The structural evolution of organic matter during maturation of coals and its impact on petroleum potential and feedstock for the deep biosphere

The structural evolution of coals during coalification from peat to the end of the high volatile bituminous coal rank (VR_r = 0.22–0.81%) has been studied using a natural maturity series from New Zealand. Samples were studied using a range of standard coal analyses, Rock–Eval analysis, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and pyrolysis gas chromatography (Py-GC). The structural evolution of coal during diagenesis and moderate catagenesis is dominated by defunctionalisation reactions leading to the release of significant amounts of oxygen and thereby to an enrichment of aromatic as well as aliphatic structures within the residual organic matter. Based on the evolution of pyrolysis yields and elemental compositions with maturity it can be demonstrated that oxygen loss is the major cause for increasing Hydrogen Index values or hydrocarbon generating potentials of coals at such maturity levels. For the first time, the loss of oxygen in form of CO₂ has been quantified. During maturation from peat to high volatile bituminous coal ranks ∼10–105 mg CO₂/g TOC has been released. This is equivalent to 2.50E−4 to 1.25E−3 mg CO₂ generated from every litre of sediment per year falling into the range of deep biosphere utilisation rates. Immature coals, here New Zealand coals, therefore manifest the potential to feed deep terrestrial microbial life, in contrast to more mature coals (VR_r > ∼0.81%) for which defunctionalisation processes become less important.     

Structural insights from boron tribromide ether cleavage into lignites and low maturity coals from the New Zealand Coal Band

Ether functionalities form an important cross-linking structure within the macromolecular organic matrix of lignites and coals. To obtain a deeper insight into the complex internal structure of such macromolecules and the maturation related changes of the ether compounds within the network structure, boron tribromide (BBr₃) ether cleavage was applied to a series of lignite and coal samples of different maturity (R₀ 0.27-0.80%) obtained from coal mines and natural outcrops from the North and South Island of New Zealand. Terminal ether-bound alcohols rapidly decrease during diagenesis and occur only in low amounts during the catagenetic stage. Comparison between ester- and ether-bound terminal alcohols indicates a parallel decreasing trend during the diagenetic stage, suggesting that the stability differences between both linkages are not large enough to be observed in maturation processes over geological time scales. Polyether compounds were detected with chain length up to five carbon atoms. After a small decrease during the diagenetic phase these compounds occur in relatively high concentrations, even in the main catagenetic stage. This suggests that these linkage structures represent important cross-linking substructures within the macromolecular matrix of lignites and coals being sterically protected within the macromolecular network during the maturation process. Additional cross-linking substructures were (poly)ether aromatics, esters and ketones.     

Geological and laboratory evidence for early generation of large amounts of liquid hydrocarbons from suberinite and subereous components

Suberinite, and subereous components of amorphous nature, comprise largely unrecognized, proficient sources of liquid hydrocarbons. Due to difficulties in recognizing the presence of subereous components and suberinite in organic sediments, the contributions of these liptinitic components to the organic input of source rocks are easily underestimated. Severe chemical alterations of suberinite in the vitrinite reflectance range of R_o = 0.35–0.60% are demonstrated. Organic geochemical data, obtained from samples subjected to natural maturation, reveal that subereous components/suberinite undergoes early thermal degradation to generate large amounts of hydrocarbons below R_o = 0.60%. Data obtained from laboratory maturation of immature, suberinite-rich coals indicate that about 50% of the potential of suberinite for generating C₁₂₊ hydrocarbons has already been exhausted during natural maturation of the samples, prior to the onset of the traditionally defined “oil window”. The present data (a) contradict the assumption that suberinite is mainly sourced by selective preservation/enrichment of a stable, highly aliphatic biopolymer, i.e. “suberan” and (b) suggest that suberinite contains appreciable amounts of aliphatic and aromatic moieties which are released at low thermal stress.     

Analyses of insoluble residues of altered organic matter by C CP/MAS nuclear magnetic resonance

Alteration phenomena affecting organic matter during diagenesis frequently lead to the formation of residues almost insoluble. Data from ¹³C CP/MAS nuclear magnetic resonance analyses (NMR) of these residues have been compared to those obtained by other techniques such as elemental analysis, infrared spectroscopy, Rock-Eval pyrolysis or gas chromatography. Three examples of alteration phenomena have been chosen: the artificial and natural oxidation of coals, the biodegradation of oils and solid bitumens, and the radiolytic degradation of organic matter. NMR results and those obtained by other techniques converge on similar general conclusions. Additional information can be extracted from ¹³C NMR data: e.g. definition of the phenol/carbonyl ratio, transformation of the aromatic network and aromatic ring substitution. These comparisons are good evidence for the reliability of non-destructive analysis of the insoluble fraction of altered organic material by ¹³C CP/MAS NMR.     

岩浆冷凝成岩阶段的氧逸度与华南热液型铀矿的成矿类型

文章根据多年积累的资料,着重分析了岩石氧化系数与岩浆冷凝成岩阶段的氧逸度以及与岩石铀含量之间的关系,得出岩浆冷凝成岩阶段的氧逸度,是造成华南以酸性岩浆岩为成矿母岩的铀矿床成矿类型差异的主因的结论.从理论上找出了各类型铀矿之间的内在联系,深化了成矿机理的认识,确认了华南存在岩浆热液型铀矿的可能性.     

Rock-magnetic characterization of early, redoxomorphic diagenesis in turbiditic sediments from the Madeira Abyssal Plain

Rock‐magnetic measurements of two sediment cores from the Madeira Abyssal Plain (MAP), north Atlantic, are used to investigate post‐depositional changes in the concentration, grain size and composition of magnetic minerals in the sediments that have occurred within organic‐rich turbidite horizons. The changes are associated with an initial stage of suboxic (reductive) diagenesis, following depletion of porewater O₂, and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment, as a result of diffusion of O₂ from the overlying sea water. The turbidites are of late Quaternary age (δ¹⁸O stages 1–3) and derive both from different sites on the NW African continental margin, and from the flanks of the Canary Islands. Thus, the turbidites are variable compositionally, especially in terms of carbonate, detrital magnetic mineral and organic carbon content. Diagenetic changes in these sediments have been identified using solid‐phase geochemical data (U, Mn, C_org and CaCO₃) reported previously in more than one study. Rock‐magnetic parameters of the sediments, when expressed on a carbonate‐free basis, reveal that significant depletion of detrital ferrimagnetic iron (Fe²⁺/Fe³⁺) oxide grains has occurred within organic‐rich turbidites during redoxomorphic diagenesis. Normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size‐selective process, but it has a minimal effect on the canted‐antiferromagnetic Fe³⁺ oxides in the sediment. Such components, if present, therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively, and bulk magnetic concentration is thus depleted. There is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic‐rich turbidites, beneath both active and fossil oxidation fronts. These grains are most probably associated with populations of live magnetotactic bacteria, which commonly inhabit such organic‐rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis. These results show that simple and rapid rock‐magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep‐sea sediments, at least as effectively as more laborious, time‐consuming and sample‐destructive geochemical measurements.     

准噶尔盆地中拐地区上乌尔禾组富火山碎屑砂砾岩储层的古环境及储层机理研究

通过准噶尔盆地中拐地区二叠系上乌尔禾组的岩石矿物学特征、元素地球化学特征、沉积环境特征、物性特征、成岩作用特征等综合分析,研究了该区富火山碎屑砂砾岩储层的成因机理。结果表明,研究区乌尔禾组古气候总体较为稳定,为温暖湿润的气候,丰富的降水使上乌尔禾组发育多套厚层砂砾岩扇三角洲沉积体系,大量的火山碎屑在流域内湿润的气候条件下发生化学风化,并被富氧淡水携带入湖,水解释放大量的阳离子,使得上乌尔禾组时期湖盆水体环境为咸水的氧化条件。火山碎屑沉积埋藏后也为后期成岩作用提供了碱性的成岩流体环境以及自生矿物形成的物质基础。该区上乌尔禾组储层物性受到沉积环境和成岩作用的双重控制,埋深较浅的扇三角洲前缘水下分流河道微相的砂砾岩为研究区的主要目的储层,其储集空间主要为剩余粒间孔,以及长石、火山碎屑等溶蚀孔隙。     

A model of the diagenetic evolution of coaly sedimentary organic matter

Elemental composition was used to calculate the amounts of compounds produced during the diagenetic evolution of a coal series from the Mahakam delta (Kalimantan, Indonesia). These calculations were based on the following hypotheses: organic nitrogen does not take part in reactions and remains unchanged in the residual organic matter, the only compounds produced are water, carbon dioxide and hydrocarbons.This approach shows that carbon loss during diagenesis is mainly as CO₂, and hydrogen loss is mainly as H₂O. Hydrocarbon production is negligible, in accordance with absence of bacterial methane accumulations in the Mahakam delta.The δ¹³C of coals in the sequence becomes about 2 per mil more positive over the diagenetic depth range of coal evolution. Accounting for the coal δ¹³C change in terms of CO₂ loss requires that the CO₂ given off have δ¹³C of about ?40%.. Such negative CO₂ has not been observed in natural systems, except when CH₄ is undergoing oxidation. Several plausible causes for this effect are discussed.     

Electron Probe Microanalysis of Variable Oxidation State Oxides: Protocol and Pitfalls

Electron probe microanalysis of geological oxide materials relies on stoichiometric considerations to estimate the content of undetermined oxygen and thus calculate ZAF (atomic number, absorption, fluorescence) matrix correction factors, requiring the valences of cations in the corresponding software to be unambiguously defined. However, stoichiometric ZAF corrections may be problematic in the presence of other undetermined elements or variable valence state cations. Herein, we analyse several oxides containing such cations, that is magnetite (Fe₃O₄), haematite (Fe₂O₃), hausmannite (Mn₃O₄) and cuprite (Cu₂O). We compare data re‐calculated for incorrect valence states ( Method 1 ) with reference values, revealing incorrect results, due to an incorrect amount of oxygen used in the matrix correction. Some solid‐solution series of haematite and magnetite were also modelled in CalcZAF program to prove the relative errors when the incorrect oxygen is used. To resolve these issues, we describe two accurate methods. Method 2 uses the true valence states of analysed elements. In Method 3, all cations are analysed as metals, with the content of undetermined oxygen determined by difference. As EPMA software does not allow the use of non‐integer valences, Method 3 is applicable to cations with non‐integer or dubious valences in cases where these non‐integer valences cannot be defined.     

Metalliferous coals: A review of the main genetic and geochemical types

This paper presents a review of the genetic types and geochemical processes that have formed ‘metalliferous’ coals around the world. Primary attention is given to elements in coal that are currently being extracted from coal as raw material (Ge and U) or have, in our opinion, the best chance for such use (REE, Ag, Au, and PGE). Coals with anomalously high concentrations of other metals having potential for economic by-product recovery (Be, Sc, V, Ga, Sb, Cs, Mo, W, and Re) are briefly considered. Original data and a survey of the literature indicate that metalliferous coals are in many coal basins. Ore formation in coal-bearing structures may occur during peat accumulation, during diagenesis of the organic matter, or by epigenesis. Various metals are supplied to sedimentary basins as minerals that are transported by water and wind or as ionic species in surface water and descending and ascending underground water and may be incorporated into peat or coals. The modes of occurrence of metals in the enriched coals are diverse. The data presented in this review indicates that metalliferous coals should be regarded as promising for economic recovery for by-products in the course of coal mining and combustion.     

The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids

Differences between the δ¹⁸O values of Si- and Fe-rich immiscible liquids in the system Fe₂SiO₄-KAlSi₂O₆-SiO₂ (Fa-Lc-Q) in isothermal experiments at 0.1 MPa have been determined experimentally to be 0.6 permil. The observed partition of ¹⁸O into the Si-rich liquid is consistent with previous experience with the preferential partition of ¹⁸O into Si-rich minerals in isothermal equilibrium with minerals of less polymerized structure. Crystallochemical principles affect the distribution of oxygen isotopes in coexisting isothermal liquids in the same way as they apply to isothermally coexisting crystals. The effects of Soret (thermal) diffusion on the distribution of oxygen isotopes in silicate liquids above the solvus in the system Fa-Lc-Q under conditions of an imposed temperature gradient of ca. 250 °C over 4 mm and at 2 GPa have also been investigated experimentally. Both the magnitude and the direction of separation of oxygen isotopes as a result of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in ¹⁸O by up to 4.7 permil, and the highest δ¹⁸O values are associated with the most silica-poor compositions. The distribution of oxygen isotopes appears to be similar in each liquid, regardless of their chemical compositions, which is in contrast to the behaviour of cations whose distributions are compositionally dependent and characterized by strong crystallochemical effects wherein network-forming species such as Si and Al separate to the hot end and Mg, Fe and Ca are segregated preferentially to the cold end. Structural units in the melts are evidently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible sites in these units proceeds according to mass. Self-diffusion coefficients of oxygen in basaltic liquids estimated from the Soret experiments are in accord with those from other isotope tracer experiments, and comparable to those of Si. The possible effects of Soret diffusion on the oxygen isotopic composition of metasomatic veins in the mantle are examined in light of these data, and indicate that decay of the thermal gradients in the veins exceeds that of the diffusion of oxygen needed to produce variations in the δ¹⁸O values of mantle minerals. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are unlikely. Received: 6 January 1997 /  Accepted: 28 June 1998     

Evolution of Isotopic Carbon Dioxide–Water Systems in Lithogenesis: Communication 1. Sedimentogenesis and Diagenesis

Isotope-geochemical features of the formation of carbon and oxygen isotopic compositions in sedimentary and diagenetic carbonates are considered. Isotopic criteria for the identification of early diagenesis zone are proposed. The transition from sedimentogenesis to diagenesis (upper boundary of the early diagenesis zone) is accompanied by the alteration of carbon isotopic composition in the HCO^– _{3(hydrosphere)}–HCO^– _{3(pore water of sediment)}system. The lower boundary of early diagenesis zone is registered by the alteration of oxygen isotopic composition in the pore water of sediments and authigenic carbonates.     

The use of clay minerals and inorganic and organic geochemical indicators for evaluating the degree of diagenesis and oil generating potential of shales

A detailed geochemical study has been made on clay minerals and organic matter from two stratigraphic sequences (Sully-Lepine Series and Buckinghorse Formation) in Lower Cretaceous shales from northeastern British Columbia. The characteristics of the discrete illites, mineralogy and chemistry of the mixed layer clays, organic extract yields and kerogen composition indicate that little diagenesis has occurred in the Sully-Lepine Series whereas extensive diagenesis verging on the anchizone of metamorphism has occurred in the Buckinghorse Formation.On the basis of this study and the results of other workers, a preliminary classification of diagenesis is proposed. Diagenesis is divided into three stages: eodiagenesis (early), mesodiagenesis (middle) and telodiagenesis (late). The mesodiagenesis stage is divided into two sub-stages. Eodiagenesis corresponds to the zone in which pore water is lost from the shales, little hydrocarbon generation occurs and coals are of lignitous or sub-bituminous types. Mesodiagenesis corresponds to the main phase of oil genesis and coals are of the high volatile to low volatile bituminous type. The first stage of clay dehydration occurs during early mesodiagenesis and the second stage of clay dehydration occurs in late mesodiagenesis. During telodiagenesis, extensive cracking of the organic matter occurs; dry gas is the main hydrocarbon product and coals fall in the semi-anthracite range.The preliminary classification has been applied to the Lower Cretaceous shales used in this study. The Sully-Lepine Series falls in the eodiagenesis and early mesodiagenesis zones whereas in the Buckinghorse Formation the diagenetic zones range from early mesodiagenesis to telodiagenesis.     

Possibilities of the utilization of micropetrographic parameters in the scientific classification of bituminous coals

Proposals for new scientific classifications of bituminous coals are based on micropetrographic parameters, i.e. vitrinite reflectance as a criterion of the coalification and maceral composition, presupposed to express the connection between the genetic peculiarities and physical, chemical, and technological properties of the coal mass. In the case of coals with high inertinite contents, however, the utilizability of these parameters meets with difficulties resulting from the subjectivity of determining the different transitional material and from insufficient knowledge of inertinite behaviour at higher temperatures. In the case of the maceral-variable bituminous coals produced in the Ostrava-Karviná Coal Basin, these insufficiencies are not important since it is especially the expression of the variability of the properties of isometamorphic vitrinites, which has decisive effects up-on the course of the thermo-chemical transformations, that is of principal importance to the scientific classification of these coals.In the first approximation, the properties of isometamorphic vitrinites may be expressed by the parameter (H/O)_at, closely connected with fluidity. While the micropetrographic parameters reflect in particular the peculiarities in the chemical structure of the aromatic parts of coal macromolecules, the parameter (H/O)_at expresses the properties of the non-aromatic structures of vitrinite, significantly affecting the course of its thermal degradation. The experimental results show that the value of the parameter (H/O)_at, fluidity and the course of degassing the coal of a lower coalification are independent of the maceral composition and vitrinite reflectance; also that the caking and coking properties of low-rank coals are especially dependent on the parameter (H/O)_at and partially on the micropetrographic parameters. All these facts should be taken into consideration in preparing new scientific classifications of bituminous coals.     

Maturation related changes in the distribution of ester bound fatty acids and alcohols in a coal series from the New Zealand Coal Band covering diagenetic to catagenetic coalification levels

A rank series of lignites and coals of low to moderate maturation levels (vitrinite reflectance (R₀): 0.27–0.8%) from the New Zealand Coal Band were investigated using alkaline ester cleavage experiments to reveal compositional changes of ester bound components (fatty acids and alcohols) during increasing maturation. Ester bound alcohols are found to be present in highest amounts in the very immature lignite samples (R₀: 0.27–0.29%), but show a rapid decrease during early diagenesis. Ester bound fatty acids also show an initial exponential decrease during diagenesis, but reveal an intermittent increase during early catagenesis before decreasing again during main catagenesis. This intermittent increase was related to the short chain fatty acids. To obtain a maturity related signal and to eliminate facies related scattering in the amounts of fatty acids in the coal samples, the carbon preference index of fatty acids (CPI_FA) parameter is introduced here. For the long chain fatty acids (C₂₀–C₃₂) originating from terrigenous plant debris, the CPI_FA decreases with increasing maturity, showing a strong maturation related signal. During diagenesis, the same trend can be observed for the short chain fatty acids, but the intermittent increase in the amounts of short chain fatty acids is also accompanied by high CPI_FA values. This indicates less altered organic biomass at this advanced maturation level and is in contrast to the mature CPI_FA signal of the long chain fatty acids of the same samples. One possible reason for this discrepancy could be extremely different amounts of short and long chain fatty acids in the original source organic matter of these samples. However, another intriguing explanation could be the incorporation of immature bacterial biomass from deep microbial communities containing C₁₆ and C₁₈ fatty acids as main cell membrane components. Deep microbial life might be stimulated at this interval by the increasing release of thermally generated potential substrates from the organic matrix during early catagenesis. In contrast to the fatty acids, the high amounts of alcohols in the immature lignite samples are also visible in the alkene distribution from the open system pyrolysis experiments of the organic matrix before and after saponification.     

The geochemistry of carbonate diagenesis: The past,present and future

Stable carbon and oxygen isotopes (δ¹⁸O and δ¹³C values) and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years. As valuable as these insights have been, many problems regarding the interpretation of geochemical signals within mature rocks remain. For example, while the δ¹⁸O values of carbonate rocks are dependent both upon the temperature and the δ¹⁸O value of the fluid, and additional information including trace element composition aids in interpreting such signals, direct evidence of either the temperature or the composition of the fluids is required. Such information can be obtained by analysing the δ¹⁸O value of any fluid inclusions or by measuring the temperature using a method such as the ‘clumped’ isotope technique. Such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well‐known tendency for δ¹⁸O values of carbonate rocks to decrease with increasing age. Unlike the δ¹⁸O, δ¹³C values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time. However, many studies have not sufficiently emphasized the effects of diagenesis and other post‐depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored. Finally, many additional proxies are poised to contribute to the interpretation of carbonate diagenesis. Although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods, care should be taken to understand both old and new proxies before applying them to the ancient record.     

成矿流体活动的地球化学示踪研究综述

成矿流体活动的地球化学示踪是近年来流体地球化学研究的一个新趋势。通过流体来源示踪、运移示踪和定位示踪可以追溯流体活动的全过程,对恢复流体活动历史、演化历程具有积极意义。对成矿流体活动的地球化学示踪方法进行了一定的总结,对人们常用的地球化学示踪方法－－同位素地球化学示踪、无素地球化学示踪、包裹体地球化学示踪及气体地球化学示踪的研究现状进行了综述。     

Mineralogy and major-element geochemistry of the lower Permian Greta Seam,Sydney Basin,Australia

The mineralogy of the high-volatile bituminous coals and associated strata from the Greta seam, Sydney Basin, Australia, has been evaluated in this study. Although the seam is not immediately overlain by marine strata, percolation of marine water into the original peat bed is indicated by the petrological, mineralogical and geochemical characteristics, which resemble those of coals with marine roof strata. The upper and lower sections of the seam have contrasting mineralogy. Pyrite typically comprises 40 to 56 wt% of the mineral assemblage in the marine-influenced upper part of the seam section. The lower part contains much less pyrite (typically <5 wt%, organic-free basis), and also relatively abundant dawsonite (up to 14 wt%, organic-free basis). The minerals within most coal plies are largely of authigenic origin. These include pyrite, siderite, clay minerals (mainly kaolinite and Na-rich mixed-layer illite/smectite), and quartz, most of which have a relatively early, syngenetic origin. Minor Ti-bearing minerals, anatase or rutile, and phosphate minerals, fluorapatite and goyazite, were probably also formed during early diagenesis. Other minerals have features that indicate late-stage precipitation. These include abundant cleat- and fracture-filling dawsonite, which may be the result of reactions between earlier-precipitated kaolinite and Na₂CO₃- or NaHCO₃-bearing fluids. Minor albite may also be epigenetic, possibly precipitated from the same Ca–Al bearing fluids that formed the dawsonite. The most abundant detrital minerals in the Greta coals are quartz, poorly ordered kaolinite, illite and mixed-layer illite/smectite (I/S). These occur mainly in the floor, roof and other epiclastic horizons of the seam, reflecting periods of greater clastic influx into those parts of the original peat-forming environment. Detrital minerals are rare in the coals away from the epiclastic horizons, probably owing to almost complete sediment bypassing in the depositional system. Alternatively, any detrital minerals that were originally present may have been leached from the peat bed by diagenetic or post-diagenetic processes.     

Carbon-13 NMR spectra of macerals separated from individual coals

The density gradient centrifugation technique has recently been applied to the separation of macerals from whole coals. Sufficient quantities have been separated to permit examination of pure exinite, vitrinite and inertinite fractions by combined cross polarization and magic angle sample spinning (CP/MASS) carbon-13 NMR techniques. The similarities and differences observed in the maceral groups of two coals are discussed. Diversity in precursors or in differential chemical maturation or a combination of both of these factors can be used to account for subtle spectral differences in the two coals and macerals studied even though the gross spectral features are very similar for a given maceral type. The data show that CP/MASS is a technique that can address such interesting geochemical questions.     

Oxidation mechanisms in Al–Mg–Fe spinels. A second stage: α–Fe2O3 exsolution

A two-stage model of oxidation was devised to explain the observed variations in crystallographic parameters in two artificially oxidized natural spinels. In the first stage, oxygen is added to the crystal boundary as cations are preserved, with Fe rising in total valence and vacant sites being formed. In the second stage, oxygen is preserved and α−Fe₂O₃ intergrowth occurs, at the expense of the oxygen of the parent spinel structure. On the basis of this model, crystallochemical formulae were calculated and cations partitioned in the various conditions. It was found that, both before and after oxidation, the spinel site population varies continuously in the direction of an increase in random charge distribution, depending on the increase of heat to the crystals. This trend was found to be reversible. Cation vacancies produced during oxidation are distributed between tetrahedral site T and octahedral site M. Received: 12 June 1997 / Revised, accepted: 17 February 1998