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1.
Ákos Török 《Environmental Geology》2008,56(3-4):583-594
The travertine buildings of Budapest show extensive black crust formation, which is related to high concentration of atmospheric pollution and a continental climate. Laminar black crusts, framboidal black crusts and leached white surfaces are compared. Physical properties (Schmidt hammer rebound, Duroscope rebound), mineralogical composition and elemental composition are measured. Framboidal black crusts contain more than 50% of acicular gypsum. The crust surface displays idiomorphic rosette-like gypsum crystals with particulates, calcite and gypsum crystal aggregates. The sulphur isotopic composition of the black crusts pinpoints the involvement of rain and dust in crust formation. Thick framboidal black crust has the lowest strength while thin laminar black crust and white dissolved surfaces show minor decrease in surface strength compared to unaltered travertine. The crusts adhere to the travertine surface and rarely show mechanical decay forms that are typical features of porous limestone ashlars in Budapest. 相似文献
2.
Orsolya Farkas Siegfried Siegesmund Tobias Licha Ákos Török 《Environmental Earth Sciences》2018,77(5):211
Twenty-seven samples of black weathering crust and host carbonates were studied from seven European countries (Germany, Hungary, Belgium, Czech Republic, France, Italy and Poland) representing 11 different sites. The samples were collected for sites for which long-term air pollution records are available. The mineralogical analyses (XRD, polarizing microscopy, SEM) have shown that despite decreasing SO2 emissions crust samples are still very rich in gypsum. Further, in all host rock samples gypsum was also detected. Good correlations (R2 > 0.9) were also found between water-soluble calcium and gypsum content and between sulphate and gypsum content both for black crusts and host rocks. The black gypsum crusts are four or five times richer in sulphate than the host rock. The conductivity of dissolved crust and host rock samples also shows a positive correlation with gypsum content of the samples. LA-ICP-MS analyses allowed the detection of high Pb-levels in black crusts and a negative shift in lead concentration at the crust/host rock transition. The lead content of the host rock is 2–5 mg/kg, while that of the crust is 3–25 mg/kg in the sample collected from Germany, while in the Belgian sample these values are 2–14 mg/kg and 80–870 mg/kg for the host rock and crust, respectively. The GC–MS technique allowed to detect the PAH content of black crusts and host rocks. The former one contains 0.6–15.6 (102.5) mg/kg, while in the host rock values between 0.2 and 2.4 mg/kg were found. The present study suggests that still large amounts of air pollution-related minerals and organic pollutants are found in the black weathering crusts of European carbonate buildings despite decreasing trends in air pollution. 相似文献
3.
Urban and rural limestone weathering; the contribution of dust to black crust formation 总被引：2，自引：2，他引：0
Ákos Török Tobias Licha Klaus Simon Siegfried Siegesmund 《Environmental Earth Sciences》2011,63(4):675-693
A novel approach of studying dust, black crust and host limestones by using various techniques is presented in this paper.
Samples were collected from limestone monuments located in rural, urban and industrial areas in Germany and Hungary. The paper
focuses on differences in the mineralogical composition, major and trace element distribution of materials and their total
polyaromatic hydrocarbons (PAH) concentration having different exposure to air pollutants. High concentration of gypsum was
found in laminar and dendritic black crusts and even in the dust of both urban and rural areas, despite the low concentration
of SO2 in rural atmospheric environments. The black crusts show a typical microfabric with distinct layers of various gypsum–calcite
mineral associations, silt-sized mineral fragments and black particle content. In dust, newly formed gypsum crystals were
found along with mineral fragments, and siliceous and, less frequently, carbonaceous fly ash particles. High concentration
of lead was found both in the samples of urban and rural areas of Germany and Hungary. Lead mostly accumulates in dust and
also in the black crust. Highest lead concentrations were found not on the surface of the black crust, but close to the crust/limestone
boundary according to LA-ICPMS analyses. It indicates that despite the ban of leaded petrol, lead is still present in the
dust and the in the soiled zones of the built environment. Similarly to lead, no significant difference in the total PAH content
of dust and crust samples were found in the two countries. Dusts are equally enriched in PAH in both countries, whereas black
crusts contain fewer amounts. Most of the aqueous extracts of the samples were saturated with respect to sulphate, which also
indicate the presence of gypsum, whilst in some German samples unsaturated conditions were detected. In summary, by using
a combination of these analytical techniques, it was possible to distinguish rural and urban samples and to outline the role
of dust in bringing pollutants to the stone surface. It is suggested that the analysed dust and stone samples indicate not
only present, but also the past, pollution fluxes and as a consequence stones similar to sediments or surface waters can be
used as environment indicators. 相似文献
4.
B. Graue S. Siegesmund P. Oyhantcabal R. Naumann T. Licha K. Simon 《Environmental Earth Sciences》2013,69(4):1095-1124
Severe stone deterioration is evident at the Cologne cathedral. In particular, the “Drachenfels” trachyte, which was the building material of the medieval construction period, shows significant structural deterioration as well as massive formation of gypsum crusts. The present article investigates crust formation on limestone, sandstone, and volcanic rock from the Cologne cathedral as well as from the Xanten and Altenberg cathedrals. These three buildings, showing varying degrees of deterioration, are located in different areas and exposed to varying industrial, urban, and rural pollution. Thin laminar and black framboidal crusts form on calcareous as well as silicate stone. The lack of a significant intrinsic calcium and sulfur source for the formation of the gypsum crusts on the Drachenfels trachyte indicates major extrinsic environmental impact: a sufficient offer of SO x from pollutant fluxes as well as external calcium sources (e.g., pollution, mortars, neighboring calcite stones). Chemical analyses reveal strong gypsum enrichment within the crusts as well as higher concentrations of lead and other pollutants (arsenic, antimony, bismuth, tin, etc.), which generally can be linked to traffic and industry. The formation of weathering crusts in an industrial environment is clearly distinguishable from that in rural areas. Scanning electron microscopy observations confirm that the total amount of pollution is less at the Altenberg cathedral than at the Cologne and Xanten cathedrals. XRF analyses show that the formation of gypsum occurs in lower amounts at Altenberg. This correlates well with the measured SO2 content and the intensity of the decay at the different locations. Furthermore, the different types of crusts, e.g., framboidal and laminar, can be differentiated and assigned to the different locations. The black weathering crusts on the silicate Drachenfels trachyte contribute to the degradation of the historic building material. They enhance mechanical moisture-related deterioration processes and the decay by chemical corrosion of rock-forming minerals. Although SO2 concentrations in air have shown a strong decrease over the past 30 years, degradation in connection with weathering crusts is still observed. This indicates that not only contemporary or recent emissions, but also past pollutant concentrations have to be considered. 相似文献
5.
Structure, chemistry and origins of gypsum crusts in southern Tunisia and the central Namib Desert 总被引：1，自引：0，他引：1
ANDREW WATSON 《Sedimentology》1985,32(6):855-875
Gypsum crusts are broadly defined as accumulations at or within about 10 m of the land surface from 0.10m to 5.0 m thick containing more than 15% by weight gypsum (CaSO4·2H2O) and at least 5.0% by weight more gypsum than the underlying bedrock. The deposits are often, but not invariably, consolidated owing to cementation by gypsum. The crusts are found in many of the world's deserts where mean monthly potential evaporation exceeds mean monthly precipitation throughout the year. Using structural, fabric and textural criteria, three main types of crust may be distinguished:(1) bedded crusts, found either at or beneath the land surface, which are made up of discrete horizontal strata up to 0.10 m thick, each showing a gradation in gypsum crystal size from less than 50 μm at the top to more than 0.50 mm at the base; (2) subsurface crusts, of which there are two forms, one made up of large, lenticular crystals (up to 0.50 m in diameter)—the desert rose crusts—and the other, a mesocrystalline form, with gypsum crystals up to about 1.0 mm in diameter; and (3) surface crusts, which are subdivided into columnar, powdery and cobble forms, all of which are made up of predominantly alabastrine gypsum (crystallites less than 50 μm in diameter). In southern Tunisia and the central Namib Desert, bedded crusts are found around ephemeral lakes and lagoons. They are characterized by size-graded beds, gypsum contents of 50–80% by weight and comparatively high concentrations of sodium, potassium, magnesium and iron. They are interpreted as shallow-water evaporites which accumulate when saline pools evaporate to dryness. Desert rose crusts or croûtes de nappe generally contain 50–70% by weight gypsum, and have higher sodium concentrations than the second subsurface form. Texturally they are characterized by poikilitic inclusion of clastic material within large lenticular crystals. They are interpreted as hydromorphic accretions, which precipitate in host sediments at near-surface water tables through the evaporation of groundwater. The second form of subsurface crust—the mesocrystalline—often occurs in close association with the various surface forms. Unlike the hydromorphic crusts, they are not restricted to low-lying terrain. They are characterized by gypsum contents reaching 90% by weight, and have a close chemical and textural similarity to columnar surface crusts. This mesocrystalline form represents an illuvial accumulation; the surface forms—excluding the bedded crusts—are exhumed examples at various stages of solutional degradation. Subsurface precipitation of gypsum from meteoric waters containing salts leached from the surface, results in displacive gypsum accumulation in the soil zone. In southern Tunisia, the gypsum is derived from sand and dust deflated from evaporitic basins; in the central Namib, salts dissolved in fog water are the most likely source. Where other salts are present, differential leaching may form two-tiered crusts, calcrete—gypsum or gypsum—halite, if rainfall is sufficient to mobilize the less soluble salt yet insufficient to flush the more soluble. Gypsum crust genesis is restricted to arid environments, and if their susceptibility to post-depositional alteration is acknowledged, they can provide valuable palaeoclimatic indicators. 相似文献
6.
Siegfried Siegesmund Akos Török Andre Hüpers Christian Müller Werner Klemm 《Environmental Geology》2007,52(2):385-397
High levels of SO2 and particulate pollution enable the rapid development of gypsum-rich weathering crusts in Budapest. Two types of white crusts,
thin and thick ones, and two forms of black crusts, laminar and framboidal ones, were studied in limestone buildings of the
parliament and Citadella. The percentage of crust cover and damage categories were documented on selected walls. Petrographic,
XRD, XRF and sulphur isotope analyses were performed under laboratory conditions to understand the mechanism of crust formation.
White crusts found both on exposed and sheltered walls display a calcite-rich layer with gypsum, while black crusts are enriched
with gypsum. The sulphur isotopic composition of white and black crusts overlaps, but the crusts are slightly enriched in
heavy isotopes compared to rainwater. S content, Si/Al ratios and particulates in black crusts suggest that air pollution
(SO2, dust) contributes to black crust formation. The accumulation of sulphur and Zn enrichment of white crusts were also documented
indicating that under high pollution levels, even these compound can accumulate on exposed facades. 相似文献
7.
8.
鄂尔多斯盆地下古生界马家沟组顶部遭受长期风化剥蚀,形成了以膏云岩为主的风化壳岩溶储层。通过偏光显微和超微扫描电镜深入观察了（含）膏云岩的孔隙类型及充填规律,分析了其岩溶作用特点,研究了以膏云岩为主的岩溶作用特征与传统碳酸盐岩岩溶作用特征的区别,探讨了膏云岩发育区的岩溶作用模式。结果表明:硬石膏结核和石膏晶体以其强亲水性和远高于白云石、方解石的溶解度,极易率先吸水发生溶解形成组构选择性溶蚀孔隙,导致膏云岩层蜂窝状溶孔的形成。硬石膏的高溶解度和力学不稳定性使得研究区以膏云岩、白云岩和灰云岩互层的风化壳储层以膏模孔、扩溶膏模孔及与之伴生的胀缩微裂缝为主要储集空间,孔隙大小具有明显的自限性。岩性—（含）膏云岩和沉积微相—海平面低位期潮上带（含）膏云坪沉积是储层形成的先天物质基础和环境条件,并因此直接导致（含）膏云岩储层的成层分布特征。表层膏云岩首先遭受淡水淋滤形成蜂窝状溶孔,多层成层分布膏云岩这种独特的物理化学性质使风化壳内的岩溶水以弥散性渗透为主,这是膏云岩发育区岩溶作用特征和模式与碳酸盐岩产生重要差异的根本原因。 相似文献
9.
钙结壳是土壤层之下的岩石在出露地表渗流带环境内受大气雨水的溶解,并通过上覆土壤层的淋滤而成的产物。由于雨水的不断的溶解,使原始的岩石或沉积物发生破裂,从而形成各种大小的砾块或砾石。随着雨水不断的溶解,雨水逐渐增加 CaCO_3的含量。这些富含碳酸钙的雨水,当它流到位于潜水面之上的下渗流带时,就在土壤层之下、原来岩石之上沉积了特殊的、层纹状方解石胶结物。它们往往覆盖岩石表面或沿着砾石的外缘、呈包覆状沉积,从而形成典型的层纹状钙结壳。渗滤豆粒的外皮也属于层纹状钙结壳的性质,这些外皮有向下方加厚的趋势,这表明渗流带的含碳酸钙的雨水,受重力作用影响下,具有往下悬垂现象,类似于洞穴内的钟乳石。豆粒之间的相互衔接呈多边形构造,也表明它是受重力作用而形成的构造。 相似文献
10.
微地域搬运——碳酸盐岩红色风化壳形成过程的一种方式 总被引：1，自引：0，他引：1
由于差异溶蚀作用,由碳酸盐岩强烈风化形成的红色风化壳通常表现出独特的剖面构型:风化壳发育深厚、下伏基岩面强烈起伏波动、溶沟和石牙相间展布。以黔中岩溶台地之上发育的红色风化壳——平坝剖面为例,通过宏观地质、地球化学、粒度分析以及矿物学等方法,并以邻近的两个石灰土剖面(罗吏剖面和龙洞堡剖面)作对比,对碳酸盐岩红色风化壳的形成过程进行了讨论,论证了微地域搬运是碳酸盐岩红色风化壳尤其是厚层红色风化壳形成过程的一种方式。溶沟部位的风化壳,从风化前锋向上的一定深度范围内,为原位残积风化的产物(残积层);在此深度以上的部分,为地势较高的相邻石牙部位不同风化程度的残积物的搬运堆积(堆积层),也是导致风化指标随深度呈锯齿状波动的直接原因。后者一般组成剖面的主体。风化壳的年代地层学表现为,在残积层,从风化前锋向上,风化年龄由新到老;在堆积层,从下到上,风化年龄由老到新。风化前锋是一个重要的地球化学作用场所,在这一狭窄的界面上,伴随碳酸盐矿物的快速淋失,残余酸不溶物开始了明显分解。而风化壳的后期演化是一个缓慢而长期的过程。达到重力平衡的剖面(即风化壳表面平缓、不发生微地域搬运的剖面),在由表及里的风化作用下,从地表向下的一定深度范... 相似文献
11.
This work analyzes the stone superficial deterioration of some monuments in Rabat city. The rock is a calcarenite with porous
texture, rich in calcium carbonates and fossils. The techniques used to study the surfaces altered by blackening or black
crusts, are the SEM equipped with EDX microprobe, the X-ray diffraction, and the analysis of the soluble salts by ionic chromatography
for anions and flame spectrometer for cations. They reveal significant contents of calcium sulphates, mainly gypsum, often
associated with sodium chlorides. These salts known to have a destructive effect on the calcareous stones seem to be originated
from the contamination by atmospheric pollutants, particularly sulfur dioxide, and by the marine sprays. 相似文献
12.
Recent caliche, including nodules, pisolites, crusts, internal sediment, speleothem deposits, and spherulites, has formed within the dolomitic Cretaceous Edwards Formation of central Texas. As weathering altered the host strata, rhombic crystals of calcite were precipitated concomitantly with dissolution of the dolomite, thereby forming nodules. The highly altered dolomite (i.e. pulverulite) was then removed and spar, internal sediment, and travertine accumulated in the internodular voids. Nodular masses of calcite and dolomite are the most prominent constituent of the caliche. Some of the nodules have a well developed concentric structure as well as other characteristics similar to hypersaline pisolites. Features which appear to be useful in distinguishing caliche from hypersaline pisolites are: regional geological setting, association with other caliche and palaeosoil deposits, types of fossils present, and the presence of rhombic calcite and/or bladed sparry calcite with triangular shaped cross-section. A brick-like calcite texture and relict aragonite rays characterize hypersaline pisolites. Incipient neomorphism of the nodules and pisolites has resulted in the development of a radial pattern of spar within these structures. Geopetal deposits of internal sediment, including terra rossa soil, inhibited spar growth in the upward direction; consequently, spar is much better developed on the undersides of pisolites. Crusts and travertine flowstone (speleothem) deposits are intimately associated with the nodular masses and internal sediment. The brecciated thin crusts and travertine flowstone are end products of the same processes. The crusts formed during times of periodic desiccation of the growing surface while the flowstone formed when water was relatively abundant. Spherulitic bodies within the caliche, commonly 1–2 mm in diameter, display a radial texture and yet are composed of single crystals of calcite. The structures are the product of neomorphosed Microcodium or Microcodium-like globular bodies. 相似文献
13.
Ken Bailey Felicity Lloyd Stuart Kearns Francesco Stoppa Nelson Eby Alan Woolley 《Lithos》2005,85(1-4):15-25
Because the calciocarbonatite lavas at Fort Portal were the first ever described they have received great attention, with the pyroclastic rocks being relatively neglected. Volumetrically the lavas are minute, and the major deposit is a 2 m thick blanket of “flaggy” tuffs, long regarded as carbonatite tuff with crustal debris. Fresh examination shows these tuffs to contain melilitite previously unreported from Fort Portal. The rock is a mix of crust and mantle debris with near-isotropic lapilli, set in a matrix composed predominantly of carbonate. The low birefringence parts of the lapilli are devitrified melilitite glass. Compound lapilli are abundant, containing aggregates of globules, together with xenolithic/crystic fragments. In some, there are concentric zones of more carbonate rich material alternating with melilitite: tangential phlogopite flakes mark the outer zones, in marked contrast to their planar distribution through the enclosing rock matrix. Euhedral titano-magnetite (10–15%) is the most obvious cognate mineral. Devitrified melilitite contains abundant small crystals and microlites of melilite, apatite, magnetite, and carbonates, mostly formed during disequilibrium quench crystallisation. Because of this, and widespread fine grained accidental debris, a precise bulk melt composition is hard to obtain, but the average is close to melilitite with high P2O5. Mantle debris is largely disaggregated magnetite–phlogopite clinopyroxenite, which could give a bulk composition close to the melt. Low Mg and high Mg calcite are present in the melilitite lapilli, and in the enclosing carbonate rich matrix. Previously, high Mg calcite was reported only as cement in lapilli tuffs, while the lavas contain only low Mg calcite in the assemblage calcite–periclase (consistent with low pressure carbonate melt crystallisation). Carbonatite–melilitite magma left the mantle carrying restite debris. Melt fragmentation took place in the deep crust, with rapidly quenched droplets enclosing crust debris. Chemical covariations within the flaggy tuffs are uniform and explicable as carbonatite–melilitite plus a thoroughly mixed combination of crust and mantle debris. New links are indicated with the alkaline ultramafic-carbonate volcanism to the south, in Uganda, and parallels with that in Italy. 相似文献
14.
ALEKSANDRA KOSTECKA 《Sedimentology》1993,40(1):27-39
At Tylicz, near Krynica Spa (Polish Carpathians), spelean deposits fill fissures and caverns in Eocene flysch rocks. They occur as: (1) clastic cave sediments transformed into hard crusts due to cementation by finely crystalline low-Mg calcite, (2) drusy calcite that covers crust surfaces and fills voids in the crust and (3) colloform calcite. Two varieties of drusy calcite are distinguished: acicular and columnar. The acicular calcite is built up of crystallites forming spherulitic fans or cones. In places it is syntaxially covered with colloform calcite. The drusy calcite is low-Mg ferroan calcite with non-ferroan subzones, whereas the colloform calcite is a low-Mg non-ferroan variety. The columnar calcite crystals form fan-like bundles. Cross-sections cut perpendicular to the c-axes of columnar crystals are equilateral triangular in shape, although some have slightly curved edges. The columnar crystals have steep rhombic terminations and most have curved triangular faces, i.e. gothic-arch calcite. Saddle crystals have also been observed. The columnar crystals are composed of radially orientated crystallites whose long dimension is parallel to the c-axis. The curved crystal faces of such polycrystals are interpreted as a result of differential growth rates of the crystallites. The spelean calcites precipitated from CO2-saturated water. The high rate of CaCO3 precipitation is thought to be responsible for the formation of radial structures. Finely crystalline calcite formed within pore spaces of clastic sediments close to the water-air interface, drusy calcite crystallized beneath the water-air interface, and colloform calcite precipitated from thin films of water. 相似文献
15.
A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement-filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low-Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non-luminescent under cathodoluminescence, but both cements and host rock are cross-cut by micro-fractures filled with bright-luminescent calcite, related to late void-filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ18O(PDB)=?5‰ to ?12‰ and from δ13O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non-luminescent calcite of early diagenetic origin. The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine-meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny. 相似文献
16.
Origin of subaerial Holocene calcareous crusts: role of algae, fungi and sparmicritisation 总被引：3，自引：0，他引：3
CHARLES F. KAHLE 《Sedimentology》1977,24(3):413-435
The Pleistocene Miami Limestone that crops out on the lower Florida Keys is overlain by thin (16 cm or less), discontinuous, Holocene calcareous crusts (caliche) that are usually laminated, composed dominantly of calcite micrite and may or may not incorporate part of the underlying limestone. Both allochems and sparry calcite cement in the former unit contain endolithic algae and fungi, borings and unicellular algae. Biogenic structures identical to those in the Miami Limestone also occur in the calcareous crusts but are somewhat less abundant in the latter unit versus the former unit. The calcareous crusts were formed in the vadose diagenetic environment. Some of the CaCO3 necessary for the micrite that comprises the bulk of the crusts was probably derived from solution of carbonate from a soil cover and some from wind blown salt spray. Most of the micrite, however, was formed by replacement of the uppermost portions of the Miami Limestone. Replacement involved micritisation of allochems and a previously unreported process, sparmicritisation, the degrading recrystallization of sparry calcite to micrite. Minor sparmicritisation was caused by micrite calcification of endolithic fungi or algae within sparry calcite cement or by micrite precipitation in empty borings within such cement. Most sparmicritisation took place by dissolution of sparry calcite and concomitant precipitation of micrite in the space occupied previously by the dissolved spar. Such sparmicritisation is interpreted to be caused by chemical reactions involving the crystals, pore water which is moving slowly but steadily and organic compounds released during bacterial decomposition of fungi, algae or both. It is recognized that sparmicritisation occurs in the marine diagenetic environment and is not, therefore, necessarily indicative of vadose diagenesis. Incomplete sparmicritisation is responsible for some of the clotted textures typically found within calcareous crusts and may explain such textures in many other carbonate rock types. A combination of sparmicritisation and micritisation has probably greatly influenced the porosity of many reefs and, in some cases, led to the formation of ‘micritic reefs’. 相似文献
17.
Groundwater salinization of the Sfax superficial aquifer,Tunisia 总被引：5，自引：0，他引：5
Groundwater salinization has become a crucial environmental problem worldwide and is considered the most widespread form of groundwater contamination. The origin of salinity in the coastal aquifer of the Sfax Basin, Tunisia was investigated by means of chemical analyses of groundwater samples from 65 wells. The groundwater samples present a clear gradation from calcium sulphate salinization to that of sodium chloride. The saturation indices for calcite and gypsum, and binary diagrams of different ions, together with multivariate analysis, indicate the existence of various salinization processes such as: dissolution of gypsum and calcite dispersed through the reservoir rock; ion exchange; intensive agricultural practices that produce effluents that infiltrate to the saturated zone; and sea-water intrusion, enhanced by excessive withdrawal of groundwater. 相似文献
18.
This paper concerns the evaporite units, depositional systems, cyclicity, diagenetic products and anhydritization patterns of the Calatayud Basin (nonmarine, Miocene, central Spain). In outcrop, the sulphate minerals of these shallow lacustrine evaporites consist of primary and secondary gypsum, the latter originating from the replacement of anhydrite and glauberite. In the evaporative systems of this basin, gypsiferous marshes of low salinity can be distinguished from central, saline lakes of higher salinity. In the gypsiferous marsh facies, the dominant, massive, bioturbated gypsum was partly replaced by synsedimentary chert nodules and siliceous crusts. In the saline lake facies, either cycles of gypsiferous lutite‐laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberite, thenardite and nodular anhydrite precipitated. Early replacement of part of the glauberite by anhydrite also occurred. Episodes of subaerial exposure are represented by: (1) pedogenic carbonates (with nodular magnesite) and gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhydrite, which formed in a sabkha. Additionally, meganodular anhydrite occurs, which presumably precipitated from ascending, highly saline solutions. The timing of anhydritization was mainly controlled by the salinity of the pore solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (>200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward‐decreasing salinity is deduced from the base to the top of the evaporite succession. 相似文献
19.
《Chemie der Erde / Geochemistry》2014,74(3):443-452
In the Dragon Belly cave helictites, a special type of irregular speleothem, are found, which grew on stalactites in all vertical and horizontal directions without any affinity to gravity. Microstructural and mineralogical analyses of this stalactite–helictite system indicate that its evolution is initiated by clogging of the central stalactite channel at its tip, probably when the cave was flooded by muddy water. Clogging caused the formation of secondary channels (≈0.2 mm in diameter) for water passage through the outer surface of the stalactite, where helictites start to grow. The secondary channel passes into the central channel of the helictite.The helictites consist of stacked idiomorphic calcite crystals with uniform orientation. Growth of calcite is essentially controlled by water transfer through the central channel and via canalicules (narrow channels of ≈0.05 mm in diameter) following the crystal boundaries of the calcite mesocrystal induced by capillary hydrostatic forces. At straight parts of the helictites calcite crystals are almost uniform in size, but at bended parts crystals are significantly smaller inside (≤0.1 mm in length) than outside of the bend (≤0.5 mm). It is proposed that the difference in calcite volume (larger crystals) vs. the inside of the bend leads to a helix form, which explains the origin of the term helictite.The Sr and Ba concentrations measured by laser ablation along helictites can be explained by cation incorporation during calcite precipitation close to equilibrium. Dilution effects caused by seasonality control the elemental distribution in the helictite, which result in a positive correlation between Sr and Ba. Variability of Mg is unrelated with Sr and Ba, and is probably due to the incongruent dissolution of Mg–calcite from the host rock. 相似文献
20.
大洋富钴结壳研究现状与进展 总被引：24，自引：0，他引：24
在研究富钴结壳的地质构造背景时,发现富钴结壳的形成与板内地幔柱活动胡关,并与基岩类型和基岩的风化程度有关。由于现代分析测试技术的进步,对富钴结壳的结构造,矿物组成和地球化学特征的研究正在逐渐深入,特别是各种微区观察测试技术和同位素年代学方法的应用,使富钴结壳的高分辨率部面研究取得长足的发展。以此为基础,生长速率缓慢,在无沉积物时期生长的富钴结壳近年来被尝试用于古海洋学和全球变化的研究,它不但能反映千万年级的环境变化,而且能弥补深海沉积物所不能反映的沉积间断的历史,因此正成为该领域研究中的一个热点。在富钴结壳的成矿物质来源方面,逐渐认识到包括火山,热液和低温水岩反应等在内的内源物质供应的重要性。在结壳的成因机制研究方面发现微生物在结壳的形成过程中起到了主要作用。 相似文献