Reductive kinetics of the reaction between a natural ilmenite and carbon

The reductive degree and rate of Bama ilmenite concentrate by graphite under argon atmosphere were measured by using thermogravimetric analysis system at the temperatures from 850 °C to 1400 °C. The reduction degree of the ilmenite increased with temperature rise, the case similar to reaction rate. The higher the temperature was, the faster the reaction rate was. The phases before and after reduction and the morphology of reduced samples were identified by using X-ray diffraction and optical microscopy analysis, respectively. The reduction degree of the ilmenite decreased due to the presence of impurities. Manganese and magnesium oxide-rich zones formed which prevented complete reduction of Fe²⁺. In general, the reaction products were iron, Ti₃O₅, reduced rutiles and pseudobrookite solid solution. The kinetics was also discussed and it was found that the rate-controlling steps were different in the different temperature ranges.     

Sapphirine and spinel phase relationships in the system FeO-MgO-Al2O3-SiO2-TiO2-O2 in the presence of quartz and hypersthene

Sapphirine and spinel can accommodate significant ferric iron and therefore the mineral equilibria involving these phases must be sensitive to a(O₂). In this paper we examine the theoretical phase relationships involving sapphirine and spinel in addition to sillimanite, garnet, cordierite, rutile, hematite-ilmenite solid solution (henceforth ilmenite), and magnetite-ulvospinel solid solution (henceforth magnetite), in the presence of quartz and hypersthene in the system FeO-MgO-Al₂O₃-SiO₂-TiO₂-O₂ (FMASTO), with particular reference to the topological inversion in P-T postulated by Hensen (Hensen 1986). Documented natural associations suggest that the appropriate topology for assemblages involving magnetite and ilmenite is Hensen's higher a(O₂) one, while, in contrast, the topology for assemblages involving ilmenite and rutile is the lower a(O₂) one. The exact configuration of the inversion between these two topologies remains uncertain because of uncertainties in the ferric/ferrous iron partitioning between sapphirine and spinel-cordierite at high temperatures. By comparison with experimental data and natural occurences, the sillimanite-sapphirine-cordierite-garnet-hypersthene-quartz assemblage is in equilibrium at about 1000°–1020° C and 7–8 kbars, while sapphirine-cordierite-spinel-garnet-hypersthene-quartz occurs at temperatures in excess of those attainable during crustal metamorphism, for ilmenite-rutile buffered assemblages. This implies that sapphirine-rutil-ehypersthene-quartz assemblages, as found in the Napier Complex, Antarctica, can only occur at > 1000° C. Also, spinel-rutile-hypersthene-quartz assemblages should not be found in rocks because temperatures in excess of 1100° C are expected to be involved in their formation. The temperatures of formation of spinel-sillimanite-sapphirine-garnethypersthene-quartz, sapphirine-spinel-cordierite-sillimanite-hypersthene-quartz, and sillimanite-spinel-cordieritegarnet-hypersthene-quartz in assemblages buffered by magnetite and ilmenite are less well constrained, but are likely to be in the range 900°–1000° C. These conclusions apply to rocks with compositions close to FMASTO; the perturbing effects of substantial concentrations of additional components, in particular Ca, mainly in garnet, and Zn and Cr, mainly in spinel, may invalidate these conclusions.     

Subsolidus phase relations in the system Zr-Fe-Ti-O in equilibrium with metallic iron. Implications for lunar petrology

An experimental study of the system Zr-Fe-Ti-O in equilibrium with metallic iron has been conducted at atmospheric pressure and in the temperature range 950–1300° C, with the purpose of modelling sub-solidus relations between Zr-bearing Fe-Ti oxide phases in lunar rocks. The phase relations are governed by the coexistence of the Fe-Ti oxides with ZrO₂ at temperatures below 1247° C, but with the new cubic ZFT phase at higher temperatures. ZFT, approximately Zr Fe Ti O₅, is probably related to the so-called stabilized cubic zirconia and could represent a synthetic equivalent of some lunar Zr-Fe-Ti rich oxide minerals.The zirconium contents in all Fe-, Ti and Fe-Ti-oxide phases increase with increasing temperature. At each temperature, Zr is incorporated preferentially in the Ti-oxide (up to 11 wt% ZrO₂), followed by ferropseudobrookite (up to 6 wt% ZrO₂), ilmenite (max. 4 wt%) and wüstite (max. 2.5 wt%). Ulvöspinel coexisting with ilmenite (+Fe⁰+ ZrO₂ and/or ZFT) always contains less than 1.2 wt% ZrO₂, whereas the same mineral in assemblage with wüstite-ZrO₂-Fe⁰ displays even higher ZrO₂ contents than ilmenite (above 4 wt%). Considering that the values determined here in the synthetic parageneses represent saturation concentrations at high temperatures in a simple model system, the ZrO₂ concentrations in the corresponding lunar minerals should generally be significantly lower. This is realized, except for the so-called Cr-Zr-Ca armalcolites which display ZrO₂ contents in the range 3.5–7 wt% and are thus probably related rather to Zr-rich oxide minerals than to armalcolite itself.     

Experimental constraints on the low-pressure evolution of transitional and mildly alkalic basalts: the effect of Fe-Ti oxide minerals and the origin of basaltic andesites

One-atmosphere, melting experiments, controlled at the fayalite-magnetite-quartz oxygen buffer, on mildly alkalic and transitional basalts from Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105±5°C, apparently independently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystalline plagioclase and olivine as the first two crystalline phases, augite as the third phase, and an Fe-Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO₂ in the liquid, and a notable increase in SiO₂ and Al₂O₃, and the minor oxides K₂O and P₂O₅, with decreasing temperature. The most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites. Because the smallest amount of glass that could be analyzed with the microprobe represents 20–55 percent liquid remaining, it can be expected that more silicic liquids will occur at lower temperatures. On normative, pseudoternary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz. This is caused by a strong systematic convergence, with the appearance of Fe-Ti oxides, of the bulk solid precipitates toward the liquid compositions, as projected on the triangle plagioclase-diopside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti oxide crystallization. This causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve toward early silica saturation.     

The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates

The co-existing microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analysed with the electron microprobe. The temperatures and oxygen fugacities of the oxide equilibration have been estimated from the curves of Buddington and Lindsley (1965). The co-existing ferromagnesian silicate phenocrysts are either iron-rich olivine, or orthopyroxene or biotite and amphibole; for each of these groups of phenocrysts, the oxide equilibration data are specific and fall on three distinct curves, parallel to experimental oxygen buffer curves. Many of the investigated rhyolites were quenched at temperatures near 900°C, which may represent liquidus temperatures for those with sparse phenocrysts, and also the intrusion temperature of water-undersaturated granites. The composition of the biotite phenocrysts, which are Al-poor and Ti-rich, taken in conjunction with the oxide data, suggest that two Lassen dacites precipitated biotite at a water fugacity of approximately 400 bars. The composition of the later crystallizing ferromagnesian silicates, particularly the pyroxenes which show a wide range in Fe/Mg ratio, is strongly influenced by the prior crystallization of the oxide phases. If the biotite phenocrysts are typical of acid liquids, then they are incapable of generating by fractionation a peraluminous residual liquid; rather they would tend to make a liquid peralkaline.     

Titanium solubility in olivine in the system TiO<Subscript>2</Subscript>–MgO–SiO<Subscript>2</Subscript>: no evidence for an ultra-deep origin of Ti-bearing olivine

The finding of ilmenite rods in olivine from orogenic peridotites has sparked a discussion about the processes of incorporation and exsolution of titanium in olivine. We have experimentally investigated the solubility of Ti in olivine as a function of composition, temperature and pressure in the synthetic TiO₂–MgO–SiO₂ system. Experiments at atmospheric pressure in the temperature range 1,200–1,500°C showed that the highest concentration of TiO₂ is obtained when olivine coexists with spinel (Mg₂TiO₄). The amount of TiO₂ in olivine in the assemblages olivine + spinel + periclase and olivine + spinel + ilmenite at 1,500°C was 1.25 wt.%. Changes in the coexisting phases and decreasing temperature result in a significant reduction of the Ti solubility. Olivine coexisting with pseudobrookite (MgTi₂O₅) and a Ti–Si-rich melt at 1,500°C displays a fourfold lower TiO₂ content than when buffered with spinel. A similar decrease in solubility is obtained by a decrease in temperature to 1,200°C. There is a negative correlation between Ti and Si and no correlation between Ti and Mg in Ti-bearing olivine. Together with the established phase relations this suggests that there is a direct substitution of Ti for Si at these temperatures, such that the substituting component has the stoichiometry Mg₂TiO₄. The unit cell volume of olivine increases systematically with increasing TiO₂ content demonstrating that the measured TiO₂ contents in olivine are not caused by micro-inclusions but by incorporation of Ti in the olivine structure. Least squares fitting of 20 olivine unit cell volumes against the Ti content yield the relation: V (ų)=290.12(1) + 23.67(85) N_Ti. The partial molar volume of end-member Mg₂TiO₄ olivine (N_Ti=1) is thus 47.24±0.13 cm³. The change of the Ti solubilty in olivine coexistent with rutile and orthopyroxene with pressure was investigated by piston cylinder experiments at 1,400°C from 15 to 55 kbar. There is no increase in TiO₂ contents with pressure and in all the experiments olivine contains ~0.2 wt.% TiO₂. Moreover, a thermodynamic analysis indicates that Ti contents of olivine coexisting with rutile and orthopyroxene should decrease rather than increase with increasing pressure. These data indicate that the ilmenite exsolution observed in some natural olivine does not signify an ultra-deep origin of peridotite massifs.     

Coexisting garnets and ilmenites synthesized at high pressures from pyrolite and olivine basanite and their significance for kimberlitic assemblages

Coexisting garnets and ilmenites have been synthesized at high pressure (21–40 kb) within the temperature range between 900 and 1100 °C from pyrolite-less-40% olivine and olivine basanite with various water contents. The two compositions yield phases with a range in the 100 Mg/Mg+Fe ratio for both garnet (41–76) and ilmenite (15–47). The distribution coefficient for iron and magnesiaum (K _{D(Fe, Mg)} ^ilm-ga = 4.0±0.5) for coexisting phases does not appear to vary with change in the bulk composition or temperature of synthesis. The synthesized ilmenites are of similar composition to those of kimberlites in 100 Mg/Mg+Fe ratio and Al₂O₃ and Cr₂O₃ solid solution. Cr₂O₃ content in ilmenite is dependent on Cr₂O₃ in the bulk composition and also on Fe₂O₃ content of ilmenite. Fe₂O₃ content of ilmenite is very sensitive to f _O2 and natural ilmenites from peridotites have formed under low f _O2. Al₂O₃ solid solution in ilmenite as well as TiO₂ in coexisting garnet tend to be higher with higher temperature. All the variety of compositions of ilmenites from kimberlites may be obtained from rocks rather close in composition to those used in experiments, within the same range of pressure and temperature but at variable oxygen fugacities.     

Halite and sylvite as solid inclusions in high-grade metamorphic rocks

Solid inclusions of halite and sylvite, formed during regional and contact metamorphism have been identified by microscopy and by electron microprobe analysis in rocks from Campolungo, Switzerland and Cornone di Blumone, Italy. The solid inclusions occur in several of the major minerals crystallized during metamorphism and have been observed as idiomorphic crystals and dendrites. The compositions measured in 100 analyses from Campolungo, Switzerland and 40 analyses from Cornone di Blumone, Italy extend across the two-phase region in the system, KCl-NaCl, indicating that the salt inclusions are high temperature precipitates. In both localities compositionally zoned and unzoned crystals have been found. Measured compositions on the temperature maximum of the two-phase region indicate at least 500° C which can be compared with 500°±20° C determined by Mercolli (1982) and Walther (1983) from the Mg content of calcites from Campolungo. The solid inclusions have been trapped apart from CO₂-rich and saline, H₂O-rich fluid inclusions which have been described by Mercolli (1982) as the earliest preserved fluid inclusions in the rocks. The early precipitation of salt minerals at Campolungo indicates that fluids were saturated with NaCl and KCl at 500° C and pressures of 2,000 bars or higher. Similar relationships exist between solid and fluid inclusions in the rocks of Cornone di Blumone which formed at temperatures as high as 800° C and pressures between 0.5 and 1 kilobar (Ulmer 1983). The entrapment of halite and sylvite as solid inclusions preserves the composition of the minerals which may therefore be useful as geothermometers.     

The reaction enstatitess+sillimanite ⇌ sapphiriness+quartz in the system MgO-Al2O3-SiO2

The univariant pressure temperature curve of the reaction aluminous enstatite solid solution+sillimanitesapphirine solid solution+quartz was determined experimentally in the pressure range between 12 and 20 kb. It is defined by four reversals at 15 kb, 1140°+10°C; at 1190°C, 15.9±1 kb; at 1300°C, 17.2±1 kb; and at 1400°C, 18±1 kb. Among the coexisting phases the Al-content of the enstatites increases strongly, with rising temperatures and pressures, up to values approaching that of pyrope composition, whereas the Al-content of the sapphirine solid solution appears to increase only slightly. Concomitantly, the sillimanites, most probably of invariant composition, exhibit growing Al/Si-disorder. These compositional and structural variations, in addition to the changing stoichiometry of the reaction equation, cause the progressively decreasing positive dP/dT slope of the equilibrium curve. — The assemblage sapphirine + quartz found in natural granulites is indicative of conditions of water pressure much lower than total pressure.     

The miscibility gap between rhodonite and bustamite along the join MnSiO3-Ca0.60Mn0.40SiO3

The miscibility gap between rhodonite and bustamite has been experimentally determined at temperatures between 600° and 1,100° C. For temperatures below 700° C the resulting limbs have been extrapolated on T-X-diagram as at such low temperatures equilibrium could not be attained. According to microprobe analyses for the natural assemblages of Ravinella di Sotto (Ivrea zone, Italy) and Broken Hill (N.S.W., Australia) equilibrium temperatures are estimated to be at 500° to 550° C. However these assemblages are thought to have re-equilibrated during cooling and the compositions of equilibrium assemblages are also pressure dependent. According to experiments and to molar volume data the rhodonite structure is stabilized by high pressures whereas bustamite by high temperatures. Based on available experimental results and natural data an isobaric T-X _Ca diagram and two isotherm -X _Ca diagrams (for T=400° C and T=600° C) are given.     

Crystallization of titaniferous chromite,magnesian ilmenite and armalcolite in tholeiitic suites in the Karoo Igneous Province

Titaniferous chromite (up to 8 wt% TiO₂) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Mount Ayliff Intrusion in the Karoo Province of southern Africa, suggesting that the original magma was TiO₂-rich. Picritic lavas with 3% TiO₂ from the Lebombo monocline of the Karoo Province also contain microphenocrysts of magnesian ilmenite (up to 6 wt% mgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are atypical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO₂, ilmenite less than 3 wt% MgO and armalcolite is rarely a primary mineral. Experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of TiO₂ on the crystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO₂ content of picritic magmas increases the TiO₂ content of the spinel, mainly at the expense of Al₂O₃, whereas Cr₂O₃ is not affected. Spinel compositions in the Mount Ayliff Intrusion (with over 45 wt% Cr₂O₃, less than 10 wt% Al₂O₃ and 8 wt% TiO₂) were duplicated in experiments on a picrite at temperatures of about 1,200°C at the Ni/NiO buffer. Increasing fO₂ from fayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite. The total FeO content of the liquid has little influence on the crystallization temperature of these phases. The TiO₂ content of the liquid, when either ilmenite or armalcolite crystallizes, varies inversely with SiO₂ content. The MgO content of the liquid at which ilmenite or armalcolite crystallizes depends upon the TiO₂ content of the starting composition, with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO₂. The partition coefficent for MgO between armalcolite or ilmenite and liquid is about 1.5. Observed magnesian armalcolite and ilmenite compositions in picrite lavas (both minerals) and in the Mount Ayliff Intrusion (ilmenite only) are consistent with crystallization from a TiO₂-rich magma with approximately 5 wt% MgO. The Fe ₂ ³⁺ TiO₅ component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1,150–1,110°C and fO₂ of the Ni/NiO to Ni/NiO+1 log unit. Similarities also exist between the compositions of chromite, ilmenite and armalcolite and liquid fraction-ation trends of some Hawaiian high-TiO₂ lavas and the experimental studies presented here.     

Fluid inclusion study of Xihuashan tungsten deposit in the southern Jiangxi province,China

The Xihuashan tungsten deposit is closely related to a small highly evolved granitic intrusion. The fluid phases associated with the wolframite-bearing quartz veins have been investigated using microthermometry and the Raman microprobe; they are highly variable in density and composition. The earlier fluids are low-density and low-salinity CO₂-bearing aqueous solutions circulating at temperatures up to 420 °C, and low-salinity (2–3 equiv. wt% NaCl) aqueous solutions without traces of CO₂ circulating at high temperatures 280°–400 °C) involved in a specific hydrothermal fracturing event; limited unmixing occurs at 380 °C and 200–100 bar in response to a sudden pressure drop. The second types of fluids related to deposition of idiomorphic drusy quartz are typical CO₂-bearing aqueous solutions with low salinity (2.5 equiv. wt% NaCl) homogenizing at low to moderate temperatures (180°–340 °C). The late fluids characterize the sulfide deposition stage; they are aqueous fluids with variable salinities homogenizing in the liquid phase between 100° and 275 °C. The Xihuashan hydrothermal evolution resulted from a discontinuous sequence of specific events occurring between 420° and 150 °C and during a continuous hydrothermal evolution of the system during cooling. The role played by the CO₂-rich fluids in the transport and deposition of tungsten in the hydrothermal environment is discussed.     

Reactions of iron with calcium carbonate at 6 GPa and 1273–1873 K: implications for carbonate reduction in the deep mantle

Experimental data on Fe-CaCO₃ interaction at 6 GPa and 1273–1873 K are presented. The system models the hypothetical redox interaction in subducting slabs at the contact with the reduced mantle and a putative process at the core-mantle boundary. The reaction is accompanied by carbonatite melt formation. It also produces Fe3C and calcium wustite, which form solid or liquid phases depending on experimental conditions. In iron-containing systems at 6 GPa, calcium carbonate melts in the range 1473–1573 K, which is consistent with aragonite disappearance from complex carbonate systems. The composition of calcium carbonate liquid is not influenced by metallic Fe. It corresponds to nearly pure CaCO₃. Along the mantle adiabat or at slightly higher temperatures, nearly pure CaCO₃ coexists with metallic iron or calcium wustite. This hypothesis explains the coexistence of metallic iron and carbonate inclusions in lithospheric and superdeep diamonds.     

Incorporating solid solutions in reactive transport equations using a kinetic discrete-composition approach

A new approach is proposed for incorporating solid solution reactions into mass conservation equations describing reaction paths in both closed and open systems. The method is applicable to problems involving advective, dispersive, and diffusive transport in a porous medium. By representing the continuously variable solid solution composition with a discrete set of stoichiometric solids that span composition space, combined with a kinetic formulation of their rates of reaction, a self-determining spatial and temporal evolution of the solid solution concentration and composition is obtained. It is demonstrated that equilibrium of an aqueous solution with a stoichiometric solid derived from a solid solution corresponds to equilibrium of the solid solution itself if and only if equilibrium of the stoichiometric solid is stable. One advantage of this approach is that it is unnecessary to introduce any additional compositional variables to represent the solid solution. Discretization may be over the entire range of composition space, or over some subset depending on the system. A major consequence of the kinetic discrete-composition solid solution representation is that modeling solid solutions is similar to modeling pure mineral phases with the exception of a weighting factor applied to reaction rates of stoichiometric solids corresponding to a common solid solution. With this approach, precipitation leads to a discrete zonation of the solid solution that approximates the continuous variation in composition expected for the actual solid solution. The approach is demonstrated for a hypothetical ideal and non-ideal binary solid solution A_xB_1−xC for a reaction path formulation and reactive transport involving advection and diffusion.     

Stability of ilmenite and titanomagnetite in the presence of carbon dioxide — A thermodynamic evaluation

The stability of ilmenite and titanomagnetite in the presence of CO₂ gas was evaluated thermodynamically. Ilmenite with a composition Ilm₉₀Hm₁₀ breaks down to the assemblage siderite-hematite-anatase below 270° C at 1,000 bars and below 300° C at 2,000 bars. Titanomagnetite with a composition Usp₅₀Mt₅₀ breaks down to the same assemblage below 345° C at 1,000 bars and below 385° C at 2,000 bars. During the low-grade metamorphism of basaltic rocks, a CO₂ partial pressure of only a few hundreds bars is sufficient to alter ilmenite and titanomagnetite to the assemblage siderite-hematite-anatase.     

Stability of almandine in the system FeO-(Fe2O3)-Al2O3-SiO2-(H2O) at elevated pressures

Almandine, although decomposing in the presence of metallic iron into the anhydrous subsolidus assemblage fayalite + ferrocordierite + hercynite solid solution at low pressures, melts incongruently to hercynite_ss + quartz + liquid at 10 kb. At pressures between about 12 and 20 kb the products of incongruent melting are hercynite_ss + liquid only, and at still higher pressures almandine melts congruently. For the intermediate pressures between 2 and 10 kb not investigated a sequence of probable breakdown and melting relations involving the phases ferrocordierite, fayalite, hercynite_ss, quartz, and liquid is derived through Schreinemakers' analyses.The lower temperature stability limit of almandine in the presence of water at low oxygen fugacities and pressures of 15 to 20 kb lies between 550° and 600° C as at low pressures. It is marked, however, by the breakdown to a hydrous assemblage involving chloritoid and the new phase aluminous deerite. Since the anhydrous melting at these pressures occurs between 1300° and 1400° C, the thermal stability range of almandine increases drastically with pressure. Its upper breakdown limit shows in principle a similar behavior as those of other garnet end members.     

Armalcolite stability as a function of pressure and oxygen fugacity

The stability of synthetic armalcolite of composition (Fe_0.5Mg_0.5Ti₂O₅ was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O₂) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down ($\text{dT}\text{dP}\text{= 20°C/}\text{kbar}$), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way ($\text{dT}\text{dP}\text{= 30°C/}\text{kbar}$) to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO₂ appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O₂ was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe_0.5Mg_0.5)-Ti₂O₅, is stable over a range ?O₂ from about 10^?9.5to 10^?10.5 atm. Below this range to at least 10^?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti³⁺. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.     

Major element chemistry of surface- and ground waters in basaltic terrain, N-Iceland.: I. primary mineral saturation

This contribution describes primary basalt mineral saturation in surface- and up to 90°C ground waters in a tholeiite flood basalt region in northern Iceland. It is based on data on 253 water samples and the mineralogical composition of the associated basalts. Surface waters are significantly under-saturated with plagioclase and olivine of the compositions occurring in the study area, saturation index (SI) values ranging from −1 to −10 and −5 to −20, respectively. With few exceptions these waters are also significantly under-saturated with pigeonite and augite of all compositions (SI = −1 to −7) and with ilmenite (SI = −0.5 to −6). The surface waters are generally over-saturated with respect to the titano-magnetite of the compositions occurring in the basalts of the study area, the range in SI being from −2 to +10. For crystalline OH-apatite, SI values in surface waters range from strong under-saturation (−10) to strong over-saturation (+5) but for crystalline F-apatite they lie in the range 0 to 15. Systematic under-saturation is, on the other hand, observed for “amorphous apatite,” i.e. an apatite of the kind Clark (1955) prepared by mixing Ca(OH)₂ and H₃PO₄ solutions. Like surface waters, ground waters are under-saturated with plagioclase and olivine, its degree increasing with increasing Ca content of the plagioclase and increasing Fe content of the olivine, the SI values being −2 to −7 and 0 to −4 for the Ca-richest and Ca-poorest plagioclase, respectively, and about −3 to −18 and 0 to −15 for forsterite and fayalite, respectively. Ground waters are generally close to saturation with pigeonite and augite of all compositions. However, some non-thermal ground waters in highland areas are strongly under-saturated. Above 25°C the ground waters are ilmenite under-saturated but generally over-saturated at lower temperatures. These waters are titano-magnetite over-saturated at temperatures below 70°C, the SI values decreasing with increasing temperature from about 6 to 8 at 10°C to 0 at 70°C. The ground waters are highly over-saturated with both crystalline OH- and F-apatite, or by approximately 10 to 15 SI units but close to saturation with “amorphous apatite” containing about equal amounts of F and OH. The results presented here for the pyroxenes carry an unknown error because available thermodynamic data do not permit but a simple solid solution model for the calculation of their solubility. Published values on the dissociation constants for ferrous iron hydroxide complexes are very variable and those for ferric iron are limited. This casts an error of an unknown magnitude on the calculated SI values for all iron bearing minerals. This error may not be large for waters with a pH of less than 9 but it is apparently high for waters with a higher pH. Improved experimental data on the stability of ferrous and ferric hydrolysis constants are needed to improve the accuracy by which Fe-mineral saturation can be calculated in natural waters.     

Approach to chemical equilibrium in diagenetic chlorites

Electron microprobe analyses were made on diagenetic chlorites in sandstones and mudstones from two deep wells according to the petrographic character of the chlorite occurrence: as pseudomorphic phases, rims on quartz or glauconite or as distinct phases in the clay matrix. Chlorite compositions do not depend upon crystallization site (reacting phases) making it apparent that new chlorites can form in an approach to chemical equilibrium at or near the surface (40° C, 1 km depth). Comparison of this data with that for late diagenetic and early metamorphic chlorites indicates that the compositional range for different grains in the same thin section is similar for the samples throughout the 40°–270° C temperature span. Compositional range decreases upon further metamorphism.Al content appears to be a more reliable indicator of temperature variations than other substitutions in the chlorite structure. The clay mineral assemblage which indicates sedimentary facies affects the trends in composition (Al increase or decrease) as a function of temperature. The octahedral site occupancy show a general increase in going from diagenesis to metamorphic conditions in pelitic rocks. The range of Fe-Mg ratios seems to depend more on the chemistry of each sample than the temperature of formation of the minerals.     

Thermodynamic models in cosmochemical systems

Generalized computer methods are developed for inferring details of the formation of cosmochemical systems. Compositions of ideal gas mixtures existing in equilibrium with multicomponent solid and liquid phases are calculated. A comparison of computed results with experimental data is made for the ternary system MgO-FeO-SiO₂. While the ideal-solution approximation is shown to be inaccurate in dealing with the silicate melts, in a system where there are only solids and gas, the stable phases and compositions can be accurately calculated. A model system containing the elements H, O, Si, Mg, S, C, Cl and F is investigated over a range of compositions involving the gas and ten solid phases, to show the power of the technique in dealing with complex gas-solid equilibria. Systems close to cosmic composition are next considered, both with and without iron. When present, most of the iron is computed to be in the metallic state with little going into pyroxene or olivine solid solutions. At low hydrogen concentration and low temperature, troilite becomes the stable iron-bearing phase. The calculated equilibrium concentrations are very sensitive to the assumed ratio of magnesium to silicon. The computational method described is easily applied to complex systems of solids and gases and represents an important tool with which to investigate cosmochemical systems.