An Experimental Study on Three Quaternary Phases in the Fe-Sn-Sb-S System: Pb-Free Franckeite, Pb-Free Cylindrite and (Fe, Sb)-Ottemannite s.s.

Experimental studies on the two solid solutions franckeite s.s. and cylindrite s.s. yielded among others thePb-free endmembers of franckeite and cylindrite respectively. In these endmembers the lead content of the twomineral phases are completely substituted by bivalent tin. A third phase, ottemannite s.s. with (Fe. Sb)-bearingcompositions, was further identified in the Pb-free 4-component system Fe-Sn-Sb-S. Pb-free franckeite andcylindrite could only be synthesized as homogeneous phases at high temperatures. The ottemannite s.s. was ob-tained at 600℃ but an extensive existing range was also confirmed at temperatures below 500℃. The phase relations were discussed in the pseudoternary subsystem SnS-SnS_2-FeSb_2S_4. The following se-quence of Pb-free phases and phase assemblages were observed under increasing Sn~(4+) content at 600℃:herzenhergite (SnS)+franckeite, franckeite, franckeite+cylindrite, cylindrite, cylindrite+(Fe, Sb)-ottemannite.(Fe, Sb)-ottemannite. (Fe. Sb)-ottemannite+berndtite (SnS_2). The assemblage cylindrite+berndtite was identi-fied in synthetic systems involving Pb. The phase Fe. Sb-ottemannite s.s. appears to be stable only under Pb-de-ficient and high sulfur fugacity conditions.     

Characterization of Galena and Vein Paragenesis in the Penjom Gold Mine, Malaysia: Trace Elements, Lead Isotope Study and Relationship to Gold Mineralization Episodes

The Penjom Gold Mine is located 30 km from the Bentong-Raub Suture at the western boundary of the Central Belt in Peninsular Malaysia. Gold mineralization hosted within the vein system is associated with pyrite, arsenopyrite, and minor base metals including galena. Trace element and lead isotope analysis was undertaken on nine samples that represent two stages of galena formed during two tectonic events. Both the Pb isotopes and the trace elements show that the first stage galena within the mineralized areas at the footwall has different geochemical characteristics compared with galena in non mineralized areas in the hanging wall, suggesting that galena crystallized from two different ore fluids and probably at two different times. Higher Te, Se and Bi in the galena from the mineralized area may indicate hydrothermal fluids that migrate through the structural conduit and leached out the metal along the pathway that consist of dominant carbonaceous unit. The Pb isotopic ratio composition are transitional between the bulk crustal growth and an upper crustal growth curve, indicating that derivation was from arc rocks associated with continental crust or a crustal source that includes arc volcanic and old continental sedimentary rocks.     

Bismuthoan Galena—A New Variety of Galena

Bismuthoan galena is a variety of galena, resulting from the replacement of Bi for some Pb in galena.This mineral occurs in the No.11 orebody of the Lamo skarn-type Zn-Cu deposit in the Dachang ore field ,but only the No,11 orebody of the Lamo deposit is developed such bismuthoan galena.This is closely related to such a geochemical background that the No.11 orebody contains high Bi and Bi-sulfosalt minerals are well developed there.Eight electron microprobe analyses of five samples give 60.17-70.20%Pb(66.94% on average),10.00-16.06%Bi (12.47%),4.83-8.97%Ag(6.39%)and 13.25-13.98% S(13.65%).Its molecular formula is Pb0.76,Bi0.14,Ag0.13,1.03S.No galena so high in Bi has been reported in the literature and this is the first report in China.     

我国某矿田硫化物和硫盐类矿物差热失重分析的研究

DTA and TGA curves are presented for 117 samples of sulfide and sulfosalt minerals from a certain eassiterite-sulfide deposit. The differential thermal and thermogravimetric analyses were conducted under oxidation conditions and in inert atmospheres by use of China-made Model 4.1 Precise Differential Thermebalance. In conjunction with X-ray diffraction analysis, pyrrhotite and chaleopyrite of different crystal systems, silver-bearlng galena, jamesonite and boulangerite have been studied in more detail.     

Character of the Si and Al Phases in Coal Gangue and Its Ash

Abstract: Analysis of the Si and Al phases in coal gangue fuel and its ash is important for use of coal gangue ashes. A comprehensive study by theoretical and experimental analyses with differential thermal analysis, X-ray diffraction and Infrared Spectroscopy has been made in the present article to explore the diagram of the Si and Al phases in coal gangue fuel and its ashes. It is found that kaolinite and quartz are the main phases in coal gangue fuel. The ratio of moles Al2O3 to SiO2 (i.e., Al2O3 (mole) /SiO2 (mole)) is usually no more than 0.5 in most coal gangue fuel and its ashes. The kaolinit at about 984°C releases a large quantity of SiO2, which makes calcine coal gangue more active than coal gangue itself. The relationship between the ratio Al2O3 (mole)/SiO2(mole) and the components of coal gangue ash is analyzed, resulting in a formula to calculate the quantity of each phase. Applying the formula to the testing samples from an electric plant in north China supports the above conclusions.     

Nano-and micron-sized diamond genesis in nature: An overview

There are four main types of natural diamonds and related formation processes. The first type comprises the interstellar nanodiamond particles. The second group includes crustal nano-and micron-scale diamonds associated with coals, sediments and metamorphic rocks. The third one includes nanodiamonds and microndiamonds associated with secondary alteration and replacing of mafic and ultramafic rocks.The fourth one includes macro-, micron-and nano-sized mantle diamonds which are associated with kimberlites, mantle peridotites and eclogites. Each diamond type has its specific characteristics. Nanosized diamond particles of lowest nanometers in size crystallize from abiotic organic matter at lower pressures and temperatures in space during the stages of protoplanetary disk formation. Nano-sized diamonds are formed from organic matter at P-T exceeding conditions of catagenesis stage of lithogenesis. Micron-sized diamonds are formed from fluids at P-T exceeding supercritical water stability.Macrosized diamonds are formed from metal-carbon and silicate-carbonate melts and fluids at P-T exceeding 1150℃ and 4.5 GPa. Nitrogen and hydrocarbons play an important role in diamond formation.Their role in the formation processes increases from macro-sized to nano-sized diamond particles.Introduction of nitrogen atoms into the diamond structure leads to the stabilization of micron-and nanosized diamonds in the field of graphite stability.     

论姑山铁矿床的形成条件

On the basis of geological characteristics the Gushan iron deposit should be assigned to volcano-hydruthermal type with hematite qusrtz as its principal mineral assemblage.Iron concentration of the ore-forming fluid has been estimated from the ratio of hematite to quartz in the ores. By using experimental and thermodynamic data the soinbilities of iron minerals at elevated temperatures and pressures are calculated in the system FeO-Fe2O3-NaCl-HCl-H2O. The effect of T, P, pH, fe2 and total concentration of chlorine on the solubilities of iron minerals are discussed. Thermodynamic calculations based on presumed physicochemical conditions for the ore-forming solutions are in good agreement with geological observation. The calculation shows that iron minerals were deposited at log fo2=-21--25, log(mKCl mbl^ ) =-2,5--3, P=1-0.75 (or 0.5) Kb,T=400-350℃. It is believed that the original fluid was an acid NaCl-bearing solution of magmatic derivation. However, iron in the solution was enriched with falling temperature by dissolving pre-exist iron minerals in the consolidated rocks rather than extracted directly from the magma. Either decreasing temperature (below 400℃) or pressure is capable of depositing iron minerals from the solution, but the ratio of Fe to Si in the ore is dependent mainly upon the pH. The widespread silica vein at later stages is a reflection of decreasing acidity of the solution. Increase in fo2 will also favor the deposition of iron minerals. The hypabyssal occurrence and the existance of the Huangmaqing shale contribute greatly to the formation of hematite.     

New constraints on the P–T path of HT/UHT metapelites from the Highland Complex of Sri Lanka

We report here rare evidence for the early prograde P-Tevolution of garnet-sillimanite-graphite gneiss(khondalite)from the central Highland Complex,Sri Lanka.Four types of garnet porphyroblasts(Grt_1,Grt_2,Grt_3 and Grt_4)are observed in the rock with specific types of inclusion features.Only Grt_3 shows evidence for non-coaxial strain.Combining the information shows a sequence of main inclusion phases,from old to young:oriented quartz inclusions at core,staurolite and prismatic sillimanite at mantle,kyanite and kyanite pseudomorph,and biotite at rim in Grt_1;fibrolitic sillimanite pseudomorphing kyanite±corundum,kyanite,and spinel+sillimanite after garnet+corundum in Grt_2;biotite,sillimanite,quartz±spinel in Grt_3;and ilmenite,rulite,quartz and sillimanite in Grt_4.The pre-melting,original rock composition was calculated through stepwise re-integration of melt into the residual,XRF based composition,allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections.The earliest recognizable stage occurred in the sillimanite field at around 575℃ at 4.5 kbar.Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P-T trajectory with an average dP/dT of ~30 bar/℃ in the kyanite field,up to ~660℃ at 6.5 kbar,before crossing the wet-solidus at around 675 ℃ at 7.5 kbar.The highest pressure occurred at P 10 kbar and T around 780℃ before prograde decompression associated with further heating.At 825℃ and 10.5 kbar,the rock re-entered into the sillimanite field.The temperature peaked at 900℃ at ca.9-9.5 kbar.Subsequent near-isobaric cooling led to the growth of Grt_4 and rutile at T ~880℃.Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T400℃,which probably occurred after uplift to upper crustal levels.U-Pb dating of zircons by LAICPMS of the khondalite yielded two concordant ~(206)Pb/~(238)U age groups with mean values of 542±2 Ma(MSWD=0.24,Th/U=0.01-0.03)and 514±3 Ma(MSWD=0.50,Th/U=0.01-0.05)interpreted as peak metamorphism of the khondalite and subsequent melt crystallization during cooling.     

Mineral Composition and Metamorphic P-T Conditions of the Eclogitic Rocks in the Eastern Dabie Mountains and Their Genetic Implication

There are obvious differences in the mineral assemblage and metamorphic P-T conditions between the eclogites from the northern and southern parts of the eastern Dabie Mountains. Those from the northern part of the mountains are developed in Alpine peridotite and gneiss. They have a mineral assemblage of garnet+diopside with no quartz, and were formed at temperatures of 600℃-740℃. Those from the southern part are developed in gneiss and marble. They consist of garnet+omphacite+less quartz and were metamorphosed at temperatures in the range of 650°-800℃. These differences suggest that the former may be formed during the metamorphism of the deep subducted oceanic crust, whereas the latter may be genetically related to the subduction of the continental crust in this area.     

Measurement of the Dielectric Properties of Volcanic Scoria and Basalt at 9370 MHz

Dielectric data for volcanic scoria and basalt on the earth at microwave frequency are extremely sparse, and also crucial for volcanic terrains imaging, and development. In consideration of their similarity to lunar regolith (soils and rocks) in chemical and mineral composition, the dielectric data is significative for passive and active microwave remote sensing on the Moon. This study provides the data about the dielectric properties of three kinds of scoria and two kinds of basalt in China. The method put forward in this paper is also applicable for measuring the dielectric properties of dry rocks and other granular ground materials with low complex dielectric constants. Firstly, the authors measured the ε‘ and tanδ values of strip specimens prepared from the mixture of scoria or basalt powder and polythene with the resonant cavity perturbation method at 9370 MHz. Secondly, from the ε‘ and tang values of the mixture, the ε‘s and tanδs values of sofid scoria and basalt were calculated using Lichtenecker‘s mixture formulae. Finally, the effective complex dielectric constants, ε‘e and tanδs, of scoria at different bulk densities were calculated. The results have shown that the ε‘s and tang svalues of all solid basaltic materials measured (both solid basaltic scoria or basalt) are approximately 7 and 0.05,respectively. With increasing bulk density of scoria, the ε‘s and tanδs values of scoria increase significantly.     

Mutual Pb2+/Sn2+ substitution in sulfosalts

Summary In naturally occurring sulfides and sulfosalts, the elements tin and lead can substitute for each other and form solid solution series. The best known example is the montesite series between teallite and herzenbergite. Within a coordinated research program including ore microscopy, X-ray and microprobe studies, mineral syntheses and Mössbauer spectroscopy, other lead sulfosalts were discovered which show considerable solid solution series.Natural tin-bearing jamesonites which have been recently discovered have also been confirmed experimentally.The present work reveales a broad solid solution field for franckeite ranging from Pb-rich potosiite to Sn-rich incaite. The well-established silver-free franckeite formula exhibits an ideal composition of(Pb, Sn) ₆ ²⁺ Fe²⁺ Sn ₂ ⁴⁺ Sb ₂ ³⁺ S ₁₄ ^2– . Valencies of the Fe, Sn, and Sb in franckeite shown in this formula have been confirmed by Mössbauer spectroscopy. Frequently observed trace amounts of Ag, Zn, Ge, and In are not essential in the structure.Numerous microprobe analyses of cylindrites indicate that they form a restricted solid solution series with slight Pb/Sn²⁺ variations. Cylindrite forms at increased sulfur fugacity. Experiments and microprobe analyses on ores demonstrate the direct replacement of tin-rich franckeite or incaite by pseudomorphic lamellae of cylindrite during controlled sulfdization but no characteristic cylinders were observed. In the replacement reaction most or all of the bivalent tin was oxidized to quadrivalent tin to form cylindrite Pb₄FeSn₄Sb₂S₁₆.Contribution to the Ore Mineralogy Symposium (IMA/COM) at the 14th General Meeting of the International Mineralogical Association, at Stanford, California, in July, 1986.     

FOREWORD AND TABLE OF CONTENTS OF BOOK "PRINCIPLES OF THE TECTONICS OF CHINA"

An experimental study has been made of the system Pb-Sn-Sb-S and of its sections Pb-Sn-S and PbS-Sb₂S₃ In aqueous solutions of HCl, NH₄Cl and NaOH at T = 300–400°C and total pressure = P_H2O= 1000 atm. All lead and antimony sulfosalts known in nature have been synthesized and also the following sulfostannates: teal lite, franckeite, and cylindrite. The phase relations In a part of the system Pb-Sn-Sb-3 are discussed and an attempt is made to use the experimental data for interpretation of the conditions of deposition of “Bolivian” type ores containing sulfosalts and sulfostannates. —Authors.     

Hydrothermal synthesis of pumpellyite–okhotskite series minerals

Summary ?Hydrothermal experiments to synthesize pumpellyite group minerals of the pumpellyite–okhotskite series and to investigate their stability have been carried out at 200, 300 and 400 MPa P _fluid and 250–500 °C by using cold-seal pressure vessels and solid buffers of MnO₂–Mn₂O₃, Cu₂O–CuO and Cu₂O–Cu buffer assemblages. Okhotskite and pumpellyite rich in the okhotskite component crystallized from an oxide mixture starting material of Ca₄MgMn³⁺ ₃Al₂Si₆O_24.5-oxide+excess H₂O at P _fluid of 200, 300 and 400 MPa and temperatures of 300 and 400 °C. However, a single phase of okhotskite was not produced, and associated piemontite, hausmannite, wollastonite, clinopyroxene, corundum, braunite–neltnerite solid solution and alleghanyite also formed. Mn-pumpellyite of the okhotskite–pumpellyite join occurs as aggregates of needle crystals, rounded grains or flaky crystals. Chemical compositions are variable and range from pumpellyite-(Mn²⁺) to okhotskite: 31–36 SiO₂, 13–21 Al₂O₃, 12–25 total Mn₂O₃, 0.6–4 MgO and 20–24 wt.% CaO. Reconnaissance experiments using a starting material of synthetic Ca₂Mn³⁺Al₂Si₃O₁₂(OH)-piemontite at 300 MPa and temperatures of 250, 300, 400 and 500 °C indicate that Mn-rich pumpellyite can crystallize from piemontite at lower temperatures than the stability field of piemontite. The Mn-rich pumpellyite was accompanied by garnet, wollastonite and alleghanyite. The chemical compositions of the Mn-pumpellyites are 32–36 SiO₂, 18–27 Al₂O₃, 8–18 total Mn₂O₃ and 20–23 wt.% CaO. This study shows that the stability fields of piemontite, piemontite+Mn-pumpellyite, and Mn-pumpellyite range in this order with decreasing temperature under high fO₂ conditions. The maximum stability temperature of Mn-rich pumpellyite lies between 400 and 500 °C at 200–400 MPa in high fO₂ conditions. Received March 3, 2000; revised version accepted December 28, 2001     

Metamorphism,graphite crystallinity,and sulfide anatexis of the Rampura–Agucha massive sulfide deposit,northwestern India

Located adjacent to the Banded Gneissic Complex, Rampura–Agucha is the only sulfide ore deposit discovered to date within the Precambrian basement gneisses of Rajasthan. The massive Zn–(Pb) sulfide orebody occurs within graphite–biotite–sillimanite schist along with garnet–biotite–sillimanite gneiss, calc–silicate gneisses, amphibolites, and garnet-bearing leucosomes. Plagioclase–hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720–780°C, whereas temperatures obtained from Fe–Mg exchange between garnet and biotite (580–610°C) in the pelites correspond to postpeak resetting. Thermodynamic considerations of pertinent silicate equilibria, coupled with sphalerite geobarometry, furnished part of a clockwise P–T–t path with peak P–T of ∼6.2 kbar and 780°C, attained during granulite grade metamorphism of the major Zn-rich stratiform sedimentary exhalative deposits orebody and its host rocks. Arsenopyrite composition in the metamorphosed ore yielded a temperature [and log f(S ₂)] range of 352°C (−8.2) to 490°C (−4.64), thus indicating its retrograde nature. Contrary to earlier research on the retrogressed nature of graphite, Raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals (490 ± 43°C with a maximum at 593°C). The main orebody is mineralogically simple (sphalerite, pyrite, pyrrhotite, arsenopyrite, galena), deformed and metamorphosed while the Pb–Ag-rich sulfosalt-bearing veins and pods that are irregularly distributed within the hanging wall calc–silicate gneisses show no evidence of deformation and metamorphism. The sulfosalt minerals identified include freibergite, boulangerite, pyrargyrite, stephanite, diaphorite, Mn–jamesonite, Cu-free meneghinite, and semseyite; the last three are reported from Agucha for the first time. Stability relations of Cu-free meneghinite and semseyite in the Pb–Ag-rich ores constrain temperatures at >550°C and <300°C, respectively. Features such as (1) low galena–sphalerite interfacial angles, (2) presence of multiphase sulfide–sulfosalt inclusions, (3) microcracks filled with galena (±pyrargyrite) without any hydrothermal alteration, and (4) high contents of Zn, Ag (and Sb) in galena, indicate partial melting in the PbS–Fe_0.96S–ZnS–(1% Ag₂S ± CuFeS₂) system, which was critical for metamorphic remobilization of the Rampura–Agucha deposit.     

The stability of primary alluaudites in granitic pegmatites: an experimental investigation of the Na2(Mn2−2x Fe1+2x )(PO4)3 system

In order to assess the geothermometric potential of the Na₂(Mn_2−2x Fe_1+2x )(PO₄)₃ system (x = 0–1), which represents the compositions of natural weakly oxidized alluaudites, we performed hydrothermal experiments between 400 and 800°C, at 1 kbar, under an oxygen fugacity (f(O₂)) controlled by the Ni–NiO (NNO), Fe₂O₃–Fe₃O₄ (HM), Cu₂O–CuO (CT), and Fe–Fe₃O₄ (MI) buffers. When f(O₂) is controlled by NNO, single-phase alluaudites crystallize at 400 and 500°C, whereas the association alluaudite + marićite appears between 500 and 700°C. The limit between these two fields corresponds to the maximum temperature that can be reached by alluaudites in granitic pegmatites, because marićite has never been observed in these geological environments. Because alluaudites are very sensitive to variations of oxygen fugacity, the field of hagendorfite, Na₂MnFe²⁺Fe³⁺(PO₄)₃, has been positioned in the f(O₂)–T diagram, and provides a tool that can be used to estimate the oxygen fugacity conditions that prevailed in granitic pegmatites during the crystallization of this phosphate.     

Gold Mineralization of the Gatsuurt Deposit in the North Khentei Gold Belt,Central Northern Mongolia

Mineral assemblages, chemical compositions of ore minerals, wall rock alteration and fluid inclusions of the Gatsuurt gold deposit in the North Khentei gold belt of Mongolia were investigated to characterize the gold mineralization, and to clarify the genetic processes of the ore minerals. The gold mineralization of the deposit occurs in separate Central and Main zones, and is characterized by three ore types: (i) low‐grade disseminated and stockwork ores; (ii) moderate‐grade quartz vein ores; and (iii) high‐grade silicified ores, with average Au contents of approximately 1, 3 and 5 g t^?1 Au, respectively. The Au‐rich quartz vein and silicified ore mineralization is surrounded by, or is included within, the disseminated and stockwork Au‐mineralization region. The main ore minerals are pyrite (pyrite‐I and pyrite‐II) and arsenopyrite (arsenopyrite‐I and arsenopyrite‐II). Moderate amounts of galena, tetrahedrite‐tennantite, sphalerite and chalcopyrite, and minor jamesonite, bournonite, boulangerite, geocronite, scheelite, geerite, native gold and zircon are associated. Abundances and grain sizes of the ore minerals are variable in ores with different host rocks. Small grains of native gold occur as fillings or at grain boundaries of pyrite, arsenopyrite, sphalerite, galena and tetrahedrite in the disseminated and stockwork ores and silicified ores, whereas visible native gold of variable size occurs in the quartz vein ores. The ore mineralization is associated with sericitic and siliceous alteration. The disseminated and stockwork mineralization is composed of four distinct stages characterized by crystallization of (i) pyrite‐I + arsenopyrite‐I, (ii) pyrite‐II + arsenopyrite‐II, (iii) galena + tetrahedrite + sphalerite + chalcopyrite + jamesonite + bournonite + scheelite, and iv) boulangerite + native gold, respectively. In the quartz vein ores, four crystallization stages are also recognized: (i) pyrite‐I, (ii) pyrite‐II + arsenopyrite + galena + Ag‐rich tetrahedrite‐tennantite + sphalerite + chalcopyrite + bournonite, (iii) geocronite + geerite + native gold, and (iv) native gold. Two mineralization stages in the silicified ores are characterized by (i) pyrite + arsenopyrite + tetrahedrite + chalcopyrite, and (ii) galena + sphalerite + native gold. Quartz in the disseminated and stockwork ores of the Main zone contains CO₂‐rich, halite‐bearing aqueous fluid inclusions with homogenization temperatures ranging from 194 to 327°C, whereas quartz in the disseminated and stockwork ores of the Central zone contains CO₂‐rich and aqueous fluid inclusions with homogenization temperatures ranging from 254 to 355°C. The textures of the ores, the mineral assemblages present, the mineralization sequences and the fluid inclusion data are consistent with orogenic classification for the Gatsuurt deposit.     

Experimental study of Pd hydrolysis in aqueous solutions at 25–70°C

The hydrolysis of the Pd²⁺ ion in HClO₄ solutions was examined at 25–70°C, and the thermodynamic constants of equilibrium K ₍₁₎⁰ and K ₍₂₎⁰were determined for the reactions Pd²⁺ + H₂O = PdOH⁺ + H⁺ and Pd²⁺ + 2H₂O = Pd(OH)₂⁰ + 2H⁺, respectively. The values of log K ₍₁₎⁰ = −1.66 ± 0.5 (25°C) and −0.65 ± 0.25 (50°C) and log K ₍₂₎⁰ = −4.34 ± 0.3 (25°C) and −3.80 ± 0.3 (50°C) were derived using the solubility technique at 0.95 confidence level. The values of log K ₍₁₎⁰ = −1.9 ± 0.6 (25°C), −1.0 ± 0.4 (50°C), and −0.5 ± 0.3 (70°C) were obtained by spectrophotometric techniques. The palladium ion is significantly hydrolyzed at elevated temperatures (50–70°C) even in strongly acidic solutions (pH 1–1.5), and its hydrolysis is enhanced with increasing temperature.     

Dehydration process and structural development of cordierite ceramic precursors derived from FTIR spectroscopic investigations

 Cordierite precursors were prepared by a sol-gel process using tetraethoxysilane, aluminum sec.-butoxide, and Mg metal flakes as starting materials. The precursors were treated by 15-h heating steps in intervals of 100 °C from 200 to 900 °C; they show a continuous decrease in the analytical water content with increasing preheating temperatures. The presence of H₂O and (Si,Al)–OH combination modes in the FTIR powder spectra prove the presence of both H₂O molecules and OH groups as structural components, with invariable OH concentrations up to preheating temperatures of 500 °C. The deconvolution of the absorptions in the (H₂O,OH)-stretching vibrational region into four bands centred at 3584, 3415, 3216 and 3047 cm⁻¹ reveals non-bridging and bridging H₂O molecules and OH groups. The precursor powders remain X-ray amorphous up to preheating temperatures of 800 °C. Above this temperature the precursors crystallize to μ-cordierite; at 1000 °C the structure transforms to α-cordierite. Close similarities exist in the pattern of the 1400–400 cm⁻¹ lattice vibrational region for precursors preheated up to 600 °C. Striking differences are evident at preheating temperatures of 800 °C, where the spectrum of the precursor powder corresponds to that of conventional cordierite glass. Bands centred in the “as-prepared” precursor at 1137 and 1020 cm⁻¹ are assigned to Si–O-stretching vibrations. A weak absorption at 872 cm⁻¹ is assigned to stretching modes of AlO₄ tetrahedral units and the same assignment holds for a band at 783 cm⁻¹ which appears in precursors preheated at 600 °C. With increasing temperatures, these bands show a significant shift to higher wavenumbers and the Al–O stretching modes display a strong increase in their intensities. (Si,Al)–O–(Si,Al)-bending modes occur at 710 cm⁻¹ and the band at 572 cm⁻¹ is assigned to stretching vibrations of AlO₆ octahedral units. A strong band around 440 cm⁻¹ is essentially attributed to Mg–O-stretching vibrations. The strongly increasing intensity of the 872 and 783 cm⁻¹ bands demonstrates a clear preference of Al for a fourfold-coordinated structural position in the precursors preheated at high temperatures. The observed band shift is a strong indication for increasing tetrahedral network condensation along with changes in the Si–O and Al–O distances to tetrahedra dimensions similar to those occurring in crystalline cordierite. These structural changes are correlated to the dehydration process starting essentially above 500 °C, clearly demonstrating the inhibiting role of H₂O molecules and especially of OH groups. Received: 1 March 2002 / Accepted: 26 June 2002     

Characteristic Features of REE and Pb–Zn–Ag Mineralizations in the Na Son Deposit,Northeastern Vietnam

The Na Son deposit is a small‐scale Pb–ZnPb–Zn–Ag deposit in northeast Vietnam and consists of biotite–chlorite schist, reddish altered rocks, quartz veins and syenite. The biotite–chlorite schist is intruded by syenite. Reddish altered rocks occur as an alteration halo between the biotite–allanite‐bearing quartz veins and the biotite–chlorite schist. Allanite occurs in the biotite–allanite‐bearing quartz veins and in the proximal reddish altered rocks. Rare earth element (REE) fluorocarbonate minerals occur along fractures or at rim of allanite crystals. The later horizontal aggregates of sulfide veins and veinlets cut the earlier reddish altered rocks. The earlier Pb–Zn veins consist of a large amount of galena and lesser amounts of sphalerite, pyrite and molybdenite. The later Cu veins cutting the Pb–Zn veins include chalcopyrite and lesser amounts of tetrahedrite and pyrite. The occurrences of two‐phase H₂O–CO₂ fluid inclusions in quartz from biotite–allanite‐bearing quartz veins and REE‐bearing fluorocarbonate minerals in allanite suggest the presence of CO₂ and F in the hydrothermal fluid. The oxygen isotopic ratios of the reddish altered rocks, biotite–chlorite schist, and syenite range from +13.9 to +14.9 ‰, +11.5 to +13.3 ‰, and +10.1 to +11.6 ‰, respectively. Assuming an isotopic equilibrium between quartz (+14.6 to +15.8 ‰) and biotite (+8.6 ‰) in the biotite–allanite‐bearing quartz vein, formation temperature was estimated to be 400°C. At 400°C, δ¹⁸O values of the hydrothermal fluid in equilibrium with quartz and biotite range from +10.5 to +11.7 ‰. These δ¹⁸O values are consistent with fluid that is derived from metamorphism. Assuming an isotopic equilibrium between galena (+1.5 to +1.7 ‰) and chalcopyrite (+3.4 ‰), the formation temperature was estimated to be approximately 300°C. The formation temperature of the Na Son deposit decreased with the progress of mineralization. Based on the geological data, occurrence of REE‐bearing minerals and oxygen isotopic ratios, the REE mineralization is thought to result from interaction between biotite–chlorite schist and REE‐, CO₂‐ and F‐bearing metamorphic fluid at 400°C under a rock‐dominant condition.