Screening Effect of the Diffusive Boundary Layer in Sediments of Lake Aha in the Suburbs of Guiyang City,Guizhou Province

The redox cycle of iron and manganese is a major geochemica process at the boundary layers of lake sediments.Lake Aha,which lies in the suburbs of Guiyang City,Guizhou Province,China,is a medium-sized artificial reservoir with seasonally anoxic hypolimnion,Long-term sedimentary accumulation of iron and manganese resulted in their enrichment in the upper sediments,In the anoxic season,Fe^2 and Mn^2 ,formed by diological oxidation,would diffuse up to overlying waters from sediments.However,the concentration of oxidation,would diffuse up to overlying waters from sediments,However,the concentration of Fe^2 increased later and decreased earlier than that of Mn^2 .Generally,sulfate reduction occurred at 6 cm below the sediment-water interface.Whereas,in the anoxic season.the reduction reached upper sediments,inhibiting the release of Fe^2 ,The Fe concentration of anoxic water is quickly decreased from high to low as a result of reduction of the suplhur system.     

Petrology of a piemontite-bearing gneiss,San Gorgonio Pass,California

Equilibria between different valence states of Fe and Mn have been studied in a microcline-plagioclase-quartz gneiss which locally contains ferromagnesian minerals unusually high in Mn⁺³ and Fe⁺³ and low in Fe⁺². The compositions of coexistent minerals have been determined by chemical and microprobe analyses. The minerals in some layers were formed under highly-oxidizing conditions, as indicated by extremely low Fe⁺²/Fe⁺³ ratios in the silicates, by the presence of hematite, and by the occurrence of piemontite, which requires Mn⁺³ for its formation. The minerals in other layers were formed under less-oxidizing conditions, as indicated by the fact that epidote, rather than piemontite, crystallized with Mn-rich garnet and by the presence of biotite rather than phlogopite. In the less-oxidized layers Mn⁺³ appears to be absent. The differences in oxidation of Fe and Mn occur between adjacent layers and probably reflect sedimentary differences preserved despite the metamorphism.Iron and manganese with different valences are sharply partitioned between the coexisting phases. In highly-oxidized layers, muscovite contains more iron (as Fe⁺³) than coexistent phlogopite; in piemontite most of the manganese is Mn⁺³, while in coexistent garnet most of the manganese is Mn⁺². In less-oxidized layers, epidote contains no Mn⁺³ and contains less Mn⁺² than coexistent garnet, biotite, or amphibole. Analytical data, crystal-chemical arguments, and characteristics of Fe and Mn L-spectra indicate that in coexistent garnet and piemontite, Fe⁺², Fe⁺³, Mn⁺², and Mn⁺³ are present, in spite of the fact that trivalent manganese strongly oxidizes divalent iron in aqueous solution under normal conditions.Contribution No. 1468.     

An EPR study of Mn2+ and Fe3+ in dolomites

Electron paramagnetic resonance (EPR) measurements on dolomites from 9 different localities revealed contents of Mn²⁺ on two axial sites in all of them. The center with largerzero-field splitting (ZFS) was always present in much higher concentrations, except for a sample from Oberdorf it amounted to 95 percent or more of the total. This dolomite was the only one with a considerable content of Fe³⁺ on one axial site, almost certainly substituting for Mg²⁺. With X-ray irradiation the concentration of Fe³⁺ increased by about 30 percent showing that at least some of the divalent iron also substitutes for Mg. The ZFSs for Fe³⁺ and Mn²⁺ with larger ZFS increase with decreasing temperature in the same manner. The previous assignment of this Mn²⁺ to Mg sites is thus confirmed. An almost regular increase of the trigonal distortions at the divalent ions in different carbonates with increasing ionic radius is indicated by their crystal structure data. The very small ZFS for Mn²⁺ on Ca sites in dolomite must thus result from a strong local relaxation in the direction of a more regular octahedral arrangement. It is difficult to explain the different distribution ratios of Mn²⁺ on Ca and Mg sites with differences in growth and/or annealing temperatures alone. Thus different supply of Mg²⁺ and Ca²⁺ in the growth solutions may also contribute.     

Cathodoluminescence of synthetic and natural calcite: the effects of manganese and iron on orange emission

Summary The orange cathodoluminescence (CL) of calcite is known to be due to the presence of Mn²⁺ cations. It has been demonstrated here using CL and electron paramagnetic resonance (EPR) crossed analysis from synthetic calcite that neither Fe²⁺ nor Fe³⁺ ions influence this luminescence emission. More complex natural calcium carbonates have been investigated to check whether or not this conclusion can be applied to them. For this purpose, different white marbles from Greek quarries were analysed with CL. The data are completed with neutron activation analysis (NAA) for manganese and iron contents. Again it is shown that only manganese plays a role in the orange CL of these white marbles. This result provides an important clue in the wide field of provenance determination of calcium carbonate used in ancient art.Received February 19, 2002; revised version accepted October 22, 2002 Published online March 10, 2003     

On the distribution of iron and manganese at the sediment/water interface: thermodynamic versus kinetic control

Iron and manganese solubility at the sediment/water interface has been studied at a water depth of 20 m in Kiel Bight, Western Baltic. By means of an in situ bell jar system enclosing 3.14 m² sediment surface and 2094 l water a complete redox turn-over in the bottom water was simulated in an experiment lasting 99 days. The concentration of dissolved Fe in the bell jar water never exceeded 0.041 μmol · dm^?3during the first 50 days of the experiment and then rose abruptly as the Eh fell from +600 to ?200 mV. The concentration of dissolved Fe under oxic and anoxic conditions seems to be limited by equilibria with solid Fe-phases (hydroxides and amorphous sulphide, respectively). In contrast to Fe, manganese was released continuously from the bottom during the first 50 days of the experiment leading to exponentially increasing manganese concentrations in the bell jar water. During this time dissolved O₂ had become ready depleted and pH had dropped from 8.3 to 7.5. Contrary to iron, manganese being solubilized in reduced sediment layers can penetrate oxic strata in metastable form due to slow oxidation kinetics; when the redoxcline moves upwards Mn²⁺ is enriched in bottom waters. The maximum concentration of dissolved Mn under anoxic conditions is controlled by a solid phase with solubility properties similar to MnCO₃ (rhodochrosite). Bottom water enrichment in dissolved Mn²⁺ could be traced to originate from excess solid manganese within the top 3 cm of the sediment.     

福建云霄石榴子石宝石矿物学特征

福建云霄是我国重要的宝石级石榴子石产地,然而该区石榴子石的致色机理不清,制约了对其形成机制的理解及后续开发利用。本文选取7件福建云霄橙黄-橙红色石榴子石样品,利用傅立叶红外光谱、紫外-可见光光谱和拉曼光谱分析其谱学特征,使用电子探针及激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)分析限定其主量、微量元素组成。结果表明云霄石榴子石主要为锰铝榴石,其颜色主要与二价锰(Mn²⁺)和铁离子(Fe²⁺)对可见光的吸收有关,Mn²⁺导致其主体呈橙色,少量Fe²⁺控制其橙红色调,微量Ti⁴⁺使其呈褐色调。福建云霄石榴子石样品核部锰含量相对较低而铁、镁含量较高,锰元素含量由核部向边部逐渐升高,且具有重稀土元素富集、轻稀土元素亏损的左倾配分模式和Eu负异常,表明其形成于岩浆结晶作用晚期。     

Electron spin resonance spectra of marine and fresh-water manganese nodules

Eighty ferromanganese nodules from a wide variety of marine and fresh-water environments have been analyzed by electron spin resonance spectroscopy. The purpose has been to gain information on the forms in which the major constituents of manganese nodules are present. Contributions to ESR spectra of the nodules come mainly from Mn²⁺ and Fe³⁺. Deep-sea samples generally showed only broad resonance lines, and those with larger peaks close to g = 2.0 are believed to contain more Mn²⁺ than others. Some Antarctic and fresh-water nodules lack a strong Mn²⁺ resonance and have a peak around g = 4.0 which is most likely tetrahedral Fe³⁺. A number of smaller peaks in several samples could not be readily interpreted in terms of contributions from individual ionic species because of fundamental problems in preparing standards having the ion of interest in the same micro-environment as it experiences in the nodules.     

Defects in sodalite-group minerals determined from X-ray-induced luminescence

The luminescence spectra of a suite of natural sodium framework silicates including four different sodalite variants and tugtupite have been collected during X-ray irradiation as a function of temperature between 20 and 673 K. The origin of the emission bands observed in these samples is attributed to F-centres (360 nm), paramagnetic oxygen defects (400 and 450 nm), S₂ ^? ions (620 nm) and tetrahedral Fe³⁺ (730 nm). Luminescence in the yellow (550 nm) is tentatively attributed to Mn²⁺, and red luminescence in Cr-rich pink sodalite is possibly from Cr³⁺ activation. Sudden reduction in luminescence intensities of emission centres was observed for all minerals in the 60–120 K range. Since it is common to all the sodalite-group minerals, we infer it is a feature of the aluminosilicate framework. Sodalite luminescence has responses from substitutions on the framework (e.g. paramagnetic oxygen defects, Fe³⁺) which give sodalite properties akin to other framework silicates such as feldspar and quartz. However, the presence of the sodalite cage containing anions (such as F-centres, S₂ ^? ions) imparts additional properties akin to alkali halides. The possibility of coupling between Fe³⁺ and S₂ ^? is discussed. The overall luminescence behaviour of sodalite group can be understood in terms of competition between these centre types.     

Laser-induced time-resolved luminescence spectroscopy of minerals: a powerful tool for studying the nature of emission centres

The paper summarises new data and results referring to the characterization of the nature of luminescence centres in minerals that were published during the last 8 years. Besides well-established luminescence centres, such as Mn²⁺, Fe³⁺, Cr³⁺, divalent and trivalent rare-earth elements, S₂ ^?, and Pb²⁺, several other centres were proposed and substantiated, such as Mn³⁺, Mn⁴⁺, V²⁺, Ni²⁺, Pb⁺, Mn³⁺, Sb³⁺, Tl⁺, and radiation-induced centres. Also, a relatively new type of luminescence excitation mechanism is discussed briefly, namely plasma-induced luminescence. Here, the emission takes place when the matrix, where the formation of plasma is caused by irradiation with a beam of laser light, is capable to luminescence and contains luminescence centres.     

Electronic structures of iron(III) and manganese(IV) (hydr)oxide minerals: Thermodynamics of photochemical reductive dissolution in aquatic environments

Sunlight-induced reduction and dissolution of colloidal Fe-Mn (hydr)oxide minerals yields elevated concentrations of Fe²⁺ and Mn²⁺ in natural waters. Since these elements may be biolimiting micronutrients, photochemical reactions might play a significant role in biogeochemical cycles. Reductive photodissolution of Fe (hydr)oxide minerals may also release sorbed metals. The reactivity of Fe-Mn (hydr)oxide minerals to sunlight-induced photochemical dissolution is determined by the electronic structure of the mineral-water interface. In this work, oxygen K-edge absorption and emission spectra were used to determine the electronic structures of iron(III) (hydr)oxides (hematite, goethite, lepidocrocite, akaganeite and schwertmannite) and manganese(IV) oxides (pyrolusite, birnessite, cryptomelane). The band gaps in the iron(III) (hydr)oxide minerals are near 2.0-2.5 eV; the band gaps in the manganese (IV) oxide phases are 1.0-1.8 eV. Using published values for the electrochemical flat-band potential for hematite together with experimental pH_pzc values for the (hydr)oxides, it is possible to predict the electrochemical potentials of the conduction and valence bands in aqueous solutions as a function of pH. The band potentials enable semiquantitative predictions of the susceptibilities of these minerals to photochemical dissolution in aqueous solutions. At pH 2 (e.g., acid-mine waters), photoreduction of iron(III) (hydr)oxides could yield millimolal concentrations of aqueous Fe²⁺ (assuming surface detachment of Fe²⁺ is not rate limiting). In seawater (pH 8.3), however, the direct photo-reduction of colloidal iron(III) (hydr)oxides to give nanomolal concentrations of dissolved, uncomplexed, Fe²⁺ is not thermodynamically feasible. This supports the hypothesis that the apparent photodissolution of iron(III) (hydr)oxides in marines systems results from Fe³⁺ reduction by photochemically produced superoxide. In contrast, the direct photoreduction of manganese oxides should be energetically feasible at pH 2 and 8.3.     

The geochemistry of gem opals as evidence of their origin

Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si⁴⁺ by Al³⁺ and Fe³⁺. The induced charge imbalance is compensated chiefly by Ca²⁺, Mg²⁺, Mn²⁺, Ba²⁺, K⁺, and Na⁺. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence.     

Petrographic and geochemical characteristics of dolomite types and the origin of ferroan dolomite in the Trenton Formation, Ordovician, Michigan Basin, U.S.A.

Three major types of dolomite occur in the Trenton Formation (Mid-Ordovician) of the Michigan Basin. These are: (1) ‘regional dolomite’ which is confined to the extreme western edge of the basin; (2) ‘cap dolomite’ which occurs in the upper portion of the Trenton and is confined to the basin's southern margin; and (3) ‘fracture-related’ dolomite which occurs in association with both large- and small-scale faults and fractures. These three dolomite types can be distinguished from one another by their major element chemistry, oxygen isotope ratios and rock texture. The regional dolomite is fine-grained, has <0.34 mol% FeCO₃, and mean δ¹⁸O of ?6·8‰_OPBD. The cap dolomite is texturally similar to regional dolomite but contains 3–13·0 mol% FeCO₃ and has a mean δ¹⁸O of ?7·7‰. Fracture-related dolomites are coarse-grained, low in iron, and have the most depleted δ¹⁸O ratios (x?=–9·0%_PDB). Petrographic relationships imply that the regional dolomite, formed prior to the cap dolomite probably during early diagenesis. The cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying Utica Shale and the Trenton Limestone. Fracture-related dolomites post-date the cap dolomite and formed during deeper burial. A temperature of precipitation of approximately 80°C was calculated for fracture-related dolomites using oxygen isotope data. The distribution of the cap dolomite was controlled by the availability of Fe^2? which was in turn controlled by the availability of S^2?. In the centre of the basin Trenton-Utica deposition was continuous. The upper Trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce H_2S from seawater sulphate. The precipitation of iron sulphides (pyrite + iron monosulphide) followed and used up most of the available Fe^2?. As a result only small amounts of ferroan dolomite formed. On the periphery of the basin, subaerial exposure resulted in the oxidation of most of the available organic matter. Sulphate reducing bacteria were therefore limited and produced limited amounts of H₂S. As a result only a minor amount of iron sulphide (iron monosulphide) formed. The remaining Fe^2- was then available for the formation of the ferroan cap dolomite. This model is supported by the following: (1) In the southern margin of the basin, the contact between Trenton cap dolomite and the overlying Utica Shale is sharp and probably unconformable. In the centre of the basin the contact is gradational. (2) In the centre of the basin, the total organic carbon content in the upper Trenton is an order of magnitude higher than in the cap dolomite. (3) The whole-rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre. (4) Iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite. In the cap dolomite, iron sulphide is minor and primarily in the form of iron monosulphide.     

The uptake of cobalt into ferromanganese nodules,soils, and synthetic manganese (IV) oxides

Strong enrichments of cobalt occur in marine manganese nodules, soils, wads, and natural and synthetic minerals such as hollandite, cryptomelane, psilomelane, lithiophorite, birnessite, and δ-MnO₂. Previously, it was suggested that Co³⁺ ions in these minerals replace either Mn³⁺ or substitute for Fe³⁺ in incipient goethite epitaxially intergrown with δ-MnO₂. Neither of these interpretations is now considered to be satisfactory on account of the large discrepancy of ionic radius between octahedrally coordinated low-spin Co³⁺ and high-spin Mn³⁺ or Fe³⁺ in oxide structures. The close agreement between the ionic radii of Co³⁺ and Mn⁴⁺ suggests that some cobalt substitutes for Mn⁴⁺ ions in edge-shared [MnO₆] octahedra in many manganese(IV) oxide mineral structures. It is proposed that hydrated cations, including Co²⁺ ions, are initially adsorbed on to the surfaces of certain Mn(IV) oxides in the vicinity of essential vacancies found in the chains or sheets of edge-shared [MnO₆] octahedra. Subsequently, fixation of cobalt takes place as a result of oxidation of adsorbed Co²⁺ ions by Mn⁴⁺ and replacement of the displaced manganese by low-spin Co³⁺ ions in the [MnO₆] octahedra or vacancies.     

Contributions to the mineralogy of authigenic manganese phases from marine manganese deposits

The formation of authigenic manganese minerals and ores in the pelagic regions of the ocean is related to oxidation of Mn²⁺ extracted from basalts and other rocks with heated seawater. For littoral parts of the ocean and lakes mobilization of Mn²⁺ and Fe²⁺ is admitted finding its way to the bottom sediments (along with the organic substances) from land in the form of Mn⁴⁺. The main manganese mineral of oceanic and continental basins is vernadite. Its deposition is considered a result of the activity of microorganisms.     

Magnetic properties and neutron diffraction study of two manganese sulfosalts: monoclinic MnSb<Subscript>2</Subscript>S<Subscript>4</Subscript> and benavidesite (MnPb<Subscript>4</Subscript>Sb<Subscript>6</Subscript>S<Subscript>14</Subscript>)

Mn²⁺Sb₂S₄, a monoclinic dimorph of clerite, and benavidesite (Mn²⁺Pb₄Sb₆S₁₄) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented and compared with those of iron derivatives, berthierite (Fe²⁺Sb₂S₄) and jamesonite (Fe²⁺Pb₄Sb₆S₁₄). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb₂S₄ shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature. In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb₂S₄ interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]: jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams and persist down to 1.4 K.     

Time-resolved luminescence of Fe3+ and Mn2+ ions in hydrous volcanic glasses

 Time-resolved luminescence spectra of natural and synthetic hydrous volcanic glasses with different colors and different Fe, Mn, and H₂O content were measured, and the implications for the glass structure are discussed. Three luminescence ranges are observed at about 380–460, 500–560, and 700–760 nm. The very short-living (lifetimes less than 40 ns) blue band (380–460 nm) is most probably due to the ⁴T₂(⁴D) →⁶A₁(⁶S) and ⁴A₁(⁴G) →⁶A₁(⁶S) ligand field transitions of Fe³⁺. The green luminescence (500–560 nm) arises from the Mn²⁺ transition ⁴T₁(⁴G) →⁶A₁(⁶S). It shows weak vibronic structure, short lifetimes less than 250 μs, and indicates that Mn²⁺ is tetrahedrally coordinated, occupying sites with similar distortions and ion–oxygen interactions in all samples studied. The red luminescence (700–760 nm) arising from the ⁴T₁(⁴G) →⁶A₁(⁶S) transition of Fe³⁺ has much longer lifetimes of the order of several ms, and indicates that ferric iron is also mainly tetrahedrally coordinated. Increasing the total water content of the glasses leads to quenching of the red luminescence and decrease of the distortions of the Fe³⁺ polyhedra. Received: 30 July 2001 / Accepted: 15 November 2001     

Rare Earth Element Enrichment in Late Archean Manganese Deposits from the Iron Ore Group,East India

The major, trace and rare earth element (REE) composition of Late Archean manganese, ferromanganese and iron ores from the Iron Ore Group (IOG) in Orissa, east India, was examined. Manganese deposits, occurring above the iron formations of the IOG, display massive, rhythmically laminated or botryoidal textures. The ores are composed primarily of iron and manganese, and are low in other major and trace elements such as SiO₂, Al₂O₃, P₂O₅ and Zr. The total REE concentration is as high as 975 ppm in manganese ores, whereas concentrations as high as 345 ppm and 211 ppm are found in ferromanganese and iron ores, respectively. Heavy REE (HREE) enrichments, negative Ce anomalies and positive Eu anomalies were observed in post‐Archean average shale (PAAS)‐normalized REE patterns of the IOG manganese and ferromanganese ores. The stratiform or stratabound shapes of ore bodies within the shale horizon, and REE geochemistry, suggest that the manganese and ferromanganese ores of the IOG were formed by iron and/or manganese precipitation from a submarine, hydrothermal solution under oxic conditions that occurred as a result of mixing with oxic seawater. While HREE concentrations in the Late Archean manganese and ferromanganese ores in the IOG are slightly less than those of the Phanerozoic ferromanganese ores in Japan, HREE resources in the IOG manganese deposits appear to be two orders of magnitude higher because of the large size of the deposits. Although a reliable, economic concentration technique for HREE from manganese and ferromanganese ores has not yet been developed, those ores could be an important future source of HREE.     

Quantitative cathodoluminescence (CL) spectroscopy of minerals: possibilities and limitations

Summary ?The luminescence spectrum of a mineral contains complex information related to the intrinsic crystal and the defect structure. For quantitative analysis of cathodoluminescence (CL) the spectra have to be deconvoluted by fitting and filtering procedures to identify and measure individual peaks. Peak-width, peak-position and transition probability of the luminescence centres are influenced by effects such as interactions within the defects themselves, and interaction between defects and the surrounding crystal lattice. For calcite and feldspar a linear correlation between the defect concentration of manganese and the Mn²⁺-activated CL-intensity is documented. Combined Micro-Particle Induced X-ray Emission (μ-PIXE) and CL-spectroscopy analyses of REE-doped synthetic calcite suggest a linear correlation between REE-activated CL intensity and REE-concentration at REE-concentration levels below approximately 500 ppm. Sensitising and quenching by other REE are dominant effects yielding strong variations in the correlation between the REE-activated CL-intensity and the REE-content. Received December 6, 2001; revised version accepted May 10, 2002     

Fe2+-Fe3+ interactions in tourmaline

The color and spectroscopic properties of ironbearing tourmalines (elbaite, dravite, uvite, schorl) do not vary smoothly with iron concentration. Such behavior has often been ascribed to intervalence charge transfer between Fe²⁺ and Fe³⁺ which produces a new, intense absorption band in the visible portion of the spectrum. In the case of tourmaline, an entirely different manifestation of the interaction between Fe²⁺ and Fe³⁺ occurs in which the Fe²⁺ bands are intensified without an intense, new absorption band. At low iron concentrations, the intensity of light absorption from Fe²⁺ is about the same for E∥c and E⊥c polarizations, but at high iron concentrations, the intensity of the E⊥c polarization increases more than ten times as much as E∥c. This difference is related to intensification of Fe²⁺ absorption by adjacent Fe³⁺. Extrapolations indicate that pairs of Fe²⁺-Fe³⁺ have Fe²⁺ absorption intensity ～200 times as great as isolated Fe²⁺. Enhanced Fe²⁺ absorption bands are recognized in tourmaline by their intensity increase at 78 K of up to 50%. Enhancement of Fe²⁺ absorption intensity provides a severe limitration on the accuracy of determinations of Fe²⁺ concentration and site occupancy by optical spectroscopic methods. Details of the assignment of tourmaline spectra in the optical region are reconsidered.     

Electron paramagnetic resonance study of iron(III) and manganese(II) in the glassy and crystalline environments of synthetic fayalite and tephroite

Crystals of the olivine minerals, tephroite (Mn₂SiO₄) and fayalite (Fe₂SiO₄) containing manganese(II) and iron (II and trace of III), respectively, were synthesized. Glasses were prepared from these crystalline materials by a splat-quench technique. Measurement of electron paramagnetic resonance (EPR) of all these powdered samples at room temperature show that the g-factors of Mn²⁺ in both glassy and crystalline environments (g_eff = 2.004) are the same, although the EPR linewidths (for glass, ΔH_pp = 200 G; for crystals ΔH_pp = 287 G) suggest less clustering of paramagnetic Mn²⁺ ions in the glass. Mn²⁺ probably occupies a distorted octahedral site in the tephroite crystal structure, although a four-fold coordination is suggested from other spectroscopic investigation on this glass. The EPR parameters of Fe³⁺ in synthetic fayalite glass (g_eff = 2.01 and 6.00; ΔH_pp=150 and 1375 G, respectively, for the high and low field resonances) and powdered crystals (g_eff = 3.31 and ΔH_pp = 900 G) indicated that Fe³⁺ ion in the crystals, is probably located in a distorted tetragonal site M2 and an axial environment has been proposed in the glassy system.