Dilute Waters Research Articles

Quantitative interpretation of chemical characteristics of hydrothermal systems

The paper gives a brief review of chemical reactions and equilibria which occur in hydrothermal Experimental work, reacting rocks and water at high temperature, has shown that it is not always necessary to postulate a magmatic fluid origin for chemicals in natural hot waters. Only a few elements in waters, such as Cl, B, Cs, are not involved to any great extent in temperature and pressure dependent chemical equilibria with rock minerals. These elements are useful in identifying and following flows of particular waters within a field. From knowledge of acid-base equilibria, the pH of hot water at deep levels can be obtained, and also the variation in pH with time during drillhole production. Combined with metal ion concentrations, the pH enables an interpretation in terms of water chemistry of mineral alteration assemblages, including the tendency for calcite formation. Phase diagrams, using ratios αN a/α H, α x/α H, α Ca/α 2 H, α M g /α 2 H, etc., as parameters show how mineral assemblages in hydrothermally altered rock change as waters cool and lose carbon dioxide on approaching the surface. As equilibrium exists between hot water and rock minerals at deep levels, the silica concentrations and Na/K ratios in waters flowing rapidly to the surface given an accurate measure of the deep temperature. Also, the pH of deep hot water is a function of the water salinity, except in areas where waters contact sulfur deposits. It is shown that calcium concentration in thermal waters, for a given temperature, is approximately proportional to the square of sodium ion concentrations. Estimates of m CO2 in underground waters can be made through knowledge of any two of the variables, salinity, calcium concentration, and bicarbonate concentration in surface discharges. The concentration of carbon dioxide in deep hot water is the factor determining whether or not waters will deposit calcite in drillpipes. With increasing m CO2, calcium carbonate replaces minerals such as wairakite and epidote as the equilibrium calcium phase in the rock. An outline is given of isotope exchange equilibria which can be used for determining temperatures at depth in a field. Oxygen and sulfur isotope exchange, both between solutes, and between solutes and minerals, are the most promising in this respect. Base metal sulfide deposits are not limited to hydrothermal areas of highly saline waters. Minerals such as galena, sphalerite, and chalcopyrite are found at deep levels in the Broadlands geothermal field of dilute chloride waters, while at the surface in the area precipitates of antimony and arsenic sulfides rich in gold, silver, thallium, and mercury are formed from spring and drillhole waters.

Ion association in natural brines

Natural brines, both surface and subsurface, are highly associated aqueous solutions. Ion complexes in brines may be ion pairs in which the cation remains fully hydrated and the bond between the ions is essentially electrostatic, or coordination complexes in which one or more of the hydration water molecules are replaced by covalent bonds to the anion. Except for Cl −, the major simple ions in natural brines form ion pairs; trace and minor metals in brines form mainly coordination complexes. Limitations of the Debye-Hückel relations for activity coefficients and lack of data on definition and stability of all associated species in concentrated solutions tend to produce underestimates of the degree of ion association, except where the brines contain a very high proportion of Cl −. Data and calculations on closed basin brines of highly varied composition have been coupled with electrode measurements of single-ion activities in an attempt to quantify the degree of ion association. Such data emphasize the role of magnesium complexes. Trace metal contents of closed basin brines are related to complexes formed with major anions. Alkaline sulfo- or chlorocarbonate brines (western Great Basin) carry significant trace metal contents apparently as hydroxides or hydroxy polyions. Neutral high chloride brines (Bonneville Basin) are generally deficient in trace metals. With a knowledge of the thermodynamic properties of a natural water, many possible reactions with other phases (solids, gases, other liquids) may be predicted. A knowledge of these reactions is particularly important in the study of natural brines which may be saturated with many solid phases (silicates, carbonates, sulfates, etc.), which may have a high pH and bring about dissolution of other phases (silica, amphoteric hydroxides, CO 2, etc.), and which because of their high density may form relatively stable interfaces with dilute waters.

Evaporite Deposition from Layered Solutions

It is widely accepted that carbonates, especially those which originate as banks or reefs, and the evaporites with which they are closely associated represent mutually exclusive, successive phases of deposition: (1) a stage of carbonate bank or reef deposition under conditions of normal marine circulation, followed by (2) a time of sulfate deposition under conditions of restricted circulation, (3) an episode of further restriction leading to chloride precipitation, and, in some instances, ending in (4) an ultimate stage of bittern-salt accumulation. A multistage history of carbonate-evaporite deposition requires paleogeographic interpretations that appear bizarre in view of the epeiric setting of most deposits. An alternative is sought in deposition from density-layered waters with an upper layer of nearly seawater salinity and a lower layer or layers saturated with respect to one or more evaporite salts. Brine is supplied to such a system by evaporation on shelf areas surrounding a basin, and saturation is maintained by contact with salts on the basin floor. In this manner layered solutions persist for geologically significant spans of time, in spite of diffusion and mixing, while carbonate deposition continues in the upper dilute waters. Episodic exposure of the lower brine to evaporation is required for evaporite deposition. S ch exposure may be the result of displacement of the upper layer by wind, or may accompany long-period, high-amplitude, internal seiches. Layered systems and the effects of winds and internal seiches are demonstrable in present-day saline lakes; the processes involved are amenable to laboratory investigation.

Hyperfiltration studies XIII. Parametric study of hyperfiltration in tubular systems with high permeability membranes ( 1)

A parametric study of hyperfiltration was carried out which emphasizes high-flux membranes. Relative costs have been computed at optimum flow conditions by use of a Fortran computer program (“HYPFIL”) which deals with a single-stage hyperfiltration system with tubular membranes under turbulent flow conditions. The parametric study dealt largely with the following variables: membrane permeability and rejection, feed concentration (restricted to relatively dilute brackish waters), plant geometry, temperature, and economic parameters. High permeability membranes can lead to low cost water, even if salt rejection of the membrane is relatively modest, provided the intrinsic rejection of the membrane is well above the minimum theoretically required. High-temperature operation may lead to substantially lower hyperfiltration treatment costs. Use of operating pressures below optimum pressures (often required by membrane characteristics) causes a cost penalty: one advantage of high-flux membranes is that they could permit matching of membrane characteristics to optimum pressure operation.

The concentrations of caesium-137 and strontium-90 in the flesh of brown trout taken from rivers and lakes in the British isles between 1961 and 1966: the variables determining the concentrations and their use in radiological assessments

Concentrations of caesium-137 and strontium-90 in the flesh of brown trout are reported from two complementary studies: a detailed study over a six-year period of Lake Trawsfynydd (Merioneth, North Wales), comparing quantities of radioactive fallout deposited and water and flesh concentrations for both radionuclides, and general surveys of waters throughout the British Isles. From the British Isles surveys caesium-137 concentrations are related to potassium concentrations in the water and strontium-90 concentrations to calcium concentrations in the water, both as specific activities and as concentration factors, for the whole range of conditions from very dilute calcium bicarbonate surface waters to the marine conditions of coastal waters. It is demonstrated that concentrations of caesium-137 in brown trout flesh from some Scottish waters were probably as high as any recorded from arctic ecosystems. Concentrations of both contaminants in trout flesh can be expressed as functions of water conductivity in calcium bicarbonate surface waters from Britain, and it is suggested that this may be a useful general approach to radiological assessments involving fish from many such surface waters. The radiological implications of the principal conclusions are discussed and simple “risk diagrams” are produced as a guide to the assessment of different situations. It is shown that strontium-90, at or below I.C.R.P. public drinking water concentrations, is unlikely to be a hazard as a result of the human consumption of trout from any area in the British Isles, but that caesium-137 is a potential hazard in some areas down to consumption rates as low as 0.4 g of flesh per day. The results of previous work with these two radionuclides in fresh-water ecosystems are briefly discussed in the context of the studies reported.

UNUSUAL MONOMIXIS IN TWO SALINE ARIZONA PONDS1

Red Pond and Green Pond are shallow, saline pools in Apache County, Arizona, that stratify chemically during winter and circulate in midsummer. Stability is brought about by addition of dilute runoff and seep water to the surfaces; circulation occurs because evaporation increases the salinity of the upper waters and lowers the pond surfaces. Isothermy, dichothermy, mesothermy, and poikilotherny occur during the year. Winter stabilities are in excess of 200 g‐cm/cm2 in spite of anomalous temperature profiles and the shallow nature of the ponds.The waters are chloro‐carbonates derived by concentration from dilute waters relatively high in sulfate and carbonate. At the overturn, Red Pond and Green Pond have salinities of 22% and 11%. Phosphate concentrations are remarkably high—up to 500 mg/liter in Red Pond.Ctenocladus circinnatus is present in both ponds; this may be the fourth North American record for this green alga.

Role of Membrane Hyperfiltration on Origin of Thermal Brines, Imperial Valley, California: ABSTRACT

Unique saturated Na-Ca-K Cl thermal brines (380°C.) are recovered from geothermal wells near the Salton Sea, Imperial Valley, California. The reservoir chamber is fractured metamorphic rock (mainly greenschist facies) at 3,900-8,000 feet. The shallow waters are relatively dilute NaHCO3-Cl, high in B; NH4, I, and F are present; the Na/K ratio is less than in the brines; CO2 gas is abundant. The waters of these brines are dominantly meteoric, as evidenced by End_Page 454------------------------------ the previously reported deuterium content of the brines and local surficial waters. The most critical geochemical questions concern the mechanism for brine concentration, the origin of the Cl ion, and the surprisingly high Ca/Mg, K/Na, and Ca/K ratios of the brine. The arkosic sedimentary fill of the Imperial Valley graben contains ample material to provide by solution every chemical in the brines except for the Cl ion. Models of the brine column versus original material contained in the encompassing rock column suggest a deficiency of Cl ion. A high degree of interchange between the host rock and the thermal waters exists, as evidence by previously reported data (18O and B content) and the similarity of the Rb/K ratios of the brines to those of arkosic materials. The Cl ion must be either juvenile in origin, or a result of the concentration of meteoric interstitial water of the sedimentary fill, or derived from Cl absorbed onto silicate surfaces. The similarity of the Br/Cl ratio of the brines to local meteoric surface and ground waters, and its complete dissimilarity to those of Cl evaporites, suggest that the brines are dominantly interstitial meteoric water concentrated manyfold. Doubtless some of these brine halogens also were obtained by desorption of absorbed material at high temperatures. Hyperfiltration of relatively dilute hydrothermal solutions through electrostatic semi-permeable membranes composed of abundant montmorillonitic and illitic clays in the sedimentary fill provides the best mechanism for concentrating the brines within the proposed thermal convection cell and of affecting the relative composition of the brine and the relatively dilute waters underlying the thermal anomaly. In particular, this mechanism best explains the Ca/Na ratios of the brine; the relative abundance of Sr, Ca, and Mg within the brines possibly may be a result of this mechanism; the increase of HCO3/Cl, F/Cl, and B/Cl ratios in the dilute overlying waters, which would be effluent to the proposed membrane system, probably is a result of such hyperfiltration. High-temperature metastable equilibria between the thermal brine and its enclosing rocks strongly affect the specific composition of the brine. Such reactions probably control completely the trace-element metal content of the brines. The relative abundance of the alkali-metals appears to be strongly influenced by such rock-water reactions as well as by relative hyperfiltration. Experimental investigations are needed to understand further the origin of these waters. End_of_Article - Last_Page 455------------

Proposed Origin of Subsurface Thermal Brines, Imperial Valley, California: ABSTRACT

depth) of unique chemistry (reported by D. E. White) are recovered by geothermal wells near the Salton Sea in the Imperial Valley--a tectonically active graben area of high heat flow at the north end of the Gulf of California. The reservoir chamber consists of alternating fractured greenschist and zeolitic facies metamorphic rocks at depths of 3,900-8,000 feet. The shallow waters adjacent to and overlying this and many other thermal anomalies are dilute NaHCO3-Cl waters, high in B. NH4, I, and F are present; the Na/K ratio is less than in the brines. CO2 is abundant. The similarity of the deuterium content of these brines and various surficial waters of the Imperial Valley as determined by H. Craig and reported by White indicates that the waters of these brines are dominantly meteoric. The most critical geochemical questions concern the mechanism by which the brines are concentrated to such a high degree, the origin of the Cl ion within the brine, and the surprisingly high Ca/Na, K/Na, and Cs/K ratios of the brine. The arkosic sedimentary fill of the graben contains ample material to provide by solution every chemical found within these thermal brines with the exception of the Cl ion. The high 18O of the brines and its impoverishment End_Page 644------------------------------ in the reservoir rocks (Craig via White), the impoverishment of B in the reservoir rocks (White), and the similarity of the Rb/K ratios of the brines to those of arkosic materials indicate the high degree of interchange between the host rock and the thermal waters. The Cl ion either must be introduced at depth as juvenile Cl transported solely by diffusion in the gaseous phase from a magmatic source or must result simply from the concentration of meteoric interstitial water of the sedimentary fill. Strong evidence suggests that no Cl-evaporites are present at depth in the graben. The similarity of the Br/Cl ratio of the thermal brines to all of the meteoric surface and ground waters of the Imperial Valley area (Chevron Research data) and its complete dissimilarity to ratios found within Cl-evaporites suggest that the brines are merely the meteoric water of the graben fill concentrated many-fold. No exotic source is needed. Hyperfiltration of relatively dilute hydrothermal solutions through electrostatic semi-permeable membranes, composed of abundant montmorillonitic and illitic clays in the sedimentary fill, and probable zeolites overlying and laterally bounding the thermal anomaly, provides the best mechanism for concentrating the brines as well as determining their relative composition and that of the surface effluent waters overlying the thermal anomaly. Such high concentrations could be achieved only by a very large volumetric transfer of dilute hydrothermal waters through the membrane material due to the progressive decrease of hyperfiltration efficiency of semi-permeable membranes with increasing concentrations. Relative hyperfiltration of Ca with respect to Na and the relative increase of B, NH4, F, I, and HCO3 in solutions effluent from membranes has been observed by White in subsurface waters at lower temperatures. The relative increase of K/Na and Cs/K by selective membrane transport in a hyperfiltrated solution is consistent with the known behavior of solutions through ion-exchange columns where the smaller hydrated ion is adsorbed preferentially in the double layer, thereby permitting preferential membra e transport for the larger and less hydrated ion. A steadily expanding dome-shaped zone of brittle, fractured rocks metamorphosed by the hydrothermal solutions ascending by convective transport from a high heat source at depth, presumably a silica melt, and surrounded by relatively unmetamorphosed membrane materials (zeolites and clays) is assumed as a model. Hyperfiltration would occur within the dome by passage of solutions through the bordering membrane materials. Brines whose composition would have increased steadily through time until reaching an equilibrium would be found in the dome within which a convection cell characterized by channel flow should exist. Relatively dilute effluent solutions of a particular chemistry would emerge continuously from the membrane material to form the dilute shallow waters of specific chemical co position that typically occur near the surface at the Salton Sea and other thermal anomalies. Occasional fractures would permit leakage of the concentrated brine outward from the dome where it would mix with effluent waters. Meteoric interstitial water of the sedimentary fill would mix with the membrane-effluent and leakage waters on the borders of this hydrochemical system. End_of_Article - Last_Page 645------------

The measurements of total carbon dioxide in dilute tropical waters

A shipboard conductivity technique to measure the total carbon dioxide in dilute tropical water is described. The standard error of a single determination in the range 40-100 mg CO2/I is 6.0 mg/l. There were 41-102 mg CO2/l in waters of the Gulf of Thailand. In these waters, primary production determined with the usually assumed value of 90 mg CO2 gave errors ranging from 11 to 120% for the individual depth samples and from 6 to 92 % for column values as compared to the values obtained using the measured CO2 concentration.

The Chemical Composition of Some Waters from Lowland Lakes in Shropshire, England

Analyses are presented for p H, Na, K, Ca, Mg, HCO 3 , Cl, SO 4 , NO 3 , PO 4 and SiO 2 in surface waters from eight Shropshire meres subject to dense water blooms of blue-green algae. Samples were taken in November 1954 and June 1955. It appears likely that the chemistry of the waters reflects increasing alteration of rain water composition by solution of the products of rock and soil weathering, chiefly calcium bicarbonate. Magnesium was the only major ion observed to vary greatly with season of sampling. Potassium was anomalous in being more abundant in the dilute waters, where it accounted for an unusually high proportion of total salts. An attempt has been made to relate the chemical composition of the various waters to local geological and physiographical variations. DOI: 10.1111/j.2153-3490.1957.tb01870.x

Studies on the Quality of Fresh Waters of Puerto Rico Relative to the Occurrence of Australorbis Glabratus (Say)

Summary In Puerto Rico, Australorbis glabratus occurs throughout the range of pH and temperature found. It occurs in the maximum concentrations of CO3, HCO3, Cl, SO4, Ca and Mg found in fresh waters here. It is generally but not invariably absent from situations normally having less than 150 ppm. dissolved solids. Yet experimentally, the snail can be maintained for at least two weeks in redistilled water. Small amounts (about 0.1 ppm) of copper in distilled water produce distinct, incapacitating distress in the snails within a few hours time. Copper concentrations in natural waters range from 0.000 to 0.330 ppm. Most concentrations of copper greater than 0.020 occur in waters of less than 150 ppm. It is suggested that copper, and perhaps other heavy metals, may be the factors responsible for the absence of this snail in most dilute waters. A large excess of weak acid radicals (CO3 and HCO3) over strong acid radicals (Cl and SO4) is suggested as related to the absence of this snail in flowing waters on limestone.

The Chemical Composition of Some Waters from Lowland Lakes in Shropshire, England

Analyses are presented for pH, Na, K, Ca, Mg, HCO3, Cl, SO4, NO3, PO4 and SiO2 in surface waters from eight Shropshire meres subject to dense water blooms of blue-green algae. Samples were taken in November 1954 and June 1955. It appears likely that the chemistry of the waters reflects increasing alteration of rain water composition by solution of the products of rock and soil weathering, chiefly calcium bicarbonate. Magnesium was the only major ion observed to vary greatly with season of sampling. Potassium was anomalous in being more abundant in the dilute waters, where it accounted for an unusually high proportion of total salts. An attempt has been made to relate the chemical composition of the various waters to local geological and physiographical variations.

On some factors affecting the chemical composition of Swedish fresh waters

An attempt is made to relate variations in the chemical composition of Swedish fresh waters (mainly analyzed by Lohammar, 1938) to topographical, geochemical, and biological factors. The following are among the more general conclusions: 1. (1) In Uppland lake waters, and certain of those in Dalarna, total salt concentration is clearly related to altitude. 2. (2) There are distinct differences in ionic proportions between richer and poorer lake waters in both Dalarna and Uppland. In Dalarna the preponderance of moraine or Dalălven river sediments appears to be the important factor. In Uppland the high proportions of Cl, Na, and Mg in the waters of the lowest and richest lakes bear witness to the marine submergence of the district at the end of the glacial period. 3. (3) A comparison of Dalarna drainage and seepage lakes reveals that the waters of the latter are proportionally very rich in K but low in Ca. It is suggested that this may be due to a lesser degree of biotic influence on the waters reaching the seepage lakes. 4. (4) Dalarna seepage lakes exhibit low SO 4 levels in their waters, which may be connected with a slow rate of water transfer through them. 5. (5) The cations of bog waters are dominated by Na, those of fen waters by Ca. This illustrates the relative importance of atmospheric precipitation in the former, and mineral soil leaching in the latter. 6. (6) The frequency distributions of dissolved plus particulate minor constituents in fresh waters may be far from normal, low values predominating. Medians are therefore desirable in place of averages. 7. (7) Fe is most abundant in the more dilute waters, especially in Norrland. Mn is higher in the richer waters within each district, except in the Siljan area of limestone, where the lowest median occurs. Both elements tend to be highest (in respect of surface waters) in the shallower lakes. 8. (8) P is lowest in the Siljan limestone lakes, and highest in the Dalarna seepage lakes. 9. (9) N is lowest in the most dilute waters, Norrland being particularly low, and higher in the richer waters within each district (except in the Siljan area). 10. (10) Sr, which like the major constituents is highly correlated with total ion concentration, forms a slightly larger part of the total ions in the more dilute waters, especially in Norrland.

THE KINETICS OF PENETRATION

The rate of entrance of water into impaled cells of Halicystis Osterhoutii, Blinks and Blinks, has been determined directly by measurements of the rise of sap in a capillary for dilute sea waters (containing between 90 and 30 per cent sea water). The velocity constant remains reasonably constant down to 50 per cent sea water but it decreases markedly in lower concentrations.

A piezo electric method for the instantaneous measurement of high pressures

The solubility of silver sulphide (acanthite/argentite) has been measured in aqueous sulphide solutions between 25 and 400°C at saturated water vapour pressure and 500 bar to determine the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave, connected to a high-performance liquid chromatographic pump, titanium sampling loop, and a back-pressure regulator on line. Samples for silver determination were collected via the titanium sampling loop at experimental temperatures and pressures. The solubilities, measured as total dissolved silver, were in the range 1.0 × 10−7 to 1.30 × 10−4 mol kg−1 (0.01 to 14.0 ppm), in solutions of total reduced sulphur between 0.007 and 0.176 mol kg−1 and pHT,p of 3.7 to 12.7. A nonlinear least squares treatment of the data demonstrates that the solubility of silver sulphide in aqueous sulphide solutions of acidic to alkaline pH is accurately described by the reactions0.5Ag2S(s) + 0.5H2S(aq) = AgHS(aq) Ks,1110.5Ag2S(s) + 0.5H2S(aq) + HS− = Ag(HS)2− Ks,122Ag2S(s) + 2HS− = Ag2S(HS)22− Ks,232where AgHS(aq) is the dominant species in acidic solutions, Ag(HS)2− under neutral pH conditions and Ag2S(HS)22− in alkaline solutions. With increasing temperature the stability field of Ag(HS)2− increases and shifts to more alkaline pH in accordance with the change in the first ionisation constant of H2S(aq). Consequently, Ag2S(HS)22− is not an important species above 200°C. The solubility constant for the first reaction is independent of temperature to 300°C, with values in the range logKs,111 = −5.79 (±0.07) to −5.59 (±0.09), and decreases to −5.92 (±0.16) at 400°C. The solubility constant for the second reaction increases almost linearly with inverse temperature from logKs,122 = −3.97 (±0.04) at 25°C to −1.89 (±0.03) at 400°C. The solubility constant for the third reaction increases with temperature from logKs,232 = −4.78 (±0.04) at 25°C to −4.57 (±0.18) at 200°C. All solubility constants were found to be independent of pressure within experimental uncertainties. The interaction between Ag+ and HS− at 25°C and 1 bar to form AgHS(aq) has appreciable covalent character, as reflected in the exothermic enthalpy and small entropy of formation. With increasing temperature, the stepwise formation reactions become progressively more endothermic and are accompanied by large positive entropies, indicating greater electrostatic interaction. The aqueous speciation of silver is very sensitive to fluid composition and temperature. Below 100°C silver(I) sulphide complexes predominate in reduced sulphide solutions, whereas Ag+ and AgClOH− are the dominant species in oxidised waters. In high-temperature hydrothermal solutions of seawater salinity, chloride complexes of silver(I) are most important, whereas in dilute hydrothermal fluids of meteoric origin typically found in active geothermal systems, sulphide complexes predominate. Adiabatic boiling of dilute and saline geothermal waters leads to precipitation of silver sulphide and removal of silver from solution. Conductive cooling has insignificant effects on silver mobility in dilute fluids, whereas it leads to quantitative loss of silver for geothermal fluids of seawater salinity.

The decarburization of ferro-chromium by hydrogen

The solubility of silver sulphide (acanthite/argentite) has been measured in aqueous sulphide solutions between 25 and 400°C at saturated water vapour pressure and 500 bar to determine the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave, connected to a high-performance liquid chromatographic pump, titanium sampling loop, and a back-pressure regulator on line. Samples for silver determination were collected via the titanium sampling loop at experimental temperatures and pressures. The solubilities, measured as total dissolved silver, were in the range 1.0 × 10−7 to 1.30 × 10−4 mol kg−1 (0.01 to 14.0 ppm), in solutions of total reduced sulphur between 0.007 and 0.176 mol kg−1 and pHT,p of 3.7 to 12.7. A nonlinear least squares treatment of the data demonstrates that the solubility of silver sulphide in aqueous sulphide solutions of acidic to alkaline pH is accurately described by the reactions0.5Ag2S(s) + 0.5H2S(aq) = AgHS(aq) Ks,1110.5Ag2S(s) + 0.5H2S(aq) + HS− = Ag(HS)2− Ks,122Ag2S(s) + 2HS− = Ag2S(HS)22− Ks,232where AgHS(aq) is the dominant species in acidic solutions, Ag(HS)2− under neutral pH conditions and Ag2S(HS)22− in alkaline solutions. With increasing temperature the stability field of Ag(HS)2− increases and shifts to more alkaline pH in accordance with the change in the first ionisation constant of H2S(aq). Consequently, Ag2S(HS)22− is not an important species above 200°C. The solubility constant for the first reaction is independent of temperature to 300°C, with values in the range logKs,111 = −5.79 (±0.07) to −5.59 (±0.09), and decreases to −5.92 (±0.16) at 400°C. The solubility constant for the second reaction increases almost linearly with inverse temperature from logKs,122 = −3.97 (±0.04) at 25°C to −1.89 (±0.03) at 400°C. The solubility constant for the third reaction increases with temperature from logKs,232 = −4.78 (±0.04) at 25°C to −4.57 (±0.18) at 200°C. All solubility constants were found to be independent of pressure within experimental uncertainties. The interaction between Ag+ and HS− at 25°C and 1 bar to form AgHS(aq) has appreciable covalent character, as reflected in the exothermic enthalpy and small entropy of formation. With increasing temperature, the stepwise formation reactions become progressively more endothermic and are accompanied by large positive entropies, indicating greater electrostatic interaction. The aqueous speciation of silver is very sensitive to fluid composition and temperature. Below 100°C silver(I) sulphide complexes predominate in reduced sulphide solutions, whereas Ag+ and AgClOH− are the dominant species in oxidised waters. In high-temperature hydrothermal solutions of seawater salinity, chloride complexes of silver(I) are most important, whereas in dilute hydrothermal fluids of meteoric origin typically found in active geothermal systems, sulphide complexes predominate. Adiabatic boiling of dilute and saline geothermal waters leads to precipitation of silver sulphide and removal of silver from solution. Conductive cooling has insignificant effects on silver mobility in dilute fluids, whereas it leads to quantitative loss of silver for geothermal fluids of seawater salinity.

A standardized method for the determination of solidification points, especially of naphthalene and paraffin

This work has made it possible to obtain two new Na/Li geothermometric relationships in addition to the three already known (Fouillac and Michard, 1981; Kharaka et al., 1982) and confirms that the Na/Li geothermometer, unlike the Na/K, Na/K/Ca, K/Mg and silica geothermometers, or the isotope δ18O (H2O–SO4) geothermometer, also depends on the fluid salinity and the nature of the reservoir rocks reacting with the geothermal water. One of the relationships concerns the fluids derived from seawater–basalt interaction processes existing in emerged rifts such as those of Iceland (Reykjanes, Svartsengi, and Seltjarnarnes geothermal fields) and Djibouti (Asal-Ghoubbet and Obock geothermal areas), or in numerous oceanic ridges and rises (Mid-Atlantic and Middle-Valley ridges, East Pacific rise, etc.). The best adapted Na/Li relationship for geothermal fluids discharged from emerged rifts between 0 and 365 °C is:where Na and Li are the aqueous concentrations of these elements given in mol/L.The other Na/Li relationship was determined using dilute waters collected from wells located in different high-temperature (200–325 °C) volcanic geothermal areas of Iceland (Krafla, Námafjall, Nesjavellir and Hveragerdi). This relationship can be expressed as follows:T(K) = 2002/ [log(Na/Li) + 1.322] (r2 = 0.967, n = 17).These two relationships give estimations of temperature with an uncertainty close to ± 20 °C. The second Na/Li relationship was also successfully applied to HT dilute geothermal waters from the East African Rift (Ethiopia, Kenya).Some case studies in the literature and thermodynamic considerations suggest that the Na/Li ratios for this type of fluids could be controlled by full equilibrium reactions involving a mineral assemblage constituting at least albite, K-feldspar, quartz and clay minerals such as kaolinite, illite (or muscovite) and Li-micas. Unlike the Na/Li ratios, no thermometric relationship using Li isotopes could be determined for this type of water. However, it was noticed that δ7Li values higher than 16‰ are always associated with low- to medium-temperature waters.