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1.
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2.
The Campanian-Maastrichtian Agbaja Ironstone Formation of the Nupe basin, Nigeria, forms a major part of the about 2 billion tons of iron ore reserves of the Middle Niger Embayment. The ironstone deposits were previously reported to be similar to the Minette-type ironstones because of their depositional patterns, composition and inferred origin. Four rock-types are recognized within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, mud-ironstone and bog iron ore. In the ironstones, kaolinite of both the groundmass and the ooids/pisoids is of lateritic origin, whereas the associated quartz, mica and heavy minerals are of detrital origin. Ooids and pisoids were formed by mechanical accretion of platy kaolinite crystals by rolling on the sea floor in a near-shore environment, and were subsequently transported and deposited together with a fine-grained kaolinitic groundmass. Pyrite (mainly framboidal) and siderite (both exclusively occurring as pseudomorphs of goethite and/or hematite) are diagenetic whereas goethite is post-diagenetic in origin, resulting from the ferruginization of the kaolinitic precursor. Crandallite-gorxeicite-goyazite, bolivarite and boehmite are also post-diagenetic in origin. Hematite was formed from the dehydration of goethite, whereas gibbsite (restricted to the upper part of the deposit) is of recent and in situ lateritic origin. The presence of newly formed authigenic pyrite and siderite (now replaced by hematite and goethite) are indicators of a reducing environment during diagenesis. The absence of diagenetic chamositic clay minerals, evidently caused by a low Mg concentration, suggests that fully marine conditions were not established during sedimentation. This is supported by the lack of fossils, brecciated shell materials and bioturbation features in the deposit. Reworking and redeposition of the primary constituents are inferred from broken pisoids, nuclei of pisoidal/ooidal fragments in pisoids and high iron concentrations present in the pisoids and ooids compared to that of the groundmass. These observations indicate that the Agbaja ironstone deposits of the Lokoja study area exhibit some environmental and mineralogical characteristics that are markedly different from other known deposits of Minette-type, where primary chamositic clay minerals generally form the protore for the ironstones. The recognition of kaolinite as the precursor constituent and the occurrence of similar deposits of the same age (Late Cretaceous) in Nigeria, Sudan and Egypt have implications for the paleoenvironmental interpretations of Phanerozoic ironstone deposits. Received: 16 February 1998 / Accepted: 8 July 1998  相似文献   

3.
The Coniacian-Santonian high-phosphorus oolitic iron ore at Aswan area is one of the major iron ore deposits in Egypt. However, there are no reports on its geochemistry, which includes trace and rare earth elements evaluation. Texture, mineralogy and origin of phosphorus that represents the main impurity in these ore deposits have not been discussed in previous studies. In this investigation, iron ores from three localities were subjected to petrographic, mineralogical and geochemical analyses. The Aswan oolitic iron ores consist of uniform size ooids with snowball-like texture and tangentially arranged laminae of hematite and chamosite. The ores also possess detrital quartz, apatite and fine-grained ferruginous chamosite groundmass. In addition to Fe2O3, the studied iron ores show relatively high contents of SiO2 and Al2O3 due to the abundance of quartz and chamosite. P2O5 ranges from 0.3 to 3.4 wt.% showing strong positive correlation with CaO and suggesting the occurrence of P mainly as apatite. X-ray diffraction analysis confirmed the occurrence of this apatite as hydroxyapatite. Under the optical microscope and scanning electron microscope, hydroxyapatite occurred as massive and structureless grains of undefined outlines and variable size (5–150 μm) inside the ooids and/or in the ferruginous groundmass. Among trace elements, V, Ba, Sr, Co, Zr, Y, Ni, Zn, and Cu occurred in relatively high concentrations (62–240 ppm) in comparison to other trace elements. Most of these trace elements exhibit positive correlations with SiO2, Al2O3, and TiO2 suggesting their occurrence in the detrital fraction which includes the clay minerals. ΣREE ranges between 129.5 and 617 ppm with strong positive correlations with P2O5 indicating the occurrence of REE in the apatite. Chondrite-normalized REE patterns showed LREE enrichment over HREE ((La/Yb)N = 2.3–5.4) and negative Eu anomalies (Eu/Eu* = 0.75–0.89). The oolitic texture of the studied ores forms as direct precipitation of iron-rich minerals from sea water in open space near the sediment-water interface by accretion of FeO, SiO2, and Al2O3 around suspended solid particles such as quartz and parts of broken ooliths. The fairly uniform size of the ooids reflects sorting due to the current action. The geochemistry of major and trace elements in the ores reflects their hydrogenous origin. The oolitic iron ores of the Timsha Formation represent a transgressive phase of the Tethys into southern Egypt during the Coniacian-Santonian between the non-marine Turonian Abu Agag and Santonian-Campanian Um Barmil formations. The abundance of detrital quartz, positive correlations between trace elements and TiO2 and Al2O3, and the abundance mudstone intervals within the iron ores supports the detrital source of Fe. This prediction is due to the weathering of adjacent land masses from Cambrian to late Cretaceous. The texture of the apatite and the REE patterns, which occurs entirely in the apatite, exhibits a pattern similar to those in the granite, thus suggesting a detrital origin of the hydroxyapatite that was probably derived from the Precambrian igneous rocks. Determining the mode of occurrence and grain size of hydroxyapatite assists in the maximum utilization of both physical and biological separation of apatite from the Aswan iron ores, and hence encourages the use of these ores as raw materials in the iron making industry.  相似文献   

4.
Gypsum (CaSO4·2H2O), alunite (KAl3(SO4)2(OH)6), and rare phosphate–sulphate sanjuanite Al2(PO4)(SO4)(OH) 9(H2O) and rossiantonite (Al3(PO4)(SO4) 2(OH)2(H2O)14) have recently been identified as secondary mineral deposits in different quartz‐sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz‐sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho–sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz‐sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non‐sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H2S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co‐occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz‐sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments.  相似文献   

5.
Summary The crystal structure of synthetic holtedahlite, Mg12(PO3OH, PO4)(PO4)5(OH,O)6,P31m,a = 11.186(3),c = 4.977(1) Å,Z = 1, has been refined toR = 0.033 for 718 observed reflections. Natural holtedahlite, Mg12(PO3OH, CO3)(PO4)5(OH, O)6,a = 11.203(3),c = 4.977(1) Å, was refined toR = 0.031 for 1202 observed reflections. The structure contains pairs of face-sharing Mg-octahedra linked by edge-sharing to form double chains alongc. Hydrogen phosphate groups on three-fold axes are partially replaced by carbonate groups in natural holtedahlite. Structural similarities with ellenbergerite are pointed out.
Die Kristallstruktur von natürlichem und synthetischem Holtedahlit
Zusammenfassung Die Kristallstruktur von synthetischem Holtedahlit, Mg12(PO3OH,PO4)(PO4)5(OH,O)6,P31m,a = 11,186(3),c = 4,977(1) Å,Z = 1, wurde für 718 beobachtete Reflexe aufR = 0,033 verfeinert. Natürlicher Holtedahlit, Mg12(PO3OH, CO3)(PO4)5(OH, O)6,a = 11,203(3),c = 4,977(1) A wurde für 1202 beobachtete Reflexe aufR = 0,031 verfeinert. Die Struktur enthält Paare von über eine Fläche verknüpften Mg-Oktaedern, die weiter durch Kantenverknüpfung Doppelketten nachc bilden. Saure Phosphatgruppen auf dreizähligen Achsen sind im natürlichen Holtedahlit teilweise durch Karbonatgruppen ersetzt. Strukturelle Ähnlichkeiten zu Ellenbergerit werden aufgezeigt.

With 4 Figures  相似文献   

6.
Aalenian and lower Bajocian rocks in the central and northern Swiss Jura mountains comprise a series of parasequences that mainly reflect a shallowing-upward trend in a shallow, mixed carbonate/siliciclastic depositional environment. Within a parasequence, ooidal ironstones may occur at three specific types of horizons. These are: regressional discontinuities and transgressional discontinuities formed by sediment bypassing, and omissional discontinuities formed by starvation. Ooidal ironstones, which principally are autochthonous, accumulated during both sea-level rises and falls in a relatively broad bathymetric and hydrodynamic spectrum. The key physical factor for ferruginous ooid genesis is non-deposition. Ferruginous ooids and microbialites consist of goethite, chamosite and mixtures thereof, with subordinate amounts of apatite and silica. Ferruginous ooids grew stepwise on the sediment surface in an oxygenated marine environment. Ferruginous microbialites, being the product of benthic microbial communities, grew ? partly in cavities ? in aerated moderate- to high-energy environments. Thus, chamosite evolved from a precursor substance stable under oxidizing conditions. The close mineralogical and micromorphological resemblance of ferruginous microbialites and ooids suggests a common biogenic origin. Structural rearrangement of a biologically accreted gel-like precursor substance consisting of various amorphous hydroxides is considered a probable mode of mineral genesis in both ferruginous ooids and microbialites.  相似文献   

7.
Summary The crystal structure of arsentsumebite, ideally, Pb2Cu[(As, S)O4]2(OH), monoclinic, space group P21/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?3, Z = 2, dcalc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb2 Me(XO4)2(Z) where Me = Cu2+, Mn2+, Zn2+, Fe2+, Fe3+; X = S, Cr, V, As, P; Z = OH, H2O. Members of this group include tsumebite, Pb2Cu(SO4)(PO4)(OH), vauquelinite, Pb2Cu(CrO4)(PO4)(OH), brackebuschite, Pb2 (Mn, Fe)(VO4)2(OH), arsenbracke buschite, Pb2(Fe, Zn)(AsO4)2(OH, H2O), fornacite, Pb2Cu(AsO4)(CrO4)(OH), and feinglosite, Pb2(Zn, Fe)[(As, S)O4]2(H2O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO6 octahedra and M–T represents corner sharing between octahedra and XO4 tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)2(AsO4)2 (OH, H2O)2 and tsumcorite-group minerals Me(1)Me(2)2(XO4)2(OH, H2O)2. Received June 24, 2000; revised version accepted February 8, 2001  相似文献   

8.
The crystal structure of bonshtedtite, Na3Fe(PO4)(CO3) (monoclinic, P21/m, a = 5.137(4), b = 6.644(4), c = 8.908(6) Å, β = 90.554(14)°, V = 304.0(4) Å3, Z = 2) has been refined to R 1 = 0.041 on the basis of 1314 unique reflections. The structure is similar to that of other minerals of the bradleyite group. It is based on the [Fe(PO4)(CO3)]3? layers oriented parallel to (001). The layers are formed by corner-sharing PO4 tetrahedra and FeO4(CO3) complexes, where FeO6 tetrahedra and CO3 triangles are edge-shared. The topology of the octa-tetrahedral layer in bonshtedtite is similar to that of the autunite-group minerals, but it differs from the latter in terms of local topological properties.  相似文献   

9.
Lead bromapatite [Pb10(PO4)6Br2] has been synthesized via solid-state reaction at pressures up to 1.0 GPa, and its structure determined by single-crystal X-ray diffraction at ambient temperature and pressure. The large bromide anion is accommodated in the c-axis channel by lateral displacements of structural elements, particularly of Pb2 cations and PO4 tetrahedra. The compressibility of bromapatite was also investigated up to about 20.7 GPa at ambient temperature, using a diamond-anvil cell and synchrotron X-ray radiation. The compressibility of lead bromapatite is significantly different from that of lead fluorapatite. The pressure–volume data of lead bromapatite (P < 10 GPa) fitted to the third-order Birch-Murnaghan equation yield an isothermal bulk modulus (K T ) of 49.8(16) GPa and first pressure derivative (KT￠ K_{T}^{\prime } ) of 10.1(10). If KT￠ K_{T}^{\prime } is fixed at 4, the derived K T is 60.8(11) GPa. The relative difference of the bulk moduli of these two lead apatites is thus about 12%, which is about two times the relative difference of the bulk moduli (~5%) of the calcium apatites fluorapatite [Ca10(PO4)6F2], chlorapatite [Ca10(PO4)6Cl2] and hydroxylapatite [Ca10(PO4)6(OH)2]. Another interesting feature apparently related to the replacement of F by Br in lead apatite is the switch in the principle axes of the strain ellipsoid: the c-axis is less compressible than the a-axis in lead bromapatite but more compressible in lead fluorapatite.  相似文献   

10.
A novel complex continuous system of solid solutions involving vauquelinite Pb2Cu(CrO4)(PO4)(OH), bushmakinite Pb2Al(VO4)(PO4)(OH), ferribushmakinite Pb2Fe3+(VO4)(PO4)(OH), and a phase with the endmember formula Pb2Cu(VO4)(PO4)(H2O) or Pb2Cu(VO4)(РО3ОН)(ОН) is studied based on samples from the oxidation zone of the Berezovskoe, Trebiat, and Pervomaisko-Zverevsky deposits in the Urals, Russia. This is the first natural system in which chromate and vanadate anions show a wide range of substitutions and the most extensive solid solution system involving (CrO4)2– found in nature. The major couple substitution is Cr6+ + Cu2+ ? V5+ + M3+, where M = Fe, Al. The correlation coefficients calculated from 125 point analyses are: 0.96 between V and (Fe + Al), 0.96 between Cr and (Cu + Zn),–0.96 between V and (Cu + Zn),–0.97 between Cr and (Fe + Al), and–0.97 between (Fe + Al) and (Cu + Zn). The substitutions V5+ ? Cr6+ (correlation coefficient–0.98) and to a lesser extent P5+ ? As5+ (correlation coefficient–0.86) occur at two types of tetrahedral sites, whereas the metal–nonmetal/metalloid substitutions, i.e., V or Cr for P or As, are minor. The substitution Fe3+ ? Al3+ is also negligible in this solid solution system.  相似文献   

11.
Gold extraction at the Macraes gold mine in New Zealand involves concentration of pyrite and arsenopyrite, oxidation of those sulphides, then cyanidation. The ore concentrate is predominantly Otago Schist host rock (andesitic composition) with up to 15% sulphides. The oxidation step is conducted on ore concentrate slurry in an autoclave at 225°C and 3,800 kPa oxygen gas pressure with continuous feed. The slurry takes ca. 1 h to pass through the autoclave, during which time the sulphides are almost completely oxidised. Sulphide oxidation causes strong acidification of the slurry, which is maintained at pH of 1–2 by addition of CaCO3. Scales form on walls in the autoclave, with minerals reflecting progressive oxidation and alteration of the ore through the system. The schist in the ore feed has mineralogy similar to propylitically altered andesite: quartz, albite, muscovite, chlorite, and pyrite. Muscovite undergoes almost complete dissolution, with associated precipitation of quartz and alunite (KAl3(SO4)2(OH)6). Other principal minerals deposited and discharged include anhydrite (and/or gypsum), jarosite (KFe3(SO4)2(OH)6), hematite (and/or amorphous iron oxyhydroxide), and amorphous arsenates. Dissolved ferrous iron passes right through the autoclave, and variably hydrated Fe2+and Fe3+sulphate minerals, including rozenite and szomolnokite (both FeSO4.hydrate) and ferricopiapite (Fe5(SO4)6O(OH).hydrate), are formed along the way. The autoclave chemical system resembles acid–sulphate hydrothermal activity in geothermal systems and high-sulphidation epithermal mineral deposits formed in arc environments. These natural acid–sulphate systems are pervaded by volcanic vapours in the near-surface environment, where widespread dissolution of host rocks occurs and deposition of quartz, alunite, and anhydrite is common. Some of the volume loss associated with these natural systems may be due to dissolution of soluble sulphate minerals by later-stage groundwater incursion.  相似文献   

12.
The reactions of secondary lead orthophosphate with approximately 10?1 M sodium fluoride and sodium bromide solutions have been investigated at 25°C. Interpretation of the solubility data resulted in solubility product constants for fluoropyromorphite and bromopyromorphite of 10?71.6 and 10?78.1, respectively. According to these constants, the stability sequence for lead pyromorphites is Pb5(PO4)3Cl > Pb5(PO4)3Br > Pb5(PO4)3OH > Pb5(PO4)3F. The derived free energy data have been used to evaluate the respective stabilities of fluoro-pyromorphite and bromopyromorphite within the systems PbF2-PbO-P2O5-H2O and PbBr2-PbO-P2O5-H2O and to predict the equilibrium behavior of the Pb5(PO4)3F-Pb5(PO4)3OH solid solution under aqueous conditions.  相似文献   

13.
Summary. ?In the Nurra region, NW Sardinia, oolitic ironstones are interlayered within coarsening upward metasedimentary sequences of siltstone, sandstone, breccia, and conglomerate. A Caradoc-Early Ashgill age is suggested by the analogies with metasediments of Central-Southern Sardinia following the Sardinian tectonic phase. The sequences including oolitic ironstones are overlain by black metapelite of inferred Hirnantian to early Silurian age. The ooids consist of chamosite, siderite or magnetite or, rarely, stilpnomelane. Chamosite ooids consist of up to 30 lamellae and sporadically show clustering of magnetite grains in core and rim, and widespread late replacement by siderite. The alternation of chamosite and Fe-oxide observed in a few samples points to an ooid transport over the crests and hollows of megaripples in a continental shelf at 0–60 m depth, and/or a random displacement of the littoral environments in a rapidly evolving continent-sea transition zone producing an alternation of oxidizing and reducing conditions. Black phosphate clasts, including older Fe-ooids, frequently occur. The oolitic ironstones of Nurra are variable mixtures of an Al-Si-Ti rich- detrital component and a Fe-rich chemical component. The abundance of chamosite and siderite explains the high values of LOI and the high Fetot, Fe2+ and Al contents and the low Si amounts in comparison with other Phanerozoic oolitic ironstones. The detrital elements are Al, Si, Ti, Mg, Zr, Th. Chemical precipitation processes supplied Fe, Ca, P, Sr, HREE. The chondrite-normalized pattern shows a slight LREE enrichment, a clear negative Eu anomaly, and a flat HREE trend typical of many Ordovician oolitic ironstones. The NASC-normalized pattern has a convex shape, with peaks for Sm, Gd, like in all the pre-Devonian phosphate deposits. The numerous phosphate clasts, pyrite pockets, diffuse organic matter, and lack of glauconite suggest an upward extension of the oxygen minimum layer in a stratified basin, up to a depth of 60 m, and allow the estimation of log fO2 = − 72 to − 80 and pH = 9.0–9.5. for the underlying pyrite zone (depth > 60 m). Here the pore waters leached Eu2+ from the bottom sediments giving the observed negative anomaly of the chondrite-normalized REE pattern.
Résumé ?Dans la région de la Nurra, Sardaigne nord-ouest, des niveaux ferrugineux oolithiques sont intercalés dans des séquences métasédimentaires composées de silts, grès, brèches et conglomerats. Un age Caradoc-Ashgill inférieur est suggéré par les analogies avec les métasédiments de la Sardaigne centrale-méridionale postérieurs à la phase tectonique Sarde. Les séquences qui contiennent les niveaux ferrugineux oolithiques sont surmontées par des métapélites noires pour lesquelles on suppose un age Hirnantien à Siliurien inférier. Les oolithes sont constituées de chamosite, siderite ou magnétite ou rarement, stilpnomelane. Les oolithes de chamosite peuvent avoir jusq’à 30 enveloppes dans le cortex et sporadiquement elles montrent une concentration de grains de magnétite au centre et sur le bord et un vaste replacement tardif par de la sidérite. L’alternation de enveloppes à chamosite et à oxydes de fer observée dans quelques échantillons indique un transport des oolithes sur la crête et dans la dépression de rides géantes sur une plateforme continentale à 0–60 m de profondeur et/ou bien un déplacement casuel des milieux c?tiers dans une zone de transition entre mer et continent en rapide évolution, ce qui produisait une alternation de conditions oxydantes et réduisantes. On trouve fréquemment des intraclastes noirs de phosphate qui contiennent des oolithes ferrugineuses plus anciennes. Les niveaux oolithiques ferrugineux de la Nurra sont le résultat d’un mélange en proportions variables entre une composante détritique riche en Al, Si,Ti et une composante chimique riche en fer. L’abondance de chamosite et sidérite explique les hautes valeurs de perte au feu et la haute teneur en Fetot, Fe2+ et Al et la basse teneur en Si en comparaison avec d’autres formations oolithiques ferrugineuses Phanérozo?ques. Les éléments détritiques sont Al, Si, Ti, Mg, Zr, Th. Les processus de précipitation chimique ont fourni Fe, Ca., P, Sr, HREE. Les teneurs de terres rares normalisées aux chondrites montrent un léger enrichissement en LREE, une évidente anomalie négative de Eu, et une disposition en plateau des HREE, qui est tipique de beaucoup de formations oolithiques ferrugineuses Ordoviciennes. Les teneurs normalisées aux NASC forment une courbe convexe avec deux maxima pour Sm et Gd, comme dans tout les dép?ts phosphatés pré-Dévoniens. Les nombreux intraclastes de phosphate, les cavités pleines de pyrite, l’abondance de matière organique et l’absence de glacounie suggèrent une extension vers plus faible profondeurs ( jusq’à 60 m) de la couche d’eau marine ayant la moindre teneur en oxygène dans un bassin stratifié. Pour la zone à pyrite qui est en dessous de la zone oolithique (profondeur > 60 m) on estime les conditions suivantes: log fO2 = − 72 à− 80 et pH = 9.0–9.5. Les eaux intergranulaires solubilisent et emportent le Eu2+ des sediments du fond et donnent l’anomalie négative de Eu pour les teneurs de terres rares normalisées aux chondrites.

Received February 10, 1999;/revised version accepted June 30, 1999  相似文献   

14.
The ion activity product of Fe and phosphate in interstitial waters from four sediment cores taken from Greifensee, Switzerland indicate the presence of vivianite [Fe3(PO4)2 · 8 H2O] in the solid phase. Analysis of the sediment using an electron microprobe and by electron microscopy revealed P-rich grains to be also enriched in Fe. The combined methods provide strong evidence that vivianite is forming authigenically in the sediments. Thermodynamic stability calculations demonstrate that the most stable Fe and phosphate minerals (pyrite, siderite and apatite) are not the ones controlling the pore water chemistry. The results emphasize the importance of rate processes of mineral formation in early diagenesis.Calculations based on the sediment phosphate concentration, and the degree of supersaturation of Fe and phosphate in the upper portion (0–15 cm) of the pore waters, indicate that the rate of vivianite mineral growth is controlled by a surface reaction rather than a diffusion mechanism. The response time of dissolved phosphate in the sediment pore waters with respect to mineral precipitation is on the order of 1–20 days. Less than 15% of the phosphate released by organic matter degradation at the sediment-water interface and below is retained in the sediments.  相似文献   

15.
In the oxidation zone of the Berezovskoe gold deposit in the middle Urals, Russia, minerals of the beudantite–segnitite series (idealized formulas PbFe3 3+ AsO4)(SO4)(OH)6 and PbFe3 3+ AsO4)(AsO3OH)(OH)6, respectively) form a multicomponent solid solution system with wide variations in the As, S, Fe, Cu, and Sb contents and less variable P, Cr, Zn, Pb, and contents K. The found minerals of this system correspond to series from beudantite with 1.25 S apfu to S-free segnitite, with segnitite lacking between 1.57 and 1.79 As apfu. Segnitite at the Berezovskoe deposit contains presumably pentavalent Sb (up to 15.2 wt % Sb2O5 = 0.76 Sb apfu, the highest Sb content in the alunite supergroup minerals), which replaces Fe3+. The Sb content increases with increasing As/S value. On the contrary, beudantite is free of or very poor in Sb (0.00–0.03 Sb apfu). Many samples of segnitite are enriched in Cu (up to 8.2 wt% CuO = 0.83 Cu apfu, uncommonly high Cu content for this mineral) and/or in Zn (up to 2.0 wt% ZnO = 0.19 Zn apfu). Both Cu and Zn replace Fe. The generalized formula of a hypothetic end member of the segnitite series with 1 Sb apfu is Pb(Fe3+ M 2+Sb5+)(AsO4)2(OH)6, where M = Cu, Zn, Fe2+. The chemical evolution of beudantite–segnitite series minerals at the Berezovskoe deposit is characterized by an increase in the S/As value with a decrease in the Sb content from early to late generations.  相似文献   

16.
The Benjamin River apatite prospect in northern New Brunswick, Canada, is hosted by the Late Silurian Dickie Brook plutonic complex, which is made up of intrusive units represented by monzogranite, diorite and gabbro. The IOA ores, composed mainly of apatite, augite, and magnetite at Benjamin River form pegmatitic pods and lenses in the host igneous rocks, the largest of which is 100 m long and 10–20 m wide in the diorite and gabbro units. In this study, 28 IOA ore and rock samples were collected from the diorite and gabbro units. Mineralogical observations show that the apatite–augite–magnetite ores are variable in the amounts of apatite, augite, and magnetite and are associated with minor amounts of epidote‐group minerals (allanite, REE‐rich epidote and epidte) and trace amounts of albite, titanite, ilmenite, titanomagnetite, pyrite, chlorite, calcite, and quartz. Apatite and augite grains contain small anhydrite inclusions. This suggests that the magma that crystallized apatite and augite had high oxygen fugacity. In back scattered electron (BSE) images, apatite grains in the ores have two zones of different appearance: (i) primary REE‐rich zone; and (ii) porous REE‐poor zone. The porous REE‐poor zones mainly appear in rims and/or inside of the apatite grains, in addition to the presence of apatite grains which totally consist of a porous REE‐poor apatite. This porous REE‐poor apatite is characterized by low REE (<0.84 wt%), Si (<0.28 wt%), and Cl (<0.17 wt%) contents. Epidote‐group minerals mainly occur in grain boundary between the porous REE‐poor apatite and augite. These indicate that REE leached from primary REE‐rich apatite crystallized as allanite and REE‐rich epidote. Magnetite in the ores often occurs as veinlets that cut apatite grains or as anhedral grains that replace a part of augite. These textures suggest that magnetite crystallized in the late stage. Pyrite veins occur in the ores, including a large amount of quartz and calcite veins. Pyrite veins mainly occur with quartz veins in augite. These textures indicate pyrite veins are the latest phase. Apatite–augite–magnetite ore, gabbro–quartz diorite and feldspar dike collected from the Benjamin River prospect contain dirty pure albite (Ab98Or2–Ab100) under the microscope. The feldspar dikes mainly consist of dirty pure albite. Occurrences of the dirty pure albite suggest remarkable albitization (sodic alteration) of original plagioclase (An25.3–An60 in Pilote et al., 2012) associating with intrusion of monzogranite into gabbro and diorite. SO42? bearing magma crystallized primary REE‐rich apatite, augite and anhydrite reacted with Fe in the sodic fluids, which result in oxidation of Fe2+ and release of S2? into the sodic fluids. REE, Ca and Fe from primary REE‐rich apatite, augite and plagioclase altered by the sodic fluids were released into the fluids. Then Fe3+ in the sodic fluids precipitated as Fe oxides and epidote‐group minerals in apatite–augite–magnetite ores. Finally, residual S2? in sodic fluids crystallized as latest pyrite veins. In conclusion, mineralization in Benjamin River IOA prospect are divided into four stages: (1) oxidized magmatic stage that crystallized apatite, augite and anhydrite; (2) sodic metasomatic stage accompanying alteration of magmatic minerals; (3) oxidized fluid stage (magnetite–epidote group minerals mineralization); and (4) reduced fluid stage (pyrite mineralization).  相似文献   

17.
In the context of the deep waste disposal, we have investigated the respective stabilities of two iron-bearing clay minerals: berthierine ISGS from Illinois [USA; (Al0.975FeIII0.182FeII1.422Mg0.157Li0.035Mn0.002)(Si1.332Al0.668)O5(OH)4] and chlorite CCa-2 from Flagstaff Hill, California [USA; (Si2.633Al1.367)(Al1.116FeIII0.215Mg2.952FeII1.712Mn0.012Ca0.011)O10(OH)8]. For berthierine, the complete thermodynamic dataset was determined at 1 bar and from 2 to 310 K, using calorimetric methods. The standard enthalpies of formation were obtained by solution-reaction calorimetry at 298.15 K, and the heat capacities were measured by heat-pulse calorimetry. For chlorite, the standard enthalpy of formation is measured by solution-reaction calorimetry at 298.15 K. This is completing the entropy and heat capacity obtained previously by Gailhanou et al. (Geochim Cosmochim Acta 73:4738–4749, 2009) between 2 and 520 K, by using low-temperature adiabatic calorimetry and differential scanning calorimetry. For both minerals, the standard entropies and the Gibbs free energies of formation at 298.15 K were then calculated. An assessment of the measured properties could be carried out with respect to literature data. Eventually, the thermodynamic dataset allowed realizing theoretical calculations concerning the berthierine to chlorite transition. The latter showed that, from a thermodynamic viewpoint, the main factor controlling this transition is probably the composition of the berthierine and chlorite minerals and the nature of the secondary minerals rather than temperature.  相似文献   

18.
There is now evidence that naturally occurring acid–water is more abundant than previously thought and that it has been important in the geologic past. Understanding the processes leading to the formation of such systems is required to appreciate the role of acid systems in geologic processes and to develop indicators for recognizing the geologic/environmental importance of these systems in the past. This paper characterizes the hydrogeology, hydrogeochemistry, microbial biogeochemistry and landscape attributes of acid–groundwater surface water systems in Australia with an emphasis on a well studied playa-lake system, Lake Tyrrell, Murray Valley. A model for the origin of these acid brines is presented and the importance of acid-brine producing systems is speculated upon. Data include porewater and groundwater geochemical measurements (collected during a 10 day field campaign) and results from geochemical modeling and graphics (e.g., Piper diagrams and x–y plots of seawater evaporation trajectories). Key characteristics of the system are (1) aquifer materials have low acid buffering capacities, (2) saline groundwater flowing onto playa surfaces is an oxic, H2SO4 solution, (3) authigenic minerals include combinations of jarosite [KFe3(SO4)2(OH)6], alunite [KAl3(SO4)2(OH)6] and Fe oxides that can form as evaporite minerals, (4) a source for solutes can be marine aerosols and (5) the formation of ironstones. Groundwater acidification by various processes including sulfide oxidation and ferrolysis, and at many different times, are the unique aspects for evolution of these acid brines and they can be considered another end member of the Eugster–Jones–Hardie model for the evolution of brines in closed basins. Acid–hypersaline groundwater and playa systems such as Lake Tyrrell may be an example of expected changes in the hydrogeochemistry of terrestrial water during late-stage continental denudation under arid conditions. Historically these systems may have been integral to the formation of opal, bauxite, some low temperature ore deposits, of authigenic K-feldspars, and continental redbeds. Natural acid saline systems, such as those in Australia, may also be representative of acid saline systems on Mars.  相似文献   

19.
Apatite is a concentrator of F and Cl, which play a significant role in the formation of minerals of platinum-group elements of pneumatolitic origin. There are three apatite generations in Norilsk magmatic sulfide ores. Apatite I occurs in sulfide bodies and rims of fluid alteration above sulfide droplets in disseminated ores. Its composition evolved from hydroxyl-chlorapatite to chlorapatite. Apatite I associates with Tiрbiotite, titanomagnetite, ilmenite with baddeleyite lamellae, anhydrite, Ti-poor kaersutite, Cl-bearing hastingsite and edenite, djerfisherite, bartonite, and minerals of the Pt and Au groups. Apatite I contains up to 2.3 wt % lanthanides, primarily Ce, La, and Nd. Apatite I is overgrown and replaced by apatite II, the composition of which evolved from hydroxyl-chlor-fluorapatite to fluorapatite. Apatite II often occurs also as individual crystals in massive sulfides and contains up to 0.9 wt % lanthanides. The pneumatolitic chlorapatite and fluorapatite contain ~0.5 wt % SiO2. The composition of apatite indicates discrete evolution of fluids released during the crystallization of Norilsk sulfide melts: from water–chloride to chloride at the first state, and from water–chlorite–fluoride to essentially fluoride at the second stage. The lanthanides released during the replacement of chlorapatite I by fluorapatite II were probably incorporated in pneumatolitic zoned orthite-(Ce). In the areas affected by prehnite–pumpellyite metamorphism, apatite I and apatite II within metamorphosed sulfide ores are partly or completely replaced by apatite III, which varies in composition from hydroxyl-chlorapatite to hydroxylapatite poor in fluorine and lanthanides. The lanthanides released during the replacement of apatite I and II by metamorphic hydroxylapatite III were probably incorporated in metamorphic unzoned orthite-(Ce).  相似文献   

20.
Experimental and predicted thermochemical constants are used to assess the formation and stability of lead phosphates in soil and sedimentary environments. For the chemical conditions likely to be encountered in oxidizing environments, the stability fields of pyromorphites [Pb5(PO4)3X, X = OH?, Cl?, Br? and F?] and plumbogummite [PbAl3(PO4)2(OH)5-H2O] predominate strongly over those of the other secondary lead minerals. The theoretical phase relationships together with several field observations are used as the basis for suggesting that the interaction of lead and phosphorus (to form pyromorphites and plumbogummite in particular) is an important buffer mechanism controlling the migration and fixation of lead in the environment. Calculations using the concentrations of lead and phosphate ions in serum indicate that the solubility of lead phosphates may be the limiting factor with regard to lead ion concentration in human body. The removal of lead from wastewater by precipitation as lead chloropyromorphite is considered a spin-off of possible industrial interest.  相似文献   

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