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1.
Oldoinyo Lengai, located in the Gregory Rift in Tanzania, is a world-famous volcano owing to its uniqueness in producing natrocarbonatite melts and because of its extremely high CO2 flux. The volcano is constructed of highly peralkaline [PI = molar (Na2O + K2O)/Al2O3 > 2–3] nephelinite and phonolites, both of which likely coexisted with carbonate melt and a CO2-rich fluid before eruption. Results of a detailed melt inclusion study of the Oldoinyo Lengai nephelinite provide insights into the important role of degassing of CO2-rich vapor in the formation of natrocarbonatite and highly peralkaline nephelinites. Nepheline phenocrysts trapped primary melt inclusions at 750–800 °C, representing an evolved state of the magmas beneath Oldoinyo Lengai. Raman spectroscopy, heating-quenching experiments, low current EDS and EPMA analyses of quenched melt inclusions suggest that at this temperature, a dominantly natritess-normative, F-rich (7–14 wt%) carbonate melt and an extremely peralkaline (PI = 3.2–7.9), iron-rich nephelinite melt coexisted following degassing of a CO2 + H2O-vapor. We furthermore hypothesize that the degassing led to re-equilibration between the melt and liquid phases that remained and involved 1/ mixing between the residual (after degassing) alkali carbonate liquid and an F-rich carbonate melt and 2/ enrichment of the coexisting nephelinite melt in alkalis. We suggest that in the geological past similar processes were responsible for generating highly peralkaline silicate melts in continental rift tectonic settings worldwide.  相似文献   

2.
《Applied Geochemistry》2002,17(10):1305-1312
The effect of different drying conditions on the stability of NaNd(CO3)·6H2O and NaEu(CO3)·6H2O and the identity of the decomposition product have been investigated. The rate of decomposition and the nature of the altered phases are dependant on the drying conditions used. When the phases are oven dried at 120 °C, the decomposition is immediate and the phase completely alters to Nd2(CO3)3 or Eu2(CO3)3 respectively. Under less severe drying conditions, the Na rare earth carbonate phases alter to Nd2(CO3)3·8H2O and Eu2(CO3)3·8H2O over a period of 24–48 h, but they can be kept indefinitely in a water saturated environment. The implications for using Nd and Eu as actinide analogues are discussed.  相似文献   

3.
《Applied Geochemistry》1999,14(1):53-65
Owens Lake in SE California became essentially dry by the 1920s after the Los Angeles Aqueduct was constructed and diversion of water from the Owens River began. Frequent dust storms at Owens Lake produce clouds of efflorescent salts which present human health hazards as a result of their small particle size and elevated concentrations of As and SO4. This study was conducted to characterize the evolution of major elements in ground water in eastern Owens Lake and to examine the factors controlling the concentrations of dissolved As and F. Evapoconcentration of shallow ground waters at the lakebed surface produces high pH, high alkalinity brines with major ion compositions that are consistent with those predicted by the Hardie–Eugster Model. Evaporite minerals identified in the surface salts using XRD were halite (NaCl), thenardite (Na2SO4), trona (Na3H(CO3)2·2H2O), pirssonite (Na2Ca(CO3)2·2H2O), and nesquehonite (MgCO3·3H2O). Significant correlations between both As and F with Li in shallow ground waters indicate that As and F are not partitioned into surface salts until very high salinities are reached (>9.0 m). Leaching experiments show that As and F can be readily released from lakebed salts when exposed to natural precipitation. Conservative behavior of As and F results from the high pH values and low Ca activities of shallow ground waters that contribute to: (1) redox stability of As(V) even at moderately reducing conditions, (2) a decrease in the adsorption affinities of As and F to mineral surfaces, (3) undersaturation with respect to fluorite (CaF2(s)).  相似文献   

4.
The Lovozero pluton (Kola Peninsula, Russia) is an unique object from the standpoint of the abundance, diversity, and originality of Th mineralization. In contrast to other igneous rocks and to such chemical elements as Ca, REE, U, and Na, Th in the hyperalkaline pegmatites and hydrothermalites of the Lovozero pluton commonly occurs as its own mineral phases. Umbozerite Na3Sr4Th(Mn,Zn,Fe,Mg)[Si8O24](OH) (7 samples), Ti-Th silicate Na0–7Sr0–1ThTi1–2Si8O22–23(OH) · nH2O (8 samples), Na-Th silicate (Na,K)4Th3[Si8(O,OH)24] · nH2O (6 samples), thorite (2 samples), steenstrupine-(Ce)-thorosteenstrupine series minerals (5 samples), and Th phosphate (Th,Na,K,Ca,Mg,U,Sr,Ba)[(P,Si, Al)1O4] · nH2O (1 sample) were investigated in this study. Ti-Th silicates and Th phosphate have been described for the first time. All of the above-mentioned minerals have been examined with electron microprobe, IR spectroscopy, powder diffraction, thermogravimetric and optical methods. High-Th minerals such as steenstrupine, umbozerite, Th phosphate, and Na-Th silicates crystallized mainly during the ussingite stage of the pegmatite-forming process. At the early hydrothermal high-alkaline stage, steenstrupine was replaced with REE and Th aggregates (belovite, vitusite, seidite, Na-Th silicates, Ti-Th silicates, etc.). Thorite, Ti-Th silicates, and minerals of the rhabdophane and monazite groups were formed at the late hydrothermal low-alkaline stage. Despite the metamict features of almost all samples, stoichiometric ratios of cations in umbozerites and Ti-Th silicates remain stable. Clear relationships have been revealed between umbozerites and Ti-Th silicates, on the one hand, and seidite-(Ce), a Ti-silicate that has a zeolite-like structure, on the other. This implies that, under certain conditions, these minerals may be regarded as potential suppliers of Th to the environment due to the leaching of Th from zeolite channels.  相似文献   

5.
Zabuye Salt Lake in Tibet, China is a carbonate-type salt lake, which has some unique characteristics that make it different from other types of salt lakes. The lake is at the latter period in its evolution and contains liquid and solid resources. Its brine is rich in Li, B, K and other useful minor elements that are of great economic value. We studied the concentration behavior of these elements and the crystallization paths of salts during isothermal evaporation of brine at 15°C and 25°C. The crystallization sequence of the primary salts from the brine at 25°C is halite (NaCl) → aphthitalite (3K2SO4·Na2SO4) → zabuyelite (Li2CO3)→ trona (Na2CO3·NaHCO3·2H2O) → thermonatrite (Na2CO3·H2O) → sylvite (KCl), while the sequence is halite (NaCl) → sylvite (KCl) → trona (Na2CO3·NaHCO3·2H2O) → zabuyelite (Li2CO3) → thermonatrite (Na2CO3·H2O) → aphthitalite (3K2SO4·Na2SO4) at 15°C. They are in accordance with the metastable phase diagram of the Na+, K+-Cl?, CO32?, SO42?-H2O quinary system at 25°C, except for Na2CO3·7H2O which is replaced by trona and thermonatrite. In the 25°C experiment, zabuyelite (Li2CO3) was precipitated in the early stage because Li2CO3 is supersaturated in the brine at 25°C, in contrast with that at 15°C, it precipitated in the later stage. Potash was precipitated in the middle and late stages in both experiments, while boron was concentrated in the early and middle stages and precipitated in the late stage.  相似文献   

6.
A new mineral depmeierite, the first cancrinite-group member with the species-forming extraframework anion PO 4 3? , has been found at Mt. Karnasurt in the Lovozero alkaline pluton on the Kola Peninsula in Russia. Natrolite and depmeierite are the major components of a hydrothermal peralkaline veinlet 1.5 cm thick, which cross cuts the foyaite-urtite-lujavrite complex. The associated minerals are steenstrupine-(Ce), vuonnemite, epistolite, sodalite, aegirine, serandite, natisite, and vitusite-(Ce). Depmeierite occurs as colorless transparent isometric grains up to 1 cm in size. Its luster is vitreous. The mineral is brittle, and its cleavage (100) is perfect. Its Mohs hardness is 5, and D(meas) = 2.321(1) and D(calc) = 2.313 g/cm3. Depmeierite is optically biaxial positive, ω = 1.493(2), and ? = 1.497(2). The IR spectrum is given. The chemical composition is as follows (wt %, the average of 10 microprobe analyses with the H2O and CO2 determined by selective sorption): 23.04 Na2O, 0.54 K2O, 0.03 Fe2O3, 29.07 Al2O3, 36.48 SiO2, 3.30 P2O5, 0.08 SO3, 0.97 CO2, and 5.93 H2O; the total is 99.44. The empirical formula based on (Si,Al)12O24 is (Na758K0.12)Σ7.70(Si6.19Al5.81O24)[(PO4)0.47(CO3)0.22(OH)0.02(SO4)0.01]Σ0.72 · 3.345H2O. The simplified formula is Na8[Al6Si6O24](CO3)1 ? x · 3H2O (x < 0.05). Depmeierite is hexagonal with space group P63, and the unit-cell dimensions are a = 12.7345(2), c = 5.1798(1), V = 727.46(2) Å3, and Z = 1. The strongest reflections of the X-ray powder pattern (d, Å (I, %) [hkl]) are as follows: 6.380(30) [110], 4.695(91) [101], 3.681(37) [300], 3.250(100) [211], 2.758 (33) [400], 2.596(31) [002], and 2.121(24) [330, 302]. The crystal structure was studied using a single crystal, and R hkl = 0.0362. Depmeierite differs from cancrinite in the development of wide channels containing Na cations, H2O molecules, prevailing PO 4 3? -anionic groups, and CO 3 2? . The mineral is named in honor of the German crystallographer Wulf Depmeier (born in 1944). The type specimen is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences in Moscow. The cancrinite sensu stricto subgroup separated within the cancrinite group comprises six minerals with AB frameworks, the smallest unit cell is (a ≈ 12.55–12.75, c ≈ 5.1–5.4 Å), and the chain […Na…H2O…]∞ exists in narrow channels: cancrinite, vishnevite, cancrisilite, hydroxycancrinite, kyanoxalite, and depmeierite. The P-bearing varieties of the cancrinite-group minerals are discussed, as well as the formation conditions of the noncarbonate members of the group related to intrusive alkaline complexes.  相似文献   

7.
This study formulates a comprehensive depositional model for hydromagnesite–magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite–magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre‐scale) and amalgamated mounds composed primarily of hydromagnesite [Mg5(CO3)4(OH)2·4H2O], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca–Mg‐carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post‐deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg–HCO3 groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg‐carbonate sediments (hydromagnesite mounds) overlies the Ca–Mg‐carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments ‘emerged’ from the water surface. Capillary action and evaporation draw Mg–HCO3 water up towards the ground surface, precipitating Mg‐carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO3·5H2O] which partially cements pre‐existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca–Mg‐carbonate boundary indicates that there has been ca 8000 years of continuous Mg‐carbonate deposition at a rate of 0·4 mm yr?1. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition.  相似文献   

8.
Natrocarbonatite flows in the crater of the volcano Oldoinyo Lengai (Tanzania) are the only carbonatite magmas observed to erupt and have provided strong arguments in favor of a magmatic origin for carbonatite. The currently favored explanation for the genesis of these carbonatites by liquid immiscibility between a silicate and a carbonatite melt is questioned based on the extremely low eruption temperatures of 544-593 °C and compositional and mineralogical characteristics not in agreement with experimental constraints. Experimental investigations of the relationship between Oldoinyo Lengai natrocarbonatite and related silicate rock compositions do indicate that alkali-bearing peralkaline carbonatite with liquidus calcite can form by liquid immiscibility. At the same time, these experiments result in evidence which speaks against a liquid immiscibility origin for the highly alkaline and peralkaline Oldoinyo Lengai natrocarbonatite. On the carbonatite side of the miscibility gap, fractional crystallization cannot account for a liquid evolution from alkali-bearing peralkaline carbonatite to highly alkaline natrocarbonatite. Such an evolution does not seem to be compatible with the liquidus mineral assemblages and the chemistry of Oldoinyo Lengai natrocarbonatite. No natural silicate magma is known to produce natrocarbonatite compositions by liquid immiscibility. The best interpretation of the Oldoinyo Lengai natrocarbonatite flows involves expulsion of a cognate, mobile, alkaline, and CO2-rich fluid condensate. This conclusion is supported by recent studies of silicate and carbonatite melt inclusions in minerals of ultramafic alkaline complexes, trace element partitioning, isotopic constraints, and by experimental data on major element partitioning between coexisting H2O-CO2-rich fluid and carbonatitic melt. In contrast to all other suggested modes of formation, an origin of Oldoinyo Lengai natrocarbonatite from cognate fluid appears best to be in agreement with the field observations, the petrography, mineralogy, and geochemistry of Oldoinyo Lengai natrocarbonatite and the dynamics of the Oldoinyo Lengai natrocarbonatite extrusion.  相似文献   

9.
Lake Kitagata, Uganda, is a hypersaline crater lake with Na–SO4–Cl–HCO3–CO3 chemistry, high pH and relatively small amounts of SiO2. EQL/EVP, a brine evaporation equilibrium model (Risacher and Clement 2001), was used to model the major ion chemistry of the evolving brine and the order and masses of chemically precipitated sediments. Chemical sediments in a 1.6-m-long sediment core from Lake Kitagata occur as primary chemical mud (calcite, magadiite [NaSi7O13(OH)3·3H2O], burkeite [Na6(CO3)(SO4)2]) and as diagenetic intrasediment growths (mirabilite (Na2SO4·10H2O)). Predicted mineral assemblages formed by evaporative concentration were compared with those observed in salt crusts along the shoreline and in the core from the lake center. Most simulations match closely with observed natural assemblages. The dominant inflow water, groundwater, plays a significant role in driving the chemical evolution of Lake Kitagata water and mineral precipitation sequences. Simulated evaporation of Lake Kitagata waters cannot, however, explain the large masses of magadiite found in cores and the formation of burkeite earlier in the evaporation sequence than predicted. The masses and timing of formation of magadiite and burkeite may be explained by past groundwater inflow with higher alkalinity and SiO2 concentrations than exist today.  相似文献   

10.
Nine beryl samples from Western Carpathians, Slovakia, were investigated by infrared and Raman spectroscopy and differential thermal analysis. Two types of water H2O I and H2O II were detected. Infrared spectroscopy proved the presence of water type I and II in the presence of alkali cations with several bands: (1) symmetric stretching vibration—ν1; (2) antisymmetric stretching mode—ν3; (3) bending vibration—ν2. The presence of singly and doubly coordinated type II water (IIs and IId) was confirmed by single-crystal IR spectroscopy. From Raman spectra a band at 3606 cm?1 was assigned to ν1 of water type I and the range of 3597–3600 cm?1 to water type II. The presence of doubly coordinating water indicates a relatively highly hydrated environment with the presence of alkali ions including Na as the dominant cation coordinated by H2O II. CO2 bands were detected only by single-crystal IR spectroscopy. Thermal analysis proved total water loss in the range of 1.4–2.0 wt% and three main dehydration events. Based on the study of bond-topological arrangements two molecules of H2O IId are each bound with two H···O1 bonds and one Na–OW bond with an angular distortion, and by releasing one H2O molecule more stable H2O IIs is produced. The H2O I molecule is bound only by two equivalent hydrogen bonds. The H2O IIs molecule with a Na–OW bond strength of 0.28 vu and two H···O1 bonds of 0.14 vu without any forced angular distortion is the most stable of all.  相似文献   

11.
Alkali carbonates nyerereite, ideally Na2Ca(CO3)2 and gregoryite, ideally Na2CO3, are the major minerals in natrocarbonatite lavas from Oldoinyo Lengai volcano, northern Tanzania. They occur as pheno- and microphenocrysts in groundmass consisting of fluorite and sylvite; nyerereite typically forms prismatic crystals and gregoryite occurs as round, oval crystals. Both minerals are characterized by relatively high contents of various minor elements. Raman spectroscopy data indicate the presence of sulfur and phosphorous as (SO4)2− and (PO4)3− groups. Microprobe analyses show variable composition of both nyerereite and gregoryite. Nyerereite contains 6.1–8.7 wt % K2O, with subordinate amounts of SrO (1.7–3.3 wt %), BaO (0.3–1.6 wt %), SO3 (0.8–1.5 wt %), P2O5 (0.2–0.8 wt %) and Cl (0.1–0.35 wt %). Gregoryite contains 5.0–11.9 wt % CaO, 3.4–5.8 wt % SO3, 1.3–4.6 wt % P2O5, 0.6–1.0 wt % SrO, 0.1–0.6 wt % BaO and 0.3–0.7 wt % Cl. The content of F is below detection limits in nyerereite and gregoryite. Laser ablation ICP-MS analyses show that REE, Mn, Mg, Rb and Li are typical trace elements in these minerals. Nyerereite is enriched in REE (up to 1080 ppm) and Rb (up to 140 ppm), while gregoryite contains more Mg (up to 367 ppm) and Li (up to 241 ppm) as compared with nyerereite.  相似文献   

12.
We report on the crystallization behavior and the salt weathering potential of Na2SO4, MgSO4 and an equimolar mixture of these salts in natural rock and porous stone. Geochemical modeling of the phase diagram of the ternary Na2SO4–MgSO4–H2O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Model calculations include stable and metastable solubilities of the various hydrated states of the single salts and the double salts Na2Mg(SO4)2·4H2O (bloedite), Na2Mg(SO4)2·5H2O (konyaite), Na12Mg7(SO4)13·15H2O (loeweite) and Na6Mg(SO4)4 (vanthoffite). In situ Raman spectroscopy was used to study the phase transformations during wetting of pure MgSO4·H2O (kieserite) and of the incongruently soluble salts bloedite and konyaite. Dissolution of kieserite leads to high supersaturation resulting in crystallization of higher hydrated phases, i.e. MgSO4·7H2O (epsomite) and MgSO4·6H2O (hexahydrite). This confirms the high damage potential of magnesium sulfate in salt damage of building materials. The dissolution of the incongruently soluble double salts leads to supersaturation with respect to Na2SO4·10H2O (mirabilite). However, the supersaturation was insufficient for mirabilite nucleation. The damage potential of the two single salts and an equimolar salt mixture was tested in wetting–drying experiments with porous sandstone. While the high damage potential of the single salts is confirmed, it appears that the supersaturation achieved during wetting of the double salts at room temperature is not sufficient to generate high crystallization pressures. In contrast, very high damage potentials of the double salts were found in experiments at low temperature under high salt load.1  相似文献   

13.
Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions (2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich and agpaitic (molecular Na2O + K2O/Al2O3 > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites of Oldoinyo Lengai. Received: 18 January 1996 / Accepted: 2 September 1996  相似文献   

14.
The aim of this study is the synthesis of CuSeO3·2H2O (chalcomenite analog), ZnSeO3·2H2O, and ZnSeO3·H2O and the investigation of their solubility in water. CuSeO3·2H2O has been synthesized from solutions of Cu nitrate and Na selenite, while Zn selenites were synthesized from solutions of Zn nitrate and Na selenite. The samples obtained have been examined with X-ray diffraction and infrared and Raman spectroscopy. The solubility has been determined using the isothermal saturation method in ampoules at 25°C. The solubility has been calculated using the Geochemist’s Workbench (GMB 9.0) software package. Solubility products have been calculated for CuSeO3·2H2O (10–10.63), ZnSeO3·2H2O (10–8.35), and ZnSeO3·H2O (10–7.96). The database used comprises thermodynamic characteristics of 46 elements, 47 base particles, 48 redox pairs, 551 particles in solution, and 624 solid phases. The Eh–pH diagrams of the Zn–Se–H2O and Cu–Se–H2O systems were plotted for the average contents of these elements in underground water in oxidation zones of sulfide deposits.  相似文献   

15.
Products and kinetics of ion exchange of heterophyllosilicate minerals lomonosovite and murmanite with aqueous AgNO3 solutions under low-temperature conditions have been studied using scanning electron microscopy, electron microprobe analysis, single-crystal X-ray diffraction, infrared spectroscopy, 23Na nuclear magnetic resonance spectroscopy and dynamic calorimetry. Both minerals show strong affinity for silver in cation exchange. Simplified formulae of Ag-exchanged forms of murmanite and lomonosovite are (Ag3.0Ca0.5Na0.5) (Ti,Nb,Mn,Fe)3.7?4 (Si2O7)2O4·4(H2O,OH) and (Ag8.2Na1.2Ca0.3) (Ti,Nb,Mn,Fe)3.9?4 (Si2O7)2 (PO4)1.9O4·xH2O, respectively. The reaction of ion exchange for murmanite follows the first-order kinetic model up to ca. 70–80 % conversion. The rate of the process is described by the equation k(h?1) = 107.64±0.60 exp[?(12.2 ± 0.9)·103/RT]. The average heat release value in the temperature range 39.4–72 °C is 230 J g?1. The cation exchange is limited by processes in solid state, most probably binding of silver.  相似文献   

16.

Background  

This study provides experimental evidence for biologically induced precipitation of magnesium carbonates, specifically dypingite (Mg5(CO3)4(OH)2·5H2O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia. This wetland is part of a larger hydromagnesite (Mg5(CO3)4(OH)2·4H2O) playa. Abiotic and biotic processes for magnesium carbonate precipitation in this environment are compared.  相似文献   

17.
The prevailing theory for the formation of trona [Na3(CO3)(HCO3) · 2(H2O)] relies on evaporative concentration of water produced by silicate hydrolysis of volcanic rock or volcaniclastic sediments. Given the abundance of closed drainage basins dominated by volcanics, it is puzzling that there are so few trona deposits and present-day lakes that would yield dominantly Na–CO3 minerals upon evaporation. Groundwater in the San Bernardino Basin (southeastern Arizona, USA and northeastern Sonora, Mexico) would yield mainly Na–CO3 minerals upon evaporation, but waters in the surrounding basins would not. Analysis of the chemical evolution of this groundwater shows that the critical difference from the surrounding basins is not lithology, but the injection of magmatic CO2. Many major deposits of trona and Na–CO3-type lakes appear to have had “excess” CO2 input, either from magmatic sources or from the decay of organic matter. It is proposed that, along with the presence of volcanics, addition of “excess” CO2 is an important pre-condition for the formation of trona deposits.  相似文献   

18.
This paper summarizes a new outlook on the conceptual model of Melgaço–Messegães CO2-rich cold (≈18 °C) mineral water systems, issuing in N of Portugal, based on their isotopic (2H, 3H, 13C, 14C and 18O) and geochemical features. Stable isotopes indicate the meteoric origin of these CO2-rich mineral waters. Based on the isotopic fractionation with the altitude, a recharge altitude between 513 up to 740 m a.s.l. was estimated, corroborating the tritium results. The lowest 3H content (0 TU) is found in the groundwater samples with the highest mineralization. The mineral waters circulation are mainly related to a granitic and granodioritic environment inducing two different groundwater types (Ca/Na–HCO3 and Na/Ca–HCO3), indicating different underground flow paths. Calcium dissolution is controlled by hydrolysis of rock-matrix silicate minerals (e.g. Ca-plagioclases) and not associated to anthropogenic sources. The shallow dilute groundwaters exhibit signatures of anthropogenic origins (e.g. NO3) and higher Na/Ca ratios. The stable isotopes together with the geochemistry provided no indication of mixing between the regional shallow cold dilute groundwater and mineral water systems. The heavy isotopic signatures identified in the δ13C data (δ13C = 4.7 ‰, performed on the total dissolved inorganic carbon (TDIC) of CO2-rich mineral waters) could be derived from a deep-seated (upper mantle) source or associated to methanogenesis (CH4 source). The negligible 14C content (≈2 pmC) determined in the TDIC of the mineral waters, corroborates the hypothesis of a mantle-derived carbon source to the mineral groundwater systems or dissolution of carbonate layers at depth.  相似文献   

19.
Orogenic gold mineralization at the Damang deposit, Ghana, is associated with hydrothermal alteration haloes around gold‐bearing quartz veins, produced by the infiltration of a H2O–CO2–K2O–H2S fluid following regional metamorphism. Alteration assemblages are controlled by the protoliths with sedimentary rocks developing a typical assemblage of muscovite, ankerite and pyrite, while intrusive dolerite bodies contain biotite, ankerite and pyrrhotite, accompanied by the destruction of hornblende. Mineral equilibria modelling was undertaken with the computer program thermocalc , in subsets of the model system MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–CO2–H2O–TiO2–Fe2O3, to constrain conditions of regional metamorphism and the subsequent gold mineralization event. Metapelites with well‐developed amphibolite facies assemblages reliably constrain peak regional metamorphism at ~595 °C and 5.5 kbar. Observed hydrothermal alteration assemblages associated with gold mineralization in a wide compositional range of lithologies are typically calculated to be stable within P–T–X(CO2) arrays that trend towards lower temperatures and pressures with increasing equilibrium fluid X(CO2). These independent P–T–X(CO2) arrays converge and the region of overlap at ~375–425 °C and 1–2 kbar is taken to represent the conditions of alteration approaching equilibrium with a common infiltrating fluid with an X(CO2) of ~0.7. Fluid‐rock interaction calculations with M–X(CO2) diagrams indicate that the observed alteration assemblages are consistent with the addition of a single fluid phase requiring minimum fluid/rock ratios on the order of 1.  相似文献   

20.
Albrechtschraufite, MgCa4F2[UO2(CO3)3]2?17-18H2O, triclinic, space group Pī, a?=?13.569(2), b?=?13.419(2), c?=?11.622(2) Å, α?=?115.82(1), β?=?107.61(1), γ?=?92.84(1)° (structural unit cell, not reduced), V?=?1774.6(5) Å3, Z?=?2, D c?=?2.69 g/cm3 (for 17.5 H2O), is a mineral that was found in small amounts with schröckingerite, NaCa3F[UO2(CO3)3](SO4)?10H2O, on a museum specimen of uranium ore from Joachimsthal (Jáchymov), Czech Republic. The mineral forms small grain-like subhedral crystals (≤ 0.2 mm) that resemble in appearance liebigite, Ca2[UO2(CO3)3]??~?11H2O. Colour pale yellow-green, luster vitreous, transparent, pale bluish green fluorescence under ultraviolet light. Optical data: Biaxial negative, nX?=?1.511(2), nY?=?1.550(2), nZ?=?1.566(2), 2?V?=?65(1)° (λ?=?589 nm), r < v weak. After qualitative tests had shown the presence of Ca, U, Mg, CO2 and H2O, the chemical formula was determined by a crystal structure analysis based on X-ray four-circle diffractometer data. The structure was later on refined with data from a CCD diffractometer to R1?=?0.0206 and wR2?=?0.0429 for 9,236 independent observed reflections. The crystal structure contains two independent [UO2(CO3)3]4? anions of which one is bonded to two Mg and six Ca while the second is bonded to only one Mg and three Ca. Magnesium forms a MgF2(Ocarbonate)3(H2O) octahedron that is linked via the F atoms with three Ca atoms so as to provide each F atom with a flat pyramidal coordination by one Mg and two Ca. Calcium is 7- and 8-coordinate forming CaFO6, CaF2O2(H2O)4, CaFO3(H2O)4 and CaO2(H2O)6 coordination polyhedra. The crystal structure is built up from MgCa3F2[UO2(CO3)3]?8H2O layers parallel to (001) which are linked by Ca[UO2(CO3)3]?5H2O moieties into a framework of the composition MgCa4F2[UO2(CO3)3]?13H2O. Five additional water molecules are located in voids of the framework and show large displacement parameters. One of the water positions is partly vacant, leading to a total water content of 17-18H2O per formula unit. The MgCa3F2[UO2(CO3)3]?8H2O layers are pseudosymmetric according to plane group symmetry cmm. The remaining constituents do not sustain this pseudosymmetry and make the entire structure truly triclinic. A characteristic paddle-wheel motif Ca[UO2(CO3)3]4Ca relates the structure of albrechtschraufite partly to that of andersonite and two synthetic alkali calcium uranyl tricarbonates.  相似文献   

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