increase of F content in surface and groundwater originated from the solution of apatite and more commonly from the solution of fluoride - bearing micas and amphiboles within Miocene clastics, Gölcük pyroclastics and volcanic rocks in the Isparta region. Groundwater quality is supremely important for people's health. In these basins Mn, Al, Fe, As and F contents of groundwater reached up to 433μg/l, 3540μg/l, 6190μg/l, 21.8μg/l and 5620μg/l, respectively. When compared to drinking water guidelines established by WHO [22,23] Turkey and the EPA, these elements are above the critical values in the different sampling time as locally.

鉱物科学から断層摩擦のメカニズム解明に挑む

Maximum friction coefficients of common minerals can be characterized by empirical Byerlee's rule, however, some important fault-forming minerals such as mica and clay minerals have ultimately low friction coefficient. The layered structure and high affinity to water should be related to the low friction coefficient. We have investigated the interlayer bonding energy, adsorption energy of water molecules, and physical properties of adsorbed water by using molecular simulations, surface x-ray scattering, and surface forces measurements. These results were used for interpreting the weak friction coefficients of layered minerals.

Hooked on Soils: A Day in the Life of Ed Landa

Hooked on Soils: A Day in the Life of Ed Landa

The Interlayer Regions of Sheet Silicates as a Favorable Habitat for Endolithic Microorganisms

Dark-colored rocks that attenuate photosynthetically active radiation (PAR) are generally thought to be poor substrates for endolithic colonization. In this study we show that the internal space of the common dark-colored mica mineral, biotite, is a favorable microhabitat for the growth of filamentous cyanobacteria. Laboratory incubation experiments demonstrated that filamentous cyanobacteria with a cell diameter of ≤3 μm were able to colonize biotite interlayers. Light transmittance measurements on solid biotite sheets showed that PAR (400–690 nm) transmission is sufficient to allow for cyanobacterial growth in the interlayer regions of the mineral. Of note is that damaging UV radiation is attenuated by more than one order of magnitude at the corresponding depth, most likely by the iron contained within the mineral, showing that the interlayer region represents a UV protected microhabitat with respect to the mineral surface. The interlayer region is therefore a favorable environment for small cells that cannot rely only on UV screening compounds. These results show that biotite, and by extension other layered silicate minerals, are a hitherto unrecognized habitat suitable for endolithic microorganisms with potential implications in research fields such as astrobiology.

Specular neutron reflection at the mica/water interface – irreversible adsorption of a cationic dichain surfactant

Neutron reflection from the important mineral mica at the solid/liquid interface is presented here using a new approach – a very thin mica crystal supported on a silicon substrate. This approach avoids the problems of crystal defects and surface undulations that have hindered previous work. The use of mica as a reflectivity substrate is important as it is a model surface, which is atomically smooth with a high structural charge. In this work the mica/water interface is fully characterized. In particular, a characteristic double critical edge is observed, arising from the higher scattering length densities of the mica and D2O subphase relative to the silicon support. The experimental data are modelled using a combined approach: conventional amplitude summation (matrix method) for the thin layers and reflected intensity summation with attenuation terms for the thick layers of mica and hydrocarbon adhesive. Reflection data from the adsorption of the dichain cationic surfactant didodecyldimethylammonium bromide (DDAB) to the surface of muscovite mica from aqueous solution are also presented. It is found that, at twice the critical micelle concentration, a bilayer of DDAB with a thickness of 24 Å is observed, containing essentially no water. Its partial removal by washing and ion exchange is also presented.

Mica and Heavy Minerals as Markers to Map Nile Delta Coastline Displacements during the Holocene

ABSTRACT Stanley, J.-D. and Clemente, P.L., 2014. Mica and heavy minerals as markers to map Nile Delta coastline displacements during the Holocene. Mica and heavy minerals in sediments of Egypt's Nile Delta are examined to test if measured proportions of these two mineral groups and their distributions can be used to define former coastline positions and their shifts in time and space during the Holocene. The premise of the study is based on the sufficiently different attributes of these two components, especially their shape, density, and size, which could induce their segregation and dissimilar dispersal patterns during sediment transport. To test this hypothesis, mineralogical data from more than 1400 samples from 87 sediment cores recovered across the northern third of the delta margin were analyzed. The marked contrast in both temporal and spatial distributions of high proportions of mica and of heavy minerals indicates distinct separation occurred primarily north of the central delta, in an area fro...

Seasonal changes in glacial polynya activity inferred from Weddell Sea varves

Abstract. The Weddell Sea and the associated Filchner–Rønne Ice Shelf constitute key regions for global bottom-water production today. However, little is known about bottom-water production under different climate and ice-sheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium-to-coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, Fe, Ti, Rb, and K are elevated. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity during the LGM. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. Under these conditions, lighter coarser-grained layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom-water formation and increased sediment transport. Vice versa, darker finer-grained layers were then deposited during less windier season, mainly during summer, when coastal polynya activity was likely reduced.

Mapping the alteration footprint and structural control of Taknar IOCG deposit in east of Iran, using ASTER satellite data

Taknar Fe+Cu±Zn±Pb±Au±Ag deposit in northeast of Iran is studied by Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) reflectance and emittance data. Structural and mineralogical evidences of IOCG mineralization is mapped by visual image interpretation and spectral processing techniques. The tectonic model is consistent with an extensional zone associated with a releasing bend of right-lateral regional faults, extending about 7km2 and encompassing all the known orebodies of Taknar. A combination of band ratio logical operator and matched filtering were used for spectral mapping, which lead to a series of mineral content and crystallinity maps including ferric oxide, ferrous, white mica, chlorite, silica and opaque minerals. The channel way in which hydrothermal fluids were migrating is accurately defined by abundance of white mica and ferric iron oxide maps. Rhythmic sediments of Taknar formation which was characterized by chlorite mineral map is a “reducing” environment that hosts the mineralization. This REDOX environment is also marked by a sudden change in white mica composition from acidic phases to neutral/alkaline. Subsequent field check and microscopic study indicated the accuracy of these remotely mapped minerals. Based on this finding, several new prospects for further exploration was proposed. These results indicates that ASTER data is capable of delineating alteration footprints of an IOCG mineral system in deposit scale exploration.

The sediment properties of glacial diamicts from the Jutulsessen area of Gjelsvikfjella, East Antarctica: A reflection of source materials and regional climate

The glacial diamicts deposited in the Jutulsessen area of Gjelsvikfjella, Dronning Maud Land, East Antarctica show the effects of localized meltwater channels and wind processes, as well as of the dominant glacial processes. These glacial sediments are characterized by poor sorting and a variable mean particle size, and the localized meltwater channels have removed silt- and clay-sized sediments, resulting in the relative enrichment of coarser sediments. The X-ray diffraction (XRD) patterns of clay minerals in samples collected across the study area show very similar characteristics. Biotite is the dominant mica mineral in the clay together with chlorite and K-feldspar. The presence of illite, and small amounts of smectite, demonstrates the limited extent of chemical weathering due to the cold and arid conditions. The samples from the glacial fan area contain mixed-layer clays, suggesting the effects of localized meltwater and limited chemical activity which has altered the crystal structure of biotite at lower topographic level. Scanning electron microscopy (SEM) analysis of surface microtextures from representative quartz grains suggest the dominance of glacial and glaciofluvial processes. The low rates of biogenic activity in this area are indicated by the low total organic carbon (TOC) content of the sediments.

A new application of SIMS to the analysis of nitrogen in mica minerals: tobelite

Muscovite, KAl2[AlSi3O10](OH)2, is a common rock-forming mineral in igneous and metamorphic-rocks, sediments, hydrothermal alteration and ore deposits. The site between two adjacent T-O-T (tetrahedral-octahedral-tetrahedral) layers is shared between K and NH4 in all proportions leading to the building of the "ammonium micas". In this work three tobelite crystals were investigated in terms of major and minor constituents by electron probe microanalysis (EPMA) and in terms of nitrogen, for the first time at authors' knowledge, by secondary ion mass spectrometry (SIMS). The objective was that to gain information on the presence and amount of NH4 in this matrix. The lack of standards did not allow us to provide quantitative results at the ion microprobe. Nevertheless, the SIMS data agree qualitatively with constraints resulting from EPMA analyses and charge-balance crystal chemical considerations. The results of this study emphasize the capabilities of SIMS in the in-situ analysis of N and point out the possibility to develop reference materials (standards) for quantitative SIMS analysis of nitrogen in mica.

Petrography, mineralogy and geochemistry of Cretaceous sediment samples from western Khorat Plateau, Thailand, and considerations on their provenance

At Mo Hin Khao on the western flank of Khorat Plateau, Thailand, the Phra Wihan Formation reveals litharenite and sublitharenite with some subarkose and arkose. A cuesta in the eroded sedimentary sequence exhibits spectacular rock pillars of considerable geotourist potential. The rock sequence is high in silica (SiO2 67–98wt%) and contains quartz, mica, magnetite, chert fragments and accessory minerals such as zircon and tourmaline and amphibole species. These accessory minerals suggest felsic rocks, such as granite, granodiorite and pegmatite, were sources for the sandstones. Geochemical analyses of the sedimentary sequence suggest that source rocks may lie in the passive continental margin, before sediment transport and deposition in the Khorat Basin by rivers flowing across a large flood plain. Many depositional sequences/episodes formed thick beds of cross bedded clastic rocks. A high average maturity index (>5) indicates sedimentary reworking/recycling. Chemical Index of Alteration (CIA) values range from 47 to 98, suggesting variable chemical weathering within the source area rocks, largely representing moderate to high degrees of weathering. The average CIA value of these sediments (78) suggests that relatively extreme alteration factors were involved.

Chemical composition of sediments in Baikal deep-water boreholes as a basis for reconstructions of climatic and environmental changes

Earlier reconstructions of climatic and environmental changes from data of deep-water drilling on the Akademichesky Ridge in Lake Baikal were based both on the content of biogenic silica or the abundance of diatom valves and on the results of X-ray diffraction (XRD) analysis of the sediment mineralogy. It was established that clay minerals are the main carrier of information about climatic variations in a drainage basin. The content of biogenic silica strictly correlates with two chrystallochemical parameters: the abundance of smectite beds in illite–smectite and the abundance of illite. However, detailed analysis of clay minerals calls for exclusive XRD techniques without mass determination of minerals in the sediments of long Baikal cores. We propose a new approach to determine the mineral composition of bottom sediments, based on their chemical composition. We compared the average chemical compositions of Pleistocene, Pliocene, and Miocene core sediments from the boreholes BDP-96 and BDP-98 and sediments of the Paleo-Barguzin River avandelta and recognized groups of chemical elements marking warm and cold climatic periods. However, the difference in the chemical compositions of sediments in warm and cold periods is insignificant. Since an XRD analysis of mineral composition is usually performed for short time intervals, it was necessary to identify cold and warm intervals by the mineral composition calculated from the chemical composition of sediments. The mineral contents were estimated using the Selektor software. Based on the average contents of chemical elements, we computed the mineral composition of the bottom sediments throughout the BDP-98 section and studied its warm and cold periods. We have established that feldspars weakly respond to climatic changes, their contents show minor variations in warm and cold epochs, whereas the contents of mica minerals change seriously. Thus, clay minerals, together with biogenic silica, are a good indicator of paleoclimatic environmental changes.

Diagenetic controls on evolution of porosity in the Permian Illawarra Coal Measures, Southern Sydney Basin, Australia

Petrography of the Illawarra Coal Measures was described by thin section, scanning electron microscope and X-ray diffraction techniques. Sandstone composition of the Illawarra Coal Measures includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals. The Illawarra Coal Measures consists of litharenite and rarely sublitharenite. Quartz includes monocrystalline and polycrystalline grains while the feldspar includes both K-feldspar and plagioclase. Volcanic, sedimentary and chert rock fragments are present in the Illawarra Coal Measures. Thin-section porosity occurs in all units, particularly the coarse-grained deposits. Secondary porosity is more common than primary porosity in the Illawarra Coal Measures. Thin sections and scanning electron microscopy were used to describe diagenetic alterations and their influence on porosity in Illawarra Coal Measures. These diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. In the Illawarra Coal Measures, shale, siltstone and fine-grained sandstone are common in the Wilton Formation, Bargo Claystone, Darkes Forest Sandstone, Allans Creek Formation, Unnamed Member Three and Unnamed Member Two. These units have low porosity and form lithological seals and confining layers. Medium- and coarse-grained sandstone is common in the Kembla Sandstone, Lawrence Sandstone Member and Loddon Sandstone and contains low porosity. Thus, gas or water may be present in these formations. The Tongarra Coal, Allans Creek Coal Member, Wongawilli Coal, Hargrave Coal Member, Cape Horn Coal Member, Balgownie Coal Member and Bulli Coal are the main sources for gas in the Illawarra Coal Measures.

Crystal-chemical microheterogeneity of Precambrian globular dioctahedral mica minerals

The paper presents a novel method for determining the crystal-chemical heterogeneity of finely dispersed dioctahedral 2: 1 mica mineral phases based on the modeling of their powder diffraction patterns. We used three samples, which were taken from Lower-Middle Riphean rock sections of the Anabar and Olenek uplifts in northern Siberia, and one sample from the Upper Riphean Inzer Formation in the southern Urals. Choice of globular samples was determined by contrast features of their chemical composition and different lithological types of the host terrigenous rocks that are commonly transformed at the level of deep catagenesis.Based on structural formulas, unit cell parameters, and coordinates of atoms occupying the cells, we modeled powder diffractograms that made it possible to determine the sizes of coherent scattering domains and probability parameters, which characterize the type, content, and distribution of stacking faults in each of the studied samples. The modeling results demonstrated that each sample represents a physical mixture of individual micaceous phases of different compositions. The paper discusses scales and modes of the crystalchemical heterogeneity of micaceous varieties, which make up globules in the coarse- and fine-grained terrigenous sediments, as well as their various specific crystal-chemical characteristics. The probable physicochemical settings, which were responsible for specific features of the structural and crystal-chemical heterogeneity of micaceous varieties in each of the studied four samples, are also discussed.

Evaluation of thermal mud characteristics of Erzurum (Kprky) clayey raw materials (NE Turkey)

In Turkey, there are several natural occurrences of clays/muds which are used for therapeutic, aesthetic, and pharmaceutical purposes. The aim of this study was to assess the mineralogical and physicochemical properties of Erzurum clayey raw materials from Koprukoy (Erzurum, NE Turkey) for their therapeutic purposes. Raw material samples were collected from Pliocene geological formation deposited in the lacustrine environment and located Koprukoy (Erzurum) thermal area. The samples of geological formation including some fossil were tested by X-ray powder refraction and diffraction methods to determine the mineralogical and physicochemical properties. The experimental results showed that smectite clay mineral was abundant, as well as kaolinite, illite, zeolite, mica, quartz, calcite, plagioclase, feldspar and non-clay minerals. Physicochemical properties, such as specific surface area, cation exchange capacity and plasticity index were also determined using ammonium acetate, ethylene glycol mono ethylene ether and consistency test methods, respectively. From the physicochemical, chemical and mineralogical properties, all samples have thermal properties, which make them potentially suitable for therapeutic or aesthetic purposes. Key words: Clayey material, thermal mud, clay mineralogy, technological property, pelotherapy.

국내 원전 부지 내 암석의 광물학적 특성 규명 및 풍화에 따른 방사성 세슘(137Cs)의 흡착 평가

원자력 발전소 중대사고에 의해 방사성 세슘(<TEX>$^{137}Cs$</TEX>)이 지하수계로 유출될 경우를 가정하여, 연구 지역의 깊이에 따른 암석매질의 특성을 규명하고 세슘의 흡착계수를 정량적으로 평가하였다. 대상지역인 신고리 원전 3, 4호기의 지하 암석매질은 주로 석영 및 장석류로 이루어진 화강암 계열이며, 운모류를 10~20% 함유하고 있다. 비교적 얕은 심도(6.3~7.4 m)의 파쇄대에서 2차 광물인 녹니석이 일부 포함되어 있었지만, 기반암에서는 거의 발견되지 않았다. <TEX>$^{137}Cs$</TEX>의 흡착분배계수(<TEX>$K_d$</TEX>)는 파쇄대 지역에서 약 880~960 mL/g로 기반암 지역에서의 820~840 mL/g보다 비교적 높게 나타났으며, 이는 파쇄대에 포함되어 있는 풍화생성물인 2차 광물들에 의한 영향으로 판단된다. 따라서 <TEX>$^{137}Cs$</TEX>이 지하 매질로 유출될 경우 대부분은 천부 지역에 흡착되어 세슘에 의한 오염 확산 속도가 지연될 것이라고 예상되며, 이러한 결과는 원자력 발전소 안정성 평가인자 자료로 활용될 것으로 기대된다. This study was to characterize the minerals in fractured and bedrock zone, and determine quantitatively sorption for radioactive cesium (<TEX>$^{137}Cs$</TEX>) at the Korean nuclear facility site. The rock samples were granite group that mainly consists of quartz and feldspar with 10~20% mica minerals. Chlorite was observed as secondary mineral for the rock samples collected from fractured zone, but not for bedrock samples. The <TEX>$^{137}Cs$</TEX> sorption distribution coefficients increased to <TEX>$K_d$</TEX> = 880~960 mL/g in the fractured zone because of the presence of secondary minerals formed by weathering processes, compared to the bedrock zone (<TEX>$K_d$</TEX> = 820~840 mL/g). These results suggest that the released <TEX>$^{137}Cs$</TEX> to groundwater environment could be significantly retarded in the fractured zone in the case of severe nuclear accident at the study site.

부산 장산 지역의 구과상(球課狀) 유문암에 대한 암석학적 연구

부산광역시 동부의 장산화산암체는 화산함몰체로 알려져 있고, 화산함몰체 외각부를 따라 구과상 유문암이 <TEX>$60^{\circ}{\sim}90^{\circ}$</TEX>의 각도로 수직유상구조를 보여주며 환상암맥으로 나타난다. 구과의 직경은 수 mm에서 2.8 cm 이상이며, 평균적으로 5~10 mm로 측정되었다. 이들 구과는 하나의 핵을 중심으로 방사상을 나타내는 단식 구과형과 육안상 흰색을 띠는 각을 가지면서 핵 부분을 중심으로 방사상을 나타내는 방사상 단식 구과형으로 비교적 단순한 형태를 보여주며, 이들 구과들이 서로 인접하여 합체된 포도송이와 같은 집합체를 보여주기도 한다. 구과의 동심원상 핵 부분에 대한 전자현미분석 결과, 은미정질 기질부 물질의 성분은 주로 <TEX>$SiO_2$</TEX>가 거의 82% 이상, <TEX>$Al_2O_3$</TEX>는 7~10%, <TEX>$Na_2O+K_2O$</TEX>는 8% 이하의 실리카 물질과 장석(새니딘)의 미세한 섬유상 교생으로 나타났다. 구과에 대한 X-선 회절분석 결과, 석영, 새니딘, 앨바이트 그리고 소규모의 운모, 카오린, 녹니석으로 구성되며, X-선 면분석 결과, 핵 부분은 <TEX>$SiO_2$</TEX>가 풍부하며, 각 부분은 <TEX>$Na_2O$</TEX> 또는 <TEX>$K_2O$</TEX>, <TEX>$Al_2O_3$</TEX>가 풍부하였다. 장산콜드론의 유문암 암맥에서 나타나는 구과의 깃털형과 비등형 결정 형태는 마그마 관입에 의한 이동속도보다 유리질 물질로부터 탈유리화작용이 더 빠른 속도로 진행되었고, 비교적 정체된 상태에서 급속한 냉각으로 형성되었음을 지시한다. 구과들은 고규산(75.4~75.7 wt.%) 유문암질 마그마로부터 유래되었다. Spherulitic rhyolite occur as part of ring dyke which showing a vertical flowage of <TEX>$60^{\circ}{\sim}90^{\circ}$</TEX>, of the Jangsan cauldron was studied. The spherulites range in diameter from a few millimeters to 2.8 centimeters or more, and average 5~10 millimeters. It belongs to radiated simple spherulite type. They consist of a core of moderate brown dense material encased by a thin crust, a few millimeters thick at most of white grey material. The spherulites frequently have a radiating fibrous structure, which are thought to have formed as a consequence of rapid mineral growth caused by very fast cooling of the dykes in shallow depth near the surface. EPMA examination of the concentric-zoned core of spherulites show that they are mainly composed of cryptocrystalline-fibrous intergrowth of silica minerals and alkali feldspars which have <TEX>$SiO_2$</TEX> 82% or more, <TEX>$Al_2O_3$</TEX> 7~10%, <TEX>$Na_2O+K_2O$</TEX> less than 8%. The feldspar compositions of the spherulites lie essentially within the sanidine field. XRD examination show that spherulites are mainly composed of quartz, sanidine, albite with minor mica, kaolinite and chlorite. According to X-ray mapping, the spherulites are enriched in <TEX>$SiO_2$</TEX> in the core and partly enriched <TEX>$Na_2O$</TEX> or <TEX>$K_2O$</TEX>, <TEX>$Al_2O_3$</TEX> in the shell that reflect in compositional zoning with increasing spherulitic devitrification. The feathery and non-equant crystal shapes of spherulites from rhyolite dyke of Jangsan cauldron suggest that they may have formed during the rapid cooling of dyke under the static state, or faster velocity of devitrification from glassy materials than movement velocity of the magma intrusion. The spherulitic rhyolite originated from high-silica(75.4~75.7 wt.%) rhyolite magma.