Mg-bearing Minerals Research Articles

Weathering of soil minerals by angiosperm and gymnosperm trees

AbstractWeathering of terrestrial Ca- and Mg-bearing silicate minerals is an important control on atmospheric CO2 on geological time scales. It has been determined that vascular plants can accelerate mineral weathering as compared to non-vascular plants or non-vegetated surfaces. This indicates that the evolution of vascular plants, particularly the deep-rooted trees, may play a large role in the long-term carbon cycle and its regulation of the atmosphere. The weathering impact of the separate evolutionary appearances of the gymnosperms in the Palaeozoic and the angiosperms in the Mesozoic, and the shifting ecological dominance from the former to the latter, is currently poorly understood. This study aims to contribute to our understanding of the quantitative weathering rates of the angiosperms and gymnosperms by examining plant-mineral interactions of the two tree types in a temperate field setting underlain by granodiorite. Results include determinations of soil element fluxes and etching of minerals. The observed root-mineral interactions resulted in only slightly more weathering of Ca-bearing minerals by the angiosperms. However, we observed significantly more weathering of the Mg-bearing minerals by the gymnosperms. These results suggest that increasing dominance of the angiosperms in forests in the Mesozoic may have had a small or neutral impact on accelerating overall mineral weathering and regulating CO2, but that this impact may be lithology-dependent.

Response of silicate weathering to monsoon changes on the Chinese Loess Plateau

Abstract Eolian deposits of loess on the Chinese Loess Plateau are investigated to decipher the influence of sequentially changed monsoon climate on silicate weathering process. Detailed hydrochloric acid dissolution experiments are applied to establish a sensitive proxy for silicate weathering. Combined elemental and mineralogical studies show that the minerals which are susceptible to incipient chemical weathering can be totally dissolved in hot hydrochloric acid (80°C, 3mol/L) after 4h, while other stable minerals are nearly unaffected. Thus, the hydrochloric acid dissolvable fractions (ADF) of loess and paleosol are more sensitive than bulk samples in the weathering reaction. Since Mg is tend to be leached out from the ADF while Al is conserved during the incongruent weathering, Mg/Al ratio of the ADF (Mg/Al ADF ) could indicate the weathering intensities of the Mg-bearing minerals, mainly chlorite. The Mg/Al ADF of the modern soils on the Chinese Loess Plateau is closely correlated to the local precipitation amounts, suggests that the intensity of summer monsoon is a key factor in the weathering of Mg-silicate. The silicate weathering intensity of the loess-paleosol deposits of past 130Kyrs shows strong procession cycle of ∼ 23Kyrs coupled by glacial-interglacial variation. The ∼ 23Krys cycles are coherent to the variations of the intensity of summer monsoon, while the ∼ 100Kyrs glacial-interglacial variation may relates to the changes in winter monsoon. Phased intensification of East Asian summer monsoon, and thus enhanced silicate weathering and atmosphere CO 2 drawdown, in response to the uplift of Tibetan Plateau, may be another mechanism relating the late Cenozoic tectonics to global cooling.

Major and trace elements in paddy soil contaminated by Pb–Zn mining: a case study of Kočani Field, Macedonia

The objective of this study was to assess the bulk chemical composition as well as the extent and severity of heavy metal contamination in the paddy soil of Kocani Field (eastern Macedonia). The results revealed that the paddy soil of the western part of Kocani Field is severely contaminated with Pb, Zn, As and Cd in the vicinity of the Zletovska River due to irrigation with riverine water that is severely affected by acid mine and tailing effluents from the Pb-Zn mine in Zletovo. The detected total concentrations of these metals are far above the threshold values considered to be phytotoxically excessive for surface soil. The paddy soil in the vicinity of the Zletovska River was also found to exhibit elevated levels of Ba, Th, U, V, W, Mo, Cu, Sb, Bi, Ag, Au, Hg and Tl, with concentrations above their generally accepted median concentration values obtained during this study. A correlation matrix revealed that the Mn and Fe oxides/hydroxides are the most important carrier phase for several trace elements, with the exception of rare earth elements (REEs). These also represent a major sink for the observed heavy metal pollution of the soil. REEs are mostly associated with two phases: light (L)REEs are bound to K-Al, while heavy (H)REEs are bound to Mg-bearing minerals. Although there is no direct evidence of a health risk, the paddy soil in the vicinity of Zletovska River needs further investigation and an assessment should be made of its suitability for agricultural use, particularly in view of the highly elevated concentrations of Pb, Zn, As and Cd.

Mineral matter crystallization and crack formation in tuyere coke

Polished and dry-cut sections of samples of tuyere coke were studied by scanning electron microscopy to reveal the mechanism of appearance, distribution and effects of aluminosilicate spherules and irregular segregations of slag and spinel crystals formed in the coke matrix. It was found that the formation of spherules and slag is not destructive with respect to the host coke matrix. On the contrary, the octahedral spinel crystals, which grow from the aluminosilicate spherules gradually expose {1 1 1} faces and edges and change the shape and possibly volume of the parental mineral matter. The sharp and straight edges probably cause cracks to appear in the coke matrix, leading to a weakening of its strength upon crystal growth. The proposed model for this crack formation probably reflects the negative role of some Al- and especially Mg-bearing minerals in the coal blends used for the production of coke, and partly reveals mineralogical insights into the negative influence of high ash basicity on coke strength.

Kinetics of brucite dissolution at 25°C in the presence of organic and inorganic ligands and divalent metals

Brucite (Mg(OH) 2) dissolution rate was measured at 25°C in a mixed-flow reactor at various pH (5 to 11) and ionic strengths (0.01 to 0.03 M) as a function of the concentration of 15 organic and 5 inorganic ligands and 8 divalent metals. At neutral and weakly alkaline pH, the dissolution is promoted by the addition of the following ligands ranked by decreasing effectiveness: EDTA ≥ H 2PO 4 − > catechol ≥ HCO 3 − > ascorbate > citrate > oxalate > acetate ∼ lactate and it is inhibited by boric acid. At pH >10.5, it decreases in the presence of PO 4 3−, CO 3 2−, F −, oxine, salicylate, lactate, acetate, 4-hydroxybenzoate, SO 4 2− and B(OH) 4 − with orthophosphate and borate being the strongest and the weakest inhibitor, respectively. Xylose (up to 0.1 M), glycine (up to 0.05 M), formate (up to 0.3 M) and fulvic and humic acids (up to 40 mg/L DOC) have no effect on brucite dissolution kinetics. Fluorine inhibits dissolution both in neutral and alkaline solutions. From F sorption experiments in batch and flow-through reactors and the analysis of reacted surfaces using X-ray Photoelectron Spectroscopy (XPS), it is shown that fluorine adsorption is followed by its incorporation in brucite lattice likely via isomorphic substitution with OH. The effect of eight divalent metals (Sr, Ba, Ca, Pb, Mn, Fe, Co and Ni) studied at pH 4.9 and 0.01 M concentration revealed brucite dissolution rates to be correlated with the water molecule exchange rates in the first hydration sphere of the corresponding cation. The effect of investigated ligands on brucite dissolution rate can be modelled within the framework of the surface coordination approach taking into account the adsorption of ligands on dissolution-active sites and the molecular structure of the surface complexes they form. The higher the value of the ligand sorption constant, the stronger will be its catalyzing or inhibiting effect. As for Fe and Al oxides, bi- or multidentate mononuclear surface complexes, that labilize Mg-O bonds and water coordination to Mg atoms at the surface, enhance brucite dissolution whereas bi- or polynuclear surface complexes tend to inhibit dissolution by bridging two or more metal centers and extending the cross-linking at the solid surface. Overall, results of this study demonstrate that very high concentrations of organic ligands (0.01–0.1 M) are necessary to enhance or inhibit brucite dissolution. As a result, the effect of extracellular organic products on the weathering rate of Mg-bearing minerals is expected to be weak.

Differential thermal analysis of Mg-bearing carbonates and sheet silicates

In the present paper the thermal behaviour of Mg-carbonates and -silicates is reviewed and discussed, based on own DTA investigations and data from the literature. Mg-bearing minerals in soils and sediments offer informations about the palaeoenvironment and palaeoclimatology of sediments and soils and - thus - the study of their crystal chemical compositions by means of (differential) thermal analysis informs about environmental conditions of (sedimentary Mg-) mineral (trans-) formations. The paper stresses (1) the interrelations between decomposition temperatures of (Mg-) carbonates and substitution processes and is (2) concerned with the interdependence between the dehydroxylation behaviour of Mg-bearing sheet silicates and their crystal chemical composition.

Weathering, element distribution and geochemical dispersion at Mt Keith, Western Australia: implication for nickel sulphide exploration

The komatiite–dunite-hosted Ni sulphide deposit at MKD5 (Mt Keith) has a complex regolith consisting of a residual profile (>75 m depth), partly collapsed towards the top, overlain by 40 m of exotic sediments. The weathered komatiites are composed predominantly of degraded serpentine, neo-formed hydrated Mg-silicates, carbonates, silica and Fe oxides. Magnesium concentrations decrease upwards through the lower regolith, whereas those of Fe and Si increase, reflecting degradation of Mg-bearing minerals. A sharp decline of Mg within the profile marks the Mg discontinuity , separating Mg-saprolite from the overlying Fe saprolite. The Fe saprolite, dominated by Fe oxides and silica, has pseudo-sedimentary structures indicating partial collapse of the profile. At the unconformity with the sediments, eluviation, mixing and churning have produced zones with chemical and mineralogical characteristics of the underlying, overlying and adjacent lithologies. Nickel, Co, Mn, Cr and PGE are dominantly hosted by secondary Fe oxides in the regolith, but above the Mg-discontinuity, Ni and Co are locally enriched by co-precipitation with Mn oxides. The oxidation potential, controlled by ferrolysis, ranges from −0.4 to +0.2 volts, and the pH, buffered by bicarbonate ion, irrespective of the presence of sulphides, decreases from 9.5 to 6.0 up through the profile. Copper is not significantly mobile under such conditions and precipitates as inert native metal, commonly spatially associated with Fe oxides. Separation of Ni and Cu occurs in the upper saprolite and pedolith, because metallic Cu (±Pd) is retained and residually concentrated in collapsed Fe saprolite, whereas Ni, adsorbed to goethite, is redissolved and reprecipitated below the Mg-discontinuity. High Ni concentrations in the regolith cannot alone be used to distinguish between Ni sulphide mineralized (NiS) and barren komatiitic protoliths. The strong correlation between chalcophile elements evident in mineralized protolith is lost with the oxidation of sulphides, and Ni derived from sulphides is indistinguishable from that derived from silicates. Ultramafic-derived regolith materials having Ni/Cu ratios of 7–19 and >19 may indicate the presence of massive and disseminated sulphides, respectively. The ratio at MKD5 is 60. These relationships may be distorted by preferential leaching, or by the influx of either element from wall-rocks. High concentrations of PGE, especially Pt, Pd and Ru, may also indicate mineralized protolith. However, the very low secondary mobility of PGE and Cu greatly restricts target size. Larger targets are given by Ni/Cr and Cu/Zn ratios which, either singly or multiplied together, can be used to identify mineralized komatiitic sequences and provide vectors towards ore. Their value is diminished if the ore and host sequence are decoupled during emplacement.

Effect of Ca- and Mg-bearing minerals on particle agglomeration defluidisation during fluidised-bed combustion of a South Australian lignite

Behaviour of calcium and magnesium during fluidised-bed combustion (FBC) of a South Australian lignite was investigated using a laboratory scale spouted bed combustion system. Combustion experiments were aimed at investigating the effectiveness of Ca- and Mg-bearing minerals (as alternative bed materials) in controlling particle agglomeration and bed defluidisation during FBC combustion of low-rank coals. Additional experiments performed with a Ca-treated coal investigated the role of Ca in agglomeration and defluidisation process. Experimental results indicated that both Ca/Mg-bearing minerals and Ca-treated coal were effective to different extents in reducing bed defluidisation. Tests with calcite (as the bed material) and Ca-treated coal runs (with sand as the bed material) showed trouble free operation for 8–10 h before bed defluidisation incurred. Tests with magnesite (as the bed material) showed no agglomeration and defluidisation tendencies for longer operating periods (∼12 h at 800°C). Mg-bearing compounds have been found to be effective in controlling defluidisation and allowed extended combustion operations. On the other hand, high levels of Ca either in coal or in bed material have been found to delay and decrease the severity of agglomerates formed.

The application of glacial dispersal models to the interpretation of till geochemistry in Labrador, Canada

In western Labrador, Canada, bedrock composition and glacial history are both reflected in till geochemistry. There, compositional contrasts between igneous and metamorphic bedrock of the Canadian Shield and comparatively unmetamorphosed bedrock of the Labrador Trough allow regional-scale (100s of km) definition of glacial dispersal trends through lithological and geochemical analyses. Compared to till derived from shield terrane, Labrador Trough till is depleted in chromium, but enriched in zinc, iron, and manganese. Further, the study area is situated near one or more dispersal centers of the Laurentide Ice Sheet and has been subjected to a complex sequence of ice flow, defined by striations and indicator erratic distributions. Three glacial events serve as the principal controls on drift composition, although their relative significance varies. Compositional profiles along the axis of two glacial dispersal trends show systematic changes in indicator erratic concentrations with distance along the path of ice flow. In the profile having one prominent glacial transport direction, indicator concentrations decrease exponentially with increasing distance of glacial transport. From dispersal models, Renewal Distances representing the compositional `half-life' tend to increase with decrease in grain size, i.e. 15 to 25 km for pebbles, and 30 to 40 km, or more, in silt- and finer-sized fractions. The differences between geochemical and lithological profiles reflect differences in: (1) the intensity or duration of subglacial process affecting the milling of indicator detritus to finer fractions; and (2) the physical and chemical properties of the rocks and minerals hosting the trace metals. Where regional glacial dispersal trains extend across contrasting geological terranes, indicator properties can be used to resolve till composition into `local' and `transported' components, providing a better approximation of bedrock composition for mineral exploration. The compositional expression of bedrock can be masked by transported overburden, with the extent of masking dependent on the trace metal examined. Linear relations between metals (Cr, Zn) and lithophile elements (e.g. K, Mg) indicate that Mg-bearing minerals, likely phyllosilicates, have a strong mineralogical control on till geochemistry, and metal : lithophile element ratios can be used to distinguish bedrock provenance in glacially transported materials of western Labrador.

Geological factors affecting the distribution of trace metals in glacial sediments of central Newfoundland

In central Newfoundland (NTS 12A/10, 15, 16, 2H/1), As, Pb, and Zn concentrations in the clay-sized ( 1000 ppm), exceeding levels commonly set for purposes of environmental protection. Near Pb-Zn mines at Buchans, geochemical variation with depth reflects the dispersal of detritus from mineralized bedrock, and differences in sediment type and provenance. There, surface sediments are rich in granitic debris derived from the Topsails igneous terrane 5 km north of Buchans and contain low concentrations of trace metals. These sediments are compositionally unrelated to either Buchans Group volcanic rock or an underlying, older till enriched in sulphide minerals and trace metals. Metal-rich till extending up to 10 km southwest of Buchans results from combined glacial and debris flow transport related to two distinct geological events. Trace metals are enriched (two- to fourfold) in the clay-sized fraction of till compared to the silt and clay-sized, and are associated with Al- and Mg-bearing minerals that preferentially concentrate in the clay fraction. The geochemistry of the silt and clay-sized fraction can approximate that of the <2-mm fraction. Background variations in till illustrate the important role of a geological framework to the interpretation of geochemical surveys and the origins of trace metals in the environment.

Hydration Reactions at the Mineral—Fluid Interface: Experimental and Computational Studies

Reactions that occur at the mineral-fluid interface, such as mineral dissolution, trace Aelemeat adsorption, and secondary phase precipitation, are of particular interest to geochemists. Several new surface-sensitive analytical techniques have recently been applied to the study of these reactions. In addition, recent advances in ab initio quantum mechanical calculations have made it much easier to translate experimental results into models of the actual atomic-scale reactions that occur at surfaces. The penetration depth of protons into the feldspar near-surface environment during hydrolysis has been successfully measured using Elastic Recoil Detection Analysis (ERDA) (Casey et al., 1989). Using this method, the penetration profile of H § in minerals as a function of reaction time, temperature, pH, and crystallographic orientation can be determined. The H + distribution in the mineral near-surface can be used to: [1] constrain the thicknesses of surfaceillayers that have hydrated to form distinct new crystal structures, [2] define the depth to which cation-proton exchange reactions penetrate, and [3] construct a charge balance inventory. Surface sensitive X-ray techniques, such as Xray absorption spectroscopy (electron-yield geometry), X-ray reflectivity, and low-angle surface diffraction have also been applied to mineral surface experiments and can provide unique atomic-scale structural and physical data (Chiarello et al., 1993). Detailed information about changes in surface morphology, the nature and growth dynamics of altered or recrystallized layers, and the atomic arrangements of adsorbates can all be produced by these types of measuremeats. Unambiguous results with simple oxides such as periclase will provide a foundation to our understanding of how more complicated Mgbearing minerals react with aqueous fluids. X-ray results for periclase and ERDA results for olivine (Fogo) are presented below. Accurate periodic Hartree-Fock calculations (Scamehorn et aL, 1993) and experiments (Onishi et al., 1987) show that water physisorbs onto clean (100) periclase surfaces but that there is no dissociation or chemisorption. However a reaction takes place on exposure to water over a period of several hours to produce an opaque layer at the surface (presumably brucite). A reconstructive mechanism is necessary to explain the slow reaction rate. We observe that the atomic structure at a periclase (111) surface is isomorphic to brucite (0001) with a 5% discrepancy in lattice parameter. This suggests that protonation acts to stabilize the otherwise unstable (111) surface by forming a brucite-like layer and allows the hydration reaction to proceed by growth of brucite at (111) etch pits. Theoretical calculations will demonstrate this stabilization.

Pedogenesis of a chronosequence in the Sacramento Valley, California, U.S.A., II. Elemental chemistry of silt fractions

A chronosequence of six soils from alluvium along Honcut Creek in central California has been analyzed to document pedogenic changes in the fine silt (2–20 μm) and coarse silt (20–50 μm) fractions with increasing soil age. The soils range in age from 600 yr to 1.6 million yr and have been formed in a xeric-thermic soil climate. Parent materials are moderately stratified loams, silt loams, and sandy loams derived from basic metavolcanic and granitic rocks. Parent material uniformity was quantified using particle-size distribution of non-clay fractions and X-ray fluorescence (XRF) elemental analysis of silt fractions. Pedogenic changes were quantified using XRF elemental data and a stable index calculation with Zr as the index element. Parent sediments of four of the six profiles fine upward. Parent elemental composition of the silt fractions was less variable than was texture: for example, clay-free silt/sand ratio changes by a factor of 5 down the horizons of the 600-yr-old soil whereas Ca/Zr ratio in fine silt changes by a factor of only 1.1. Comparison of C and deep BC horizons among soils demonstrate that the parent elemental composition was not constant for all chronosequence members. Silicon, K, Ti and Zr become more abundant and Na, Mg, Al, Ca and Fe less abundant in silt fractions of A and B horizons with increasing soil age due to differential weathering rates of primary minerals. All elements decrease in absolute abundance in silt fractions when Zr is used as a stable index element. About 40–50% of the Ti has been lost from the silt fractions of the 1.6-million-yr-old soil and proportional amounts from younger soils, clearly indicating that Ti is mobile in these soils. Magnesium loss can be detected over the effects of stratification in the 10,000-yr-old soil and Mg-bearing minerals are virtually absent from the silt fractions of the upper 1.5 m of soils that are 130–250,000 yr in age. Calcium, Na and Mg are almost completely weathered from the silts in the upper 4 m of the oldest soil, whereas half to more than three-fourths of K is retained. Age trends of element depletion are systematic for those elements and indices that are not influenced by stratification or change in other soil-forming factors. Age trends of depletion are linear when regressed on the log of soil age and have correlation coefficients ( r) that range from 0.57 to 0.93. A mobility sequence based on the degree of depletion of elements from fine silt of the 130,000- and 250,000-yr-old soils is: Mg ⪢ Na > Ca ≅ Fe > Al > Si > Ti > K > Zr.

Relationship Between Exchangeable and Total Magnesium in Pennsylvania Soils

AbstractThe particle size distribution, total and exchangeable Mg, and mineralogical compositions were determined on eight well-drained, noncultivated subsoils from Pennsylvania. No correlation was found between the clay content and total Mg (r =.29), or between the clay content and exchangeable Mg (r =.35). Serpentine, talc, and hypersthene were found in the very fine sand and silt fractions of soils having relatively high exchangeable Mg. Mica and 14-Å clay minerals were the only Mg-bearing minerals noted in the same fractions of soils having relatively low exchangeable Mg. Of the Mg-bearing clay minerals found in the clay fractions (smectite, vermiculite, chlorite, illite, and interstratified chlorite/vermiculite), only smectite decreased as the exchangeable Mg in the soils decreased. Two distinctly different distribution patterns of Mg were found for soils having relatively high and low exchangeable Mg. The former soils showed a decreasing Mg content as the particle size decreased, and the latter soils showed the opposite. Exchangeable Mg correlated significantly with the amount of Mg in whole soil, sand, and silt, but not with the amount of Mg in the clay, an indication that sand and silt but not clay were the important sources of exchangeable Mg in these soils.