Quantitative mineralogical analysis of carbonate sediments by X-ray diffraction: a new, automatic method for sediments with low carbonate content

A new X-ray diffraction method has been developed whereby the weight percentages of aragonite and low and high-magnesium calcite are determined from the integrated peak areas of samples. Peak areas are measured by a step scanning method. The weight percentages of MgCO₃ in calcite are determined from the angular position of the calcite peak. This technique uses a direct calculation method which simplifies the preparation of the samples and the calibration processes and increases the quality of the results. The fully automatic method uses a desk-top computer to guide the diffractometer and to carry out the necessary calculations. Tests on precision and accuracy of the method indicate that results with less than ± 4% error (mineral %) and ± 0.6% error (MgCO₃%) are obtainable for all samples even those with a low (10%) carbonate content.     

Surface carbonate and land-derived clastic marine sediments from southern Chile: mineralogical and geochemical investigation

Surface carbonate and land-derived deposits in the sea off southern Chile were investigated for their mineralogical and geochemical composition. The data were related to environmental features and compared with those of similar temperate and polar carbonate deposits from Tasmania, New Zealand, Arctica, and Antarctica. The mineralogy of the siliciclastic fraction is typical of cold areas and is mainly composed of chlorite, mica, quartz, feldspars and amphibole. The CaCO₃ content varies from 30 to 90%; carbonate mineralogy is made up of low-Mg calcite, high-Mg calcite and minor amounts of aragonite. The Ca, Mg, Sr, Fe, and Mn contents of bulk carbonates and some selected skeletal hard parts are comparable to those of carbonates from Tasmania. The elemental composition is mainly related to carbonate mineralogy, skeletal components, and seawater conditions. The δ¹³C and δ¹⁸O values of carbonates are positive, and their field falls between the “seafloor diagenesis” and “upwelling water” trend lines, because the sediments are likely to be in equilibrium with waters of Antarctic origin. The mineralogical, elemental, and isotopic compositions of carbonates from southern Chile show better similarities with the “temperate” carbonates from Tasmania and New Zealand than with the “polar” carbonates from Arctica and Antarctica. Carbonate deposition is allowed by the low terrigenous input, the low SPM concentration and, probably, the upwelling of seawater from Antarctica.     

Experimental analysis of floored interstices in clastic carbonate sediments

Laboratory tests conducted in order to investigate some of the dynamic aspects of the formation of floored interstices are described. In essence, the experiments consist of pouring a suspension of fine-grained carbonate mud (powdered clams) onto a quantity of clam shells (fragments or whole ones) under water. Settling of the carbonate suspension leads to gravity differentation and internal sedimentation phenomena. The coarser fractions, settling the fastest, will accumulate in cavities provided by some of the hollow shells, or by hollows resulting from packing of the fragments, and may even accumulate on top of some of the shells. Inside individual shells floored interstices frequently exhibiting graded bedding, were formed. Sediment pyramids on top of some of the clam shells may also exhibit graded bedding, although not as frequently as internal sediments. Sedimentation pyramids are bordered by symmetrical sets of maximum-slope angles. Closely related to the phenomena taking place during settling from suspension, appeared to be the occurrence of narrow, more or less vertical channels of clear water streaming upwards through the settling suspension. Photographs record the textural changes taking place as the result of the introduction of mud suspensions into grain-supported sedimentary fabrics. Conclusions in regard with the palaeoenvironmental significance of the experiments conducted are given.     

安徽宿松上石炭统碳酸盐岩台地沉积物定量分析与海平面变化

安徽宿松地区上石炭统碳酸盐岩发育,厚约100m,化石丰富,属于比较典型的浅海碳酸盐岩台地沉积。定量分析显示,研究区岩石类型主要为颗粒灰岩和泥粒灰岩,颗粒平均含量分别为81.2％和69.6％,以浅海生物碎屑为主;其次为粒泥灰岩和泥晶灰岩,颗粒平均含量分别为26.4％和4.1％。依据岩石学特征,研究区主要发育开阔台地相和潮坪相,并可进一步划分出5种沉积亚相,分别是台内滩、台内盆、台内坪、潮间坪和潮上坪。在研究区上石炭统中共识别出3个长期的海进—海退旋回,其表现出在海平面变化总体呈下降趋势的背景之上叠加了短期的高频海平面波动的特征。这3个旋回与扬子板块其他地区及欧美板块同时代地层中的长期海进—海退旋回相当,表明海平面的变化可能受到晚古生代冈瓦纳大陆冰川的冰期和间冰期交替活动的影响。     

The mineralogical composition of precursor sediments of calcareous rhythmites: a new approach

Previous studies in Silurian carbonates from Gotland (Sweden) have led to a model for the development of limestone-marl alternations. This model postulates that early diagenesis of precursor sediments without strong primary differences can result in a differentiation by selective dissolution of aragonite in marl beds and reprecipitation of calcite cement in limestone beds. This model is described as a set of mathematical equations that quantify the diagenetic processes (aragonite dissolution and calcite reprecipitation) that occur during the formation of limestone-marl interbeds from a hypothetical homogeneous precursor sediment. The calculations demonstrate that resulting hypothetical limestone-marl alternations show characteristic mathematical relationships between the ratios of the bed thicknesses of limestones and marls on one side, and the carbonate contents, on the other. By reversing this model, the original mineralogical composition of the precursor sediment of real-world rhythmic successions can be determined. In this study, alternations from the Silurian of Gotland, the Cambrian, Devonian, and Mississippian of North America, the Jurassic of France and Germany, and the Cretaceous of France are shown to exhibit mathematical relationships similar to those calculated for hypothetical precursor sediments without primary differences. Therefore, the mineralogical composition of their precursor sediments can be estimated. In contrast, the clear mismatch shown by the Lower Jurassic Belemnite Marls from Dorset indicates that these rhythms did not suffer an early diagenetic overprint. Our model helps to differentiate between rhythmites with strong depositional variations and those without; however, it cannot indicate whether a given alternation is the product of rhythmic diagenesis of a homogeneous precursor sediment or the result of diagenetic enhancement of subtle underlying sedimentary rhythms. For horizontally correlated patterns, such as laterally extensive beds and layers of nodules, an a priori unknown external signal has to be assumed.     

Comparison of color, chemical and mineralogical compositions of mine drainage sediments to pigment

 Forty-three untreated and actively and passively (wetland) treated coal mine drainage sediments and five yellow-red pigments were characterized using X-ray fluorescence, fusion-inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, and tristimulus colorimeter. Primary crystalline iron-bearing phases were goethite and lepidocrocite, and iron phases converted to hematite upon heating. Quartz was nearly ubiquitous except for synthetic pigments. Gypsum, bassinite, calcite, and ettringite were found in active treatment sediments. Iron concentrations from highest to lowest were synthetic pigment>wetland sediment>natural pigment>active treatment (untreated sediments varied more widely), and manganese was highest in actively treated sediments. Loss on ignition was highest for passively treated sediments. No clear trends were observed between quantified color parameters (L*, a*, b*, and Redness Index) and chemical compositions. Because sediments from passive treatment are similar in chemistry, mineralogy, and color to natural pigments, the mine drainage sediments may be an untapped resource for pigment. Received: 29 December 1997 · Accepted: 11 May 1998     

Comparison of neutron and X-ray diffraction in texture analysis of deformed carbonate rocks

The paper compares pole figure determinations with neutron diffraction and X-ray diffraction on experimentally deformed carbonate rocks, a coarse grained marble and fine grained limestone. Neutron diffraction enables determination of complete pole figures on a single spherical specimen and is advantageous for coarse grained materials. Results obtained with both diffraction techniques agree satisfactorily, although uncertainties introduced by counting statistics are more serious for neutrons than for X-rays. Continuous detectors add new possibilities which are still little explored.     

U-Th-Pb systematics of precambrian carbonate rocks: Dating of the formation and transformation of carbonate sediments

The current state in the Pb-Pb dating of the formation and epigenesis of Precambrian carbonate sediments has been reviewed. The geochemistry of admixture elements (U, Th, Pb, Rb, Sr, Mn, and Fe) constituting carbonate minerals and the redistribution of the elements in the course of early diagenesis and epigenesis have been considered. The advantages of choosing samples for Pb-Pb dating on the basis of geochemical criteria similar to those applied for Sr-isotope stratigraphy and the potentialities of applying different methods of decontamination of specimens, analyzed from surface contamination and possible epigenetic carbonate phases, were illustrated. By the example of a series of carbonate formations, distinctions in susceptibility of U-Pb and Rb-Sr systems of carbonate minerals to secondary transformations have been demonstrated. The prospects for using the method of step solution allowing noncogenetic carbonate generations to be separated and thus the accuracy and reliability of Pb-Pb dates to be upgraded were considered.     

Quantitative determination of cerussite (lead carbonate) by X-ray powder diffraction and inferences for lead speciation and transport in stream sediments from a former lead mining area in Scotland

Most current investigations of sites contaminated with heavy metals (e.g. Pb, Zn, Cu) emphasise the importance of determining the amounts of physical and chemical forms of metals rather than just the total amounts present. Chemical extraction techniques used for this purpose are inevitably operationally defined. A more direct approach to the identification of crystalline forms can be made by mineralogical techniques such as X-ray powder diffraction (XRPD), but quantitative determination of a particular form is not often attempted. Recent advances in methods of analysis and sample preparation for XRPD mean that it is now a relatively simple matter to obtain quantitative XRPD data. Here, it is applied to the quantitative determination of the forms of Pb in different size-fractions of stream sediment samples from Leadhills/Wanlockhead, SW Scotland, an historic Pb mining area. Comparison of the XRPD analyses with determinations of Pb by atomic absorption spectrophotometry demonstrates that a large proportion of the Pb present in the stream sediments is in the form of cerussite (PbCO₃). Furthermore, the cerussite tends to be concentrated in the silt fraction and is even a minor component of the clay-size fraction. However, quantitative analysis of fractions <6 μm indicates that cerussite alone cannot account for all the Pb in this size range. Indirectly, this result suggests that Pb adsorbed to clay minerals, organic matter and/or amorphous Fe and Mn oxides may be proportionally more important for the <6 μm materials. Sediment in this size range, however, typically accounts for no more than 1% by weight of the total stream bed sediment samples collected in the study area. In relation to its size distribution, the mobility of Pb within the wider environment is most likely to occur principally through physical transport of fine particles.     

麦夸特算法在X射线衍射物相定量分析中的应用

常规的X射线衍射物相定量分析方法有各自的优点,但这些方法往往也存在一些不足.在实际应用中迫切需要一种简便、高效的普适性多物相无标样定量分析方法.选择麦夸特算法、粒子群算法、遗传算法、差分进化算法这4种迭代搜索领域的经典算法构建了基于非线性模型参数估计方法的4模冗余系统,以19个配制的4相样品中各相“前三强线”的积分强度之和作为计算的原始数据,通过Matlab软件进行了含量计算.理论分析及试验结果表明,运用麦夸特算法进行定量分析具有更小的计算复杂度、更快的收敛速度及更好的全局搜索能力,各相含量的计算值与配比值的绝对误差在5%以内的约占总计算量的83%.为了验证该算法,计算了昭苏黄土剖面82个混合样品及青海湖二郎剑钻孔359个混合样品中刚玉的含量,刚玉含量的配比值与计算值的相关性分别达到0.83和0.63,刚玉含量的误差超过5%的分别占总计算量的4.88%和9.75%.基于麦夸特算法的定量分析方法在批量化处理多物相定量中具有效率高、可操作性强、准确度高等优点.      

Contribution of mineralogical and geochemical data to the study of northeast atlantic deep quaternary sediments: trace-elements as bottom-current indicators

Geochemical and mineralogical study of sixty cores from the northeast Atlantic Ocean used various analyses by X-ray diffraction and fluorescence spectrometry. Thus, we know the clay mineral, carbonate, quartz, feldspar, and trace-element content of the Quaternary sediments from the last interglacial period. These data have been treated in two ways: (1) analysis of the geographical distribution of the components calculated on a carbonate free basis; and (2) statistical analysis establishing the correlation coefficient that exists between the lithological supports and chemical elements.Thus, genetic groups are defined and are correlated to their original sources. For example, smectite-FeTi group is associated with Iceland; feldspar-NiCu with the Faeroe Islands; Illite-Chlorite-RbPb to acidic continental provinces; and SrCaCO₃ to plankton (pàrtially).The lithological supports (smectite, feldspar, carbonate, etc.) are scattered by currents, since chemical elements are associated with them, having a high degree of correlation, it should be possible to utilize these elements as paleohydrologic tracers.     

碳酸盐岩生屑颗粒定量研究*——点计数法的理论分析与应用

碳酸盐岩中的生屑丰度数据为研究化石类群分异度、重建古生态环境提供了重要依据。点计数法易于定量统计各类生屑颗粒的丰度,比目估法、图像分析方法更适于碳酸盐岩组分的定量研究。在实际应用过程中,点计数法的最佳计点数常不易确定,往往会造成统计结果的精度不足,或统计工作耗时多、工作量大。应用点计数法和图像分析方法分别对视觉百分含量对比图和碳酸盐岩显微图像进行定量统计,分析沉积组分特征对点计数法测量误差的影响,以确定点计数法在碳酸盐岩生屑丰度统计时的最佳计点数。研究结果表明,点计数法适用于碳酸盐岩生屑颗粒丰度研究。对于大部分碳酸盐岩薄片定量分析工作,500点的计点数量基本可以将点计数法的测量误差控制在±2.5％以内。但是点计数法统计结果的精度受到薄片中生屑颗粒丰度、粒径大小、分选性甚至生屑分异度等沉积组分特征的影响。因此在实际工作中,应根据具体情况,适度调整计点数,从而在统计工作量和统计数据误差之间获得平衡。     

Inhibition of dissolution within shallow water carbonate sediments: impacts of terrigenous sediment input on syn-depositional carbonate diagenesis

The early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated, shallow water, tropical carbonate sediments. The diagenetic reactions operating within carbonate sediments that fall under the influence of iron‐rich (terrigenous) sediment input are less clearly understood. Such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction, and by the reaction of any pore‐water sulphide with iron oxides, thereby minimizing sulphide oxidation and associated acidity. To test this hypothesis sediment cores were taken from sites within Discovery Bay (north Jamaica), which exhibit varying levels of Fe‐rich bauxite sediment contamination. At non‐impacted sites sediments are dominated by CaCO₃ (up to 99% by weight). Pore waters from the upper few centimetres of cores show evidence for active sulphate reduction (reduced SO₄/Cl^? ratios) and minor CaCO₃ dissolution (increased Ca²⁺/Cl^? ratios). Petrographic observations of carbonate grains (specifically Halimeda and Amphiroa) show clear morphological evidence for dissolution throughout the sediment column. In contrast, at bauxite‐impacted sites, the sediment is composed of up to 15% non‐carbonate and contains up to 6000 μg g^?1 Fe. Pore waters show no evidence for sulphate reduction, but marked levels of Fe(II), suggesting that bacterial Fe(III) reduction is active. Carbonate grains show little evidence for dissolution, often exhibiting pristine surface morphologies. Samples from the deeper sections of these cores, which pre‐date bauxite influence, commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input. Previous studies have suggested that dissolution, driven by sulphate reduction and sulphide oxidation, can account for the loss of as much as 50% of primary carbonate production in localized platform environments. The finding that chemical dissolution is minor in a terrigenous‐impacted carbonate environment, therefore, has significant implications for carbonate budgets and cycling, and the preservation of carbonate grains in such sediment systems.     

Bathymetry and sediments on the carbonate platform off western India: Significance of Halimeda bioherms in carbonate sedimentation

Bathymetry across the carbonate platform off western India indicated small-size pinnacles and their lateral coalescence into 2 -6-m high mounds landward, and linear elongated carbonate ridges and troughs, mounds and banks up to a height of 20-m seaward of the platform. Seismic data indicated that these mounds were transparent with no rigid internal structure and can be defined as bioherms. The sediments were abundantly aragonite faecal pellets, Halimeda grains and ooids and their radiocarbon ages ranged from 11 to 7.5 ka BP. It appears that the growth of Halimeda bioherms on the platform was facilitated by intense upwelling during the early Holocene. The terrigenous sediments brought by rivers were deposited in the inner shelf and have not affected the growth of bioherms. It is estimated that the platform comprises at least 1.85 Gt of mass CaCO\(_3\) accumulated during the early Holocene and comparable to those on the Great Barrier Reef. Halimeda bioherms produce abundant carbonate sediments and their growth period represents a geological carbonate sink and release of high CO\(_2\) to the atmosphere. Detailed shallow seismic studies and sediment cores are needed to quantify the exact mass content of CaCO\(_3\) and model climate change during the early Holocene.     

Quantitative compositional analysis of a Triassic carbonate platform (Southern Alps,Italy)

We have estimated abundance and distribution of automicrite, marine cements and skeletal grains in the Triassic Sella massif, an isolated platform flanked by steep (25–35°) clinoforms. 108 samples were taken at constant intervals from measured sections of the major zones of the platform edifice: the platform top, margin–upper slope, and lower slope. In a first step, carried out in the field and on hand specimen, purely detrital deposits were separated from automicrite facies, i.e. beds with automicrite, cement-filled, primary vugs and admixtures of skeletal carbonate and lithoclasts. In the second step, samples with automicrite facies were thin-sectioned and point counted. The categories used for point counting were (a) automicrite, (b) vugs and cement, (c) microspar or neomorphic spar, (d) skeletal grains and (e) internal sediments. At the platform top 46% of samples are pure detrital deposits, 27% consist of automicrite facies and 27% are too strongly altered by dolomitization to allow classification. At the margin–upper slope 68% of samples consist of automicrite facies, 22% are pure detrital sediments and 10% are strongly altered. At the lower slope 63% are detrital deposits, 10% automicrite facies and 27% are extensively dolomitized. The most important contributors to the automicrite facies are automicrite (41% on the platform top, 29% on the margin–upper slope, 28% on the lower slope) and early marine cement (35% on the platform top, 48% on the margin–upper slope, 27% on the lower slope). The amount of skeletal grains is less than 10%.The automicrite facies stabilized the platform margin and upper slope. Automicrite, abundant early marine cements and micro-organisms such as Tubiphytes, formed a rigid framework, thus substituting for the lack of a metazoan reef. On the upper slopes, the framework of automicrite facies stabilized the slope but intermittently. The automicrite layers are frequently dissected by sediment-filled fractures or are broken into clasts. We assume that they slid on the layers of loose detritus. Bigger slides turned into rubbly debris flows that formed metre-thick breccias at the lower slope and the proximal basin floor. The planar shape and steep angle of the clinoforms indicate that the large-scale geometry of the slope was not controlled by the automicrite but rather by non-cohesive layers of sand and rubble piled up to the angle of repose.The production mode of the Sella is comparable of that of a (mud) mound factory. This factory was highly productive: in 1 Ma, the platform aggraded over 300 m and prograded over 2000 m in all directions.     

Geochemical and mineralogical characteristics of recent clastic sediments from lower Godavari river: Implications of source rock weathering

The major, trace and rare earth elements geochemistry and clay mineral compositions in the river bed sediments from lower reaches of Godavari river suggest that they are derived from weathering of felsic rocks. Trace and rare earth elemental compositions indicate evidence of sedimentary sorting during transportation and deposition. Lower concentrations of transition elements, such as V, Ni and Cr imply enrichment of felsic minerals in these bed sediments. The REE pattern in lower Godavari sediments is influenced by the degree of source rock weathering. The light rare earth elements (LREE) content are indicating greater fractionation compared to the heavy rare earth elements (HREE). A striking relationship is observed between TiO₂ and gZREE content suggesting a strong control by LREE-enriched titaniferous minerals on REE chemistry. Shale-normalized REE pattern demonstrate a positive Eu anomaly, suggesting weathering of feldspar and their secondary products, which are enriched in Eu. Chondrite-normalised REE pattern is characteristic of felsic volcanic, granites and gnessic source rocks. Trace elemental compositions in sediments located near urban areas suggest influence of anthropogenic activity. Chemical Index of Alteration (CIA) is high (avg. 65.76), suggesting a moderate chemical weathering environment. X-ray diffraction analysis of clay fraction shows predominance of clay minerals that are formed because of the chemical weathering of felsic rocks.     

Chemical and mineralogical composition of fluvial sediments (Bistrita River,Romania): Geogenic vs. anthropogenic input into rivers on its way through mining areas

The upper reaches of the Bistrita drainage system were selected as a natural test site to determine the geogenic and anthropogenic input into fluvio-lacustrine systems in humid mid-latitude morphoclimatic zones. The reason for this selection lies in the complex geology and its metallogenic evolution leading to a great variety of Fe, Mn, U, and polymetallic sulfides ore deposits. It sparked an intense mining activity during the past centuries with a strong impact on the drainage system similar to many mineralized sites in the world which are still under exploitation. Sediment samples from Bistrita River were analyzed by means of X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), near-infrared (NIR) and micro-Raman spectroscopy (μ-Raman).Our results revealed that the chemical and mineralogical built-up of the stream sediments is mainly geogenic, with most of the trace elements accumulated in the river sediments derived from the source rocks exposed in catchment area of the River Bistrita. A strong input by man has been detected in the drainage system near abandoned mining sites. The trace elements are mainly accommodated in the structure of detrital minerals representative of the clastic aureole around the source rocks, and to a lesser extent adsorbed onto the surface of clay minerals. The REE incorporated into muscovite furnish evidence of having derived from the source rocks, prevalently mica schists exposed by supergene processes in the provenance area and rule out a neoformation of clay minerals on transport and deposition.     

Lithification of some modern carbonate sediments in a hypersaline lake adjacent to the Black Sea

In the hypersaline Techirghiol Lake adjacent to the Black Sea, sporadically formed lithified blocks and grapestone are found which are the result of cementation of carbonate sediments with aragonite and possibly kutnahorite (manganesian calcite). The lithified blocks are characterized by a central cavity bordered by a lithified envelope. The formation of the carbonate cement is due to subaquatic bacterial processes of calcium sulfate reduction and the synthesis of calcium carbonate at the expense of gypsum concretions derived from Pleistocene red clays cropping out along the shore of the lake. These lithified blocks have geological significance as they may be usable as criteria for the recognition of sediments deposited in the past near to the shores of hypersaline lakes.     

Quantitative analysis of calcite and Mg-calcite by X-ray diffraction: effect of grinding on peak height and peak area

Mineralogical analysis of calcite and Mg-calcite by X-ray diffraction requires that the samples be ground to a powder. Such grinding determines the particle size of the powder and the structural damage of the minerals. Both of these in turn affect the peak intensities recorded by the X-ray machine. Most carbonate sediments are inhomogeneous; they contain both calcite and Mg-calcite which are affected differently by grinding. Such differences cause quantitative analytical results to be inconsistent with the true mineralogical abundance. The two acceptable methods of analysis—(1) measurement of peak height from the base and (2) measurement of the area under the peak—were compared to determine if sample preparation affects the quantitative results. In samples with variable and relatively small amounts of calcite and Mg-calcite the measurement of peak height yields more reproducible results than does the measurement of peak areas. Different proportions of particle size of the mineralogical components in a sample powder, affect proportionally more the peak areas than the peak heights. Extensive grinding causes structural damage of the component minerals which affects much more the peak areas than the peak heights. Thus for quantitative analyses of calcite and Mg-calcite in inhomogeneous carbonate samples which require differing grinding times and have greatly variable amounts of calcite and Mg-calcite, the peak height measurement seems to be a better method than peak area measurement.