High-temperature conductivity of magnetite ores in relation to their genesis and mineral composition (by the example of the Goroblagodatskoe skarn-magnetite deposit)

Samples of magnetite ores of different parageneses and mineral compositions from the Goroblagodatskoe skarn-magnetite deposit were studied by physical and mineralogo-petrographic methods. Electrical-resistance curves were obtained for an ore containing 80–90% magnetite for the temperature range 20–800 ºC. Three groups of samples were recognized according to the pattern of temperature curves and electrical resistance: sulfide-free, sulfide-containing (pyrrhotite), and variolitic ores. The parameters of high-temperature conductivity (activation energy E0 and coefficient of electrical resistance logR0) have been determined. A linear relationship between E0 and logR0 (logR0= a – bE0) has been established for magnetite ores of all mineral types. The coefficient a varies from 1.92 to 4.80 depending on the type and mineral composition of magnetite ores. The coefficient b is nearly constant for all studied samples, 6.65. The figurative points of the samples in the E0–logR0 coordinates lie in the field bounded by the straight lines logR0= 4.80 – 6.65E0 and logR0= 1.92 – 6.65E0. The samples of magnetite ores of the skarn and hydrosilicate paragenesis with visible pyrrhotite inclusions and samples of variolitic ores form an individual group in the above field. Within the group, the ores are also subdivided into garnet-magnetite (skarn paragenesis), epidote-chlorite-magnetite (hydrosilicate paragenesis), pyroxene-magnetite, and orthoclase-magnetite (variolitic ores).     

Rapid XRD determination of the chrysotile/lizardite ratios in asbestos-bearing serpentinites

Summary The quantitative XRD determination of the most common serpentinite minerals, e.g. lizardite and chrysotile, is hampered by strongly overlapping reflections. Reconnaissance investigations indicated that the reflections 204 of lizardite and 008 of chrysotile are best suited for quantitative XRD. These lines are not interferred by other minerals such as brucite, magnesite, chlorite or talc, which are common in serpentinites. A calibration curve for the determination of the chrysotile/lizardite ratios in natural serpentinites has been constructed by means of synthetically prepared chrysotile/lizardite standards. Using this method serpentinites of the Msauli Chrysotile Asbestos Mine, South Africa, were investigated for their relative chrysotile contents. It was found, that the total amount of chrysotile in the ore zone is considerably higher than the amount of extractable chrysotile asbestos fibre.

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Die röntgenographische Bestimmung der Chrysotil/Lizardit-Verhältnisse in asbesthaltigen Serpentiniten

Zusammenfassung Die quantitative röntgenographische Bestimmung der beiden häufigsten Serpentinminerale, Lizardit und Chrysotil, ist wegen der Überlagerung ihrer stärksten Reflexe erschwert. Aufgrund von Voruntersuchungen konnte jedoch festgestellt werden, daß die Reflexe 204 von Lizardit und 008 von Chrysotil für die quantitative Bestimmung geeignet sind. Diese Reflexe werden nicht überlagert von denen anderer häufig in Serpentiniten vorkommender Minerale, wie z.B. Brucit, Magnesit, Chlorit oder Talk. Eine Eichkurve zur Bestimmung der Chrysotil/Lizardit-Verhältnisse in natürlichen Serpentiniten wurde mit Hilfe synthetisch hergestellter Standardmischungen aufgestellt. Serpentinite der Msauli Chrysotilasbest Mine, Südafrika, wurden aufgrund der hier vorgestellten Methoden auf ihren relativen Chrysotilanteil untersucht. Es ergab sich, daß der totale Gehalt an Chrysotil in der erzführenden Zone deutlich größer ist als der Gehalt an ausbringbaren Chrysotilasbestfasern.

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With 4 Figures     

Serpentinites and their tectonic signature along the Northwest Zagros Thrust Zone, Kurdistan Region, Iraq

Two types of serpentinized peridotites are distinguished within the Northwest Zagros Thrust Zone (NW-ZTZ) in Kurdistan region of Iraq. One is found as lower members of ophiolite sequences, such as the Mawat and Penjwin ophiolites of the upper Cretaceous age. The other is represented by intraformational isolated serpentinite bodies in Betwat, Qaladeza, and Qalander areas within the Walash–Naopurdan volcano-sedimentary unit of the Paleocene to Eocene paleo-arc tectonic setting. Serpentinites within the NW-ZTZ consist mainly of lizardite and chrysotile, with subordinate amounts of syn-serpentinization magnetite, carbonates, chromium chlorite, tremolite, and talc as secondary minerals, and olivine, clinopyroxene, and chromian spinel as primary minerals. Minor antigorite is also found in the sheared serpentinites often found in ophiolite sequences. Petrological and geochemical studies of serpentinites from the NW-ZTZ show that, of the original protoliths of serpentinites, those associated with ophiolites are residual depleted harzburgite and dunite. The $ {\text{Cr}}\# \left( {{{ = {\text{ Cr}}} \mathord{\left/ {\vphantom {{ = {\text{ Cr}}} {\left( {{\text{Cr}} + {\text{Al}}} \right){\text{ atomic ratio}}}}} \right. \kern-0em} {\left( {{\text{Cr}} + {\text{Al}}} \right){\text{ atomic ratio}}}}} \right) $ of chromian spinel is more than 0.6, and the forsterite content of olivine is 91–92. On the other hand, the original protolith of isolated serpentinite bodies is less depleted harzburgite or depleted lherzolite, which has spinel with Cr# less than 0.6 and olivine with 90–91 forsterite contents. Whole rock chemistry of major, trace, and rare earth elements shows that the serpentinites of ophiolite sequences are depleted in CaO, Al₂O₃, and SiO₂, Sr, and Zr, and are enriched in MgO, Ni, and Cr, in comparison with the isolated serpentinites. Cr# of the disseminated unaltered chromian spinels indicates that the serpentinites of both types had been originated from the supra-subduction zone tectonic setting; the serpentinites of ophiolite sequences obducted and thrusted over the continental margin during the obduction of the Tethyth oceanic crust onto the Arabian continental margin during the upper Cretaceous period. Isolated serpentinite bodies represent serpentinized forearc mantle wedge peridotites emplaced by diapiric upwelling into non-accretionary forearc tectonic settings during the Paleocene to Eocene age.     

Distribution of asbestos in the bedrock of the northern New Jersey area

Asbestos has been identified at fifty-five locations in the bedrock of the northern New Jersey area. Most occurrences are confined to (1) The Precambrian rocks of the New Jersey Highlands, particularly the marbles; (2) the Paleozoic serpentinites of Staten Island, New York, and Hoboken, New Jersey; and (3) the Mesozoic basaltic rocks of the Newark Basin. Chrysotile and tremolite asbestos are present in local concentrations in the marbles. In the most extensive exposure of the marble, the Franklin band, traces of tremolite-actinolite asbestos are commonly present. Crocidolite asbestos occurs in localized areas associated with fracture systems in Precambrian pegmatites and associated rocks. The Paleozoic serpentinites contain chrysotile asbestos as a major component in deformed zones. Megascopic chrysotile and anthophyllite asbestos veins occur locally in the serpentinites. Actinolite asbestos occurs in the Mesozoic basaltic rocks of the Newark Basin. Potential environmental problems associated with asbestosbearing bedrock include production and use of rock products containing asbestos, introduction of asbestos into environments surrounding excavations, and asbestos contamination of soils and water supplies.     

Asbestos in lake and reservoir waters of Staten Island,New York: Source,concentration, mineralogy,and size distribution

There is strong evidence indicating that the six commonly recognized varieties of asbestos are cancer-producing agents Epidemiological and experimental data indicate that, although response to inhalation exposure is most marked, exposure by ingestion probably also entails a risk of excess tumor incidence. The toxicity of mineral fibers can be related to fiber dimensions, mineralogy, chemistry, and surface-active properties. In the Silver Lake Reservoir in Staten Island, New York, where chrysotile from serpentinite bedrock exposures is a potential source of contamination, chrysotile asbestos levels of 15 to 86 million fibers per liter (MFL) were measured, with an average of 53 MFL Much lower levels (average 0 3 MFL) were observed in small lakes and ponds with clayey bottoms on the Staten Island Serpentinite.     

Petrographic and mineral chemistry investigation of the high-grade chrysotile asbestos-bearing Zvishavane Ultramafic Complex,south central Zimbabwe

The Zvishavane Ultramafic Complex (ZUC) in the south central part of the Zimbabwe craton is comprised of coarse-grained serpentinites, metadunites and metagabbros and hosts Africa's largest reserves, and largest mine, of high-grade chrysotile asbestos. Magnesiohornblende, actinolite, plagioclase (An_0.6–41.9), augite, diopside and clinozoisite constitute the mineralogy of the ZUC. Forsteritic olivine (>Fo₉₀) was altered to form chrysotile and antigorite minerals, although some primary olivines are preserved. Contents of Al₂O₃ range from <1 to >2.5 wt% at TiO₂ values of <0.7 wt% consistent with an island arc setting for the ZUC that originated from tholeiitic magmas. The Zvishavane Ultramafic Complex was metamorphosed at relatively high temperatures (542-779 °C) and low pressures (1.2-2kbars), consistent with contact metamorphism. Intrusion of several granitic batholiths relatively close to the ZUC likely triggered hydrothermal fluid migration which metamorphosed the ZUC. The associated asbestos deposits likely formed during hydrothermal circulation events. Zoned amphiboles, the occurrence of magnesiohornblende and actinolite, as well as cross-cutting serpentine veins are consistent with at least two stages of alteration and/or metamorphism of the ZUC. The lack of a thrust contact between the ZUC and its country rocks is consistent with the ZUC having intruded into the host Zvishavane Gneiss Complex and possibly acted as a feeder to the nearby Mberengwa greenstone belt (MGB). However, the occurrence of near end-member forsteritic olivine, the presence of zoned amphiboles, and faulting within the ZUC are all suggestive of an ophiolitic origin although forsteritic olivines also occur in intrusive layered complexes. Metamorphism of the ZUC, ascribed to intrusion of multiple batholiths and possibly the MGB, likely led to the formation of ZUC chrysotile asbestos deposits.     

Serpentinization: a review

The common alteration assemblage produced by serpentinization of ultramafic rocks is: lizardite, chrysotile, magnetite±brucite±antigorite. Lizardite-chrysotile serpentinites are more common than antigorite; the presence of antigorite indicates that the serpentinite has undergone prograde metamorphism or that the periootite was serpentinized in a higher P,T regime than lizardite and chrysotile. The iron subsitution into serpentine minerals and brucite is a function of temperature at low f_O2, with increased temperature enhancing magnetite formation. The presence of awaruite and native Fe are strong evidence for a locally very reducing environment. Isotopic studies have shown a wide variety of origins for the fluids involved in serpentinization. The increased boron content of serpentinized rocks when compared to boron contents of the parent ultramafic body indicates a possible sea water origin for the fluids. Serpentinization takes place under both constant volume and constant chemical composition conditions. The factors in evaluating the importance of the two processes for an individual serpentinite are: (1) determination of the mineral assemblage and its paragenesis, (2) the structural and tectonic relationship of the ultramafic body to its country rock, (3) fluid access to the rock in duration and amount, and (4) timing of serpentinization - before, during or after emplacement into the crust.     

Foliation development in serpentinites, Glenrock, New South Wales

At Glenrock, near the southern end of the Peel Fault System, two fault zones are delineated by mélanges in which serpentinite is the main rock type.Protogranular and mylonitic textures are present in relicts of the parent peridotite and in blocks of massive pseudomorphic serpentinite that are surrounded by schistose serpentinite. In schistose serpentinite, the earliest foliation (S₁) is defined, microscopically, by the parallel alignment of platy and fibrous serpentine minerals (lizardite and chrysotile) and by trains of magnetite and flattened serpentine pseudomorphs after olivine and pyroxene. It is considered that the schistosity formed perpendicular to the direction of maximum shortening, under conditions in which lizardite and chrysotile were ductile, but antigorite was not, by breakdown of pre-existing serpentine minerals and new growth of lizardite and chrysotile.Post-s₁ foliations (S₂andS₃) superficially resemble crenulation cleavages in the field but, microscopically, show evidence of shear displacement and are referred to as microshear sets. They probably originated in the ductile-brittle transitional field of serpentine behaviour (Raleigh and Paterson, 1965).     

Characterization of some Egyptian serpentinites used as ornamental stones

Egypt is characterized by wide occurrence of serpentinites, particularly in the central and southern parts of Eastern Desert. There are several cooperate factors that affect the serpentinites used as ornamental stones. These factors involve mineralogical characteristics （including mineral composition and microstructure parameter）, physical properties and mechanical properties. Antigorite is an essential serpentinite mineral, with a minor amount of chrysotile. Talc, carbonates （magnesite and dolomite） and tremolite are the main associated minerals. Quality and quantity of associated minerals such as talc and carbonates （dolomite and magnesite） affect the properties of serpentinites used as ornamental stones. Carbonates are resistant to weathering but suffer from acidic cleaning agents in interior use, whereas serpentinites with a high content of talc used on external faces undergo an increase in volume and a consequent rapid degradation. Studied serpentinites are characterized by relatively small grain size with foliated texture, low water absorption, low porosity, and high abrasion resistance. In the samples studied the uniaxial compressive strength （UCS） varies between 89 and 189 MPa, with an average of 152 MPa. According to the classification of Bell （1992）, sample No. B8 is very high in strength while the rest high in strength.     

The genesis of the carbonatized and silicified ultramafics known as listvenites: a case study from the Mihalıççık region (Eskişehir), NW Turkey

The Mihalıççık region (Eskişehir) in NW Turkey includes an ophiolitic assemblage with a serpentinite‐matrix mélange. The serpentinites of this mélange host silica‐carbonate metasomatites which were previously named as listvenites. Our mineralogical and geochemical studies revealed that these alteration assemblages represent members of the listvenitic series, mainly the carbonate rocks, silica‐carbonate rocks and birbirites, rather than true listvenites (sensu stricto). Tectonic activity and lithology are principal factors that control the formation of these assemblages. Carbonatization and silicification of the serpentinite host‐rock is generated by CO₂, SiO₂‐rich H₂O hydrothermal fluid which includes As, Ba, Sb and Sr. Low precious metal (Au, Ag) contents of the alteration assemblages indicate lack of these metals in the fluid. Primary assemblages of the alteration are carbonate rocks that are followed by silica‐carbonate rocks and birbirites, respectively. Petrographic studies and chemical analyses suggested an alkaline and moderate to high temperature (350–400°C) fluid with low oxygen and sulphur fugacity for the carbonatization of the serpentinites. The low temperature phases observed in the subsequent silicification indicated that the fluid cooled during progressive alteration. The increasing Fe‐oxide content and sulphur phases also suggested increasing oxygen and sulphur fugacity during this secondary process and silica‐carbonate rock formation. The occurrence of birbirites is considered as a result of reactivation of tectonic features. These rocks are classified in two sub‐groups; the Group 1 birbirites show analogous rare earth element (REE) trends with the serpentinite host‐rock, and the Group 2 birbirites simulate the REE trends of the nearby tectonic granitoid slices. The unorthodox REE trend of Group 2 birbirites is interpreted to have resulted from a mobilization process triggered by the weathering solutions rather than being products of enrichment by the higher temperature hydrothermal activity. Copyright © 2006 John Wiley & Sons, Ltd.     

苏鲁超高压变质带仰口蛇纹岩的成因——矿物化学、地球化学和年代学证据

通过矿物化学、全岩主微量元素和铂族元素研究,结合锆石年代学判断苏鲁造山带仰口蛇纹岩的原岩成因和演化历史。蛇纹岩中尖晶石经历了多阶段变质;全岩主量元素具有超基性堆晶岩的性质,代表玄武质组分含量的Ca O+Al_2O_3变化于2.0%~5.83%之间;部分不相容微量元素富集;铂族元素中Ir的含量低(0.64×10~(-9)~1.43×10~(-9)),Pd/Ir值高(1.05~3.42)。蛇纹岩中的锆石一部分为新形成的变质成因锆石(年龄平均为230±3Ma),与高压-超高压变质的年代吻合;另一部分可能是在三叠纪变质阶段古老锆石重结晶形成的。由此认为,仰口蛇纹岩的原岩可能为超基性堆晶岩,三叠纪时随着俯冲的扬子板块发生变质,在发生蛇纹石化作用之前经历了熔体/流体的改造。     

Relict chromian spinels in Tulu Dimtu serpentinites and listvenite,Western Ethiopia: implications for the timing of listvenite formation

Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary mélange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite [fo] ~96 mol%) and rare relict primary mantle olivine (Fo_90–93). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr# = 0.79–0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe²⁺) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe²⁺ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO₂-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO).     

Petrological and mineralogical studies of Pan-African serpentinites at Bir Al-Edeid area,central Eastern Desert,Egypt

The studied serpentinites occur as isolated masses, imbricate slices of variable thicknesses and as small blocks or lenses incorporated in the sedimentary matrix of the mélange. They are thrusted over the associated island arc calc-alkaline metavolcanics and replaced by talc-carbonates along shear zones. Lack of thermal effect of the serpentinites upon the enveloping country rocks, as well as their association with thrust faults indicates their tectonic emplacement as solid bodies. Petrographically, they are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of carbonates, chromite, magnetite, magnesite, talc, tremolite and chlorite. Chrysotile occurs as cross-fiber veinlets traversing the antigorite matrix, which indicate a late crystallization under static conditions. The predominance of antigorite over other serpentine minerals indicates that the serpentinites have undergone prograde metamorphism or the parent ultramafic rocks were serpentinized under higher pressure. The parent rocks of the studied serpentinites are mainly harzburgite and less commonly dunite and wehrlite due to the prevalence of mesh and bastite textures. The serpentinites have suffered regional metamorphism up to the greenschist facies, which occurred during the collisional stage or back-arc basin closure, followed by thrusting over a continental margin. The microprobe analyses of the serpentine minerals show wide variation in SiO₂, MgO, Al₂O₃, FeO and Cr₂O₃ due to different generations of serpentinization. The clinopyroxene relicts, from the partly serpentinized peridotite, are augite and similar to clinopyroxene in mantle-derived peridotites. The chromitite lenses associated with the serpentinites show common textures and structures typical of magmatic crystallization and podiform chromitites. The present data suggest that the serpentinites and associated chromitite lenses represent an ophiolitic mantle sequence from a supra-subduction zone, which were thrust over the continental margins during the collisional stage of back-arc basin.     

The zeta-potentials of natural and synthetic chrysotiles

Different specimens of the asbestos mineral chrysotile show widely different zeta-potentials. Strongly positive values are found with samples containing an excess of magnesia in the form of brucite, Mg(OH)₂. Synthetic chrysotile and natural samples containing little or no brucite give moderately positive ζ-values over the pH range 3–11. Feebly positive or weakly negative natural chrysotiles are materials which have suffered weathering; and deliberate acid-leaching of any chrysotile ultimately leaves a strongly negative pseudomorph of silica gel.Since the pH and the ambient concentration of Mg²⁺ ions near the surface are the main factors controlling ζ, and since chrysotile is readily susceptible to leaching in practically all aqueous media and does not come to a true ionic equilibrium in any practical time, the zeta-potential of an ideal chrysotile surface is a hypothetical concept.The results also explain the temporary colloidal stability of very dilute suspensions of chrysotile in weakly acidic media and the mutual coagulation of chrysotile and amosite slurries.     

Geostatistical investigation of the Kutcho Creek chrysotile deposit,Northern British Columbia

The Kutcho Creek asbestos deposit, owned by Cassiar Asbestos Corporation Ltd., has been sampled by two major exploratory programs that have provided two sets of grade data—one from the horizontal direction (wall readings) and one from the vertical direction (diamond drill cores). These data (percentage chrysotile by volume) were divided into well-defined groups on the basis of location and sample continuity, and experimental variograms for percentage of fiber content were calculated for each group. Horizontal data were all oriented in directions roughly parallel to the trend of an elongate serpentinite zone containing local centers rich in chrysotile veinlets. Spherical variogram models fitted to horizontal and vertical data sets are as follows: Vertical: $$\begin{gathered} \gamma (h) = 0.27 m^2 + 0.44 m^2 [(3h/70) - (h^3 /85,750)] h \leqslant a \hfill \\ \gamma (h) = 0.71 m^2 h \geqslant a \hfill \\ \end{gathered}$$ Horizontal: $$\begin{gathered} \gamma (h) = 0.27 m^2 + 1.20 m^2 [(h/60) - (h^3 /729,000)] h \leqslant a \hfill \\ \gamma (h) = 1.47 m^2 h \geqslant a \hfill \\ \end{gathered}$$ Wherem is the mean value of data used in the construction of a variogram,h is a lag (sample spacing), anda is the range over which the grade is autocorrelated. These two one-dimensional models can be combined to a two-dimensional model with the form γ(h)=γ(r)+γ(x) where γ(r) is an isotropic component (equivalent to the vertical model above) and γ(x) is a zonal component in the horizontal direction (equivalent to the difference between the horizontal and vertical models above). This general model describes data throughout the entire serpentinite zone in a satisfactory manner but, of course, does not contain information in the third dimension, and, thus, cannot be used as a basis for grade and tonnage calculations and corresponding error estimates. Nevertheless, the analysis has illustrated the potential of variogram analysis for such tonnage and grade calculations. Furthermore, the study has provided limiting two-dimensional information on the geometry of chrysotile-rich zones within the serpentinite belt, information that can be used to advantage in planning future exploratory drilling.     

Chlorine isotopic composition in seafloor serpentinites and high-pressure metaperidotites. Insights into oceanic serpentinization and subduction processes

Bulk-rock chlorine content and isotopic composition (δ³⁷Cl) were determined in oceanic serpentinites, high-pressure metaperidotites and metasediments in order to gain constraints on the global chlorine cycle associated with hydrothermal alteration and subduction of oceanic lithosphere. The distribution of insoluble chlorine in oceanic serpentinites was also investigated by electron microprobe. The hydrothermally-altered ultramafic samples were dredged along the South West Indian Ridge and the Mid-Atlantic Ridge. The high-pressure metamorphic samples were collected in the Western Alps: metaperidotites in the Erro-Tobbio unit and metasediments in the Schistes Lustrés nappe.Oceanic serpentinites show relatively large variations of bulk-rock Cl contents and δ³⁷Cl values with mean values of 1105 ± 596 ppm and −0.7 ± 0.4‰, respectively (n = 8; 1σ). Serpentines formed after olivine (meshes) show lower Cl content than those formed after orthopyroxene (bastites). In bastites of two different samples, Cl is positively correlated with Al₂O₃ and negatively correlated with SiO₂. These relationships are interpreted as reflecting preferential Cl-incorporation into the bastite structure distorted by Al (substituted for Si) rather than different alteration conditions between olivine and orthopyroxene minerals. High-pressure metaperidotites display relatively homogeneous Cl contents and δ³⁷Cl values with mean values of 467 ± 88 ppm and −1.4 ± 0.1‰, respectively (n = 7; 1σ). A macroscopic high-pressure olivine-bearing vein, formed from partial devolatilization of serpentinites at ∼2.5 GPa and 500-600 °C, shows a Cl content and a δ³⁷Cl value of 603 ppm and −1.6‰, respectively. Metasediments (n = 2) show low whole-rock Cl contents (<15 ppm Cl) that did not allow Cl isotope analyses to be obtained.The range of negative δ³⁷Cl values observed in oceanic serpentinites is likely to result from water-rock interaction with fluids that have negative δ³⁷Cl values. The homogeneity of δ³⁷Cl values from the high-pressure olivine-bearing vein and the metaperidotite samples implies that progressive loss of Cl inherited from oceanic alteration throughout subduction did not significantly fractionate Cl isotopes. Chlorine recycled in subduction zones via metaperidotites should thus show a range of δ³⁷Cl values similar to the range found in oceanic serpentinized peridotites.     

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

The dehydroxylation reactions of chrysotile Mg₃Si₂O₅(OH)₄ and brucite Mg(OH)₂ were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO₂ in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (‘time to a given fraction’ and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress-resolved (‘α’-resolved) course of the apparent activation energy E _a and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing E _a (α ≤ 15 %, 240–300 kJ/mol), before coming down and forming a plateau (15 % ≤ α ≤ 60 %, 300–260 kJ/mol). The reaction ends with an increasing E _a (α ≥ 60 %, 260–290 kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E _a versus α (160–110 kJ/mol). In contrast to that, the decomposition of brucite under CO₂ delivers a dynamic course with a much higher apparent E _a characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent E _a comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E _a versus α plots.     

我国西北地区不同类型纤蛇纹石石棉的电镜研究

作者对西北地区不同类型温石棉矿床的石棉进行了扫描电镜和高分辩率透射电镜研究,拍摄到清晰的晶格象和电子显微象,发现茫崖型和小八宝型的温石棉纤维的晶格条纹、晶格缺陷及纤维形貌有明显差异。这种差异与成棉地质条件和温度有关。     

Serpentinization of mantle formations in the Mauritanides Belt: regions of Agane and Gouérarate (middle-western Mauritania)

Regions of Agane and Gouérarate represent an ancient fragment of ophiolitic suture localized in the axial area of the Mauritanides Belt. These two regions are characterized by the abundance of completely serpentinized formations. In this study, we present the first use of Raman spectroscopy for identifying the species of serpentine present in the Mauritanides Belt. Serpentinites of Agane are derived from refractory peridotites composed of dunites–harzburgites; however, there are also rare serpentinites derived from ultramafic cumulates. Antigorite represents the dominant species in the serpentinite. Furthermore, chrysotile is found as post-antigorite veins. These veins are post-obduction and mark the final phase of serpentinization. The abundance of antigorite and the absence of lizardite confirm that subduction was the environment of serpentinization in these two regions, and that “the oceanic opening” responsible for the formation of ophiolitic sutures in the Mauritanides Belt was limited. The term “serpentinite” is no longer applicable to the formations of Gouérarate. As a result, these formations correspond to old serpentinites transformed to birbirites which are in phase of transformation into laterites.     

Structure of the 30-sectored polygonal serpentine. A model based on TEM and SAED studies

 Polygonal serpentine (PS) from selected serpentinite were studied using transmission electron microscopy. Fiber axis selected-area electron diffraction (SAED) patterns and electron micrographs reveal orthogonal and monoclinic lizardite polytypes. The PS models by Chisholm (1992) and Baronnet et al. (1994) do not fit SAED measurements. Experimental results are matched with calculated diffraction geometry and intensities, as well as with simulated images, indicating inversion of the tetrahedral layer at sector boundaries. The structural relationships between chrysotile and PS are discussed. Two types of 30-sectored PS are distinguished. In “regular PS” the fiber axis is [100], in “helical PS” the fiber axis points into a [uν0] direction with large u value (u≫ν). Helical PS can be regarded as a lizardite analogue of helical chrysotile. Received December 6, 1995/Revised, accepted May 8, 1996