Olivine Dissolution Research Articles

Direct measurement of the combined effects of lichen, rainfall, and temperature onsilicate weathering

A key uncertainty in models of the global carbonate–silicate cycle and long-term climate is the way that silicates weather under different climatologic conditions, and in the presence or absence of organic activity. Digital imaging of basalts in Hawaii resolves the coupling between temperature, rainfall, and weathering in the presence and absence of lichens. Activation energies for abiotic dissolution of plagioclase (23.1 ± 2.5 kcal/mol) and olivine (21.3 ± 2.7 kcal/mol) are similar to those measured in the laboratory, and are roughly double those measured from samples taken underneath lichen. Abiotic weathering rates appear to be proportional to rainfall. Dissolution of plagioclase and olivine underneath lichen is far more sensitive to rainfall.

Olivine dissolution in sulphuric acid at elevated temperatures—implications for the olivine process, an alternative waste acid neutralizing process

A kinetic model for the dissolution of olivine in sulphuric acid at elevated temperatures is presented. Such a model is essential for the design of an industrial process for the neutralization of waste acids by olivine, or for the production of precipitated silica from olivine. The effects of temperature, grainsize fraction and olivine amount on the neutralization rate have been quantified by carrying out a range of complete-neutralization experiments. Under natural weathering conditions the dissolution rate of olivine is controlled by rates of surface reactions. During neutralization of acids at elevated temperatures the reaction mixture must be stirred vigorously to prevent the precipitation of silica on the dissolving olivine grains. Under these conditions dissolution is kinetically controlled by surface reactions. For the interpretation of the experimental data a measure of the hydrogen ion activity is required. Since it is not possible to accurately measure pH in acids at high temperatures, the hydrogen ion activities were calculated using the Pitzer model. The rate equation coefficients and constants were obtained by fitting an expression for the rate equation to the experimental data. The following equation was obtained, that enables the calculation of the neutralization progress as a function of time: d [ H +] dt = − e − E act R×T × 10 8 × (1.92±0.12)×S geom × A H + 0.33 [ mol min −1] with E act the activation energy of 66.5±2 kJ mol −1, S geom the geometrical surface area of the olivine grains, calculated from the grainsize distribution and A H the hydrogen ion activity calculated with the Pitzer model, using the computer program PHRQPITZ. The equation was validated for the dissolution of Norwegian olivine, with grainsizes from 63 to 300 μm, in sulphuric acid at temperatures ranging from 60 to 90°C.

The preparation of silica from the olivine process and its possible use as a catalyst support

The production of silica from the dissolution of olivine in sulphuric acid has been studied. Variations in grainsize of the olivine result in different specific surface areas of the silica. The microporosity of the silica supports an aggregative growth model responsible for the increasing particle size. The silica is used in the preparation of a nickel catalyst. The performance of the produced catalyst is equal to a nickel catalyst based on commercially available silica

Metasomatism in mantle xenoliths from Gees, West Eifel, Germany: evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment

We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO2 when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO2), characterized by high Al2O3 (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated.

Enhancement of silicate weathering rates by vascular land plants: quantifying the effect

Theoretical arguments and biogeochemical modeling indicate that levels of paleoatmospheric CO2 depend strongly upon the degree to which the appearance, spread, and evolution of vascular plants on land accelerated the weathering of calcium and magnesium silicates (Berner, 1991, 1993). We have undertaken a series of observations and controlled experiments relating to the relative weathering effects of higher plants and their weatheringspecific responses to elevated CO2. Work to date includes a comprehensive study of Hawaiian basalt weathering beneath plant communities, along with examination of bare-rock and lichen-covered controls (modern lichens may serve as first-order analogs for terrestrial vegetation prior to the late Silurian (Retallack, 1981; Wright, 1985 ) ); analysis of the impact of plants colonizing ash and pumice from the recent eruptions of Mt. St. Helens; and growth-chamber experiments on CO2 fertilization of Loblolly pine and American beech growth on a basalt gravel substrate. Findings from these studies are presented and their implications discussed. We sampled basalt flows with ages between a few years and several thousand years from the Kilauea and Mauna Loa volcanoes on the island of Hawaii. Regions of low, moderate and high rainfall were sampled. Apparent weathering textures (leached zones, regions of partial dissolution) were not observed to penetrate unvegetated rocks less than 200 years in age to depths of more than 10/~m, even in the most humid areas. In addition, the common fruticose lichen Stereocaulon vulcani has no greater weathering effect upon its substrate than does abiotic water-rock interaction: some slight crumbling of rock surfaces might be attributable to the expansion and contraction of lichen holdfasts upon wetting, but no leaching and only negligible dissolution of substrate minerals and glass were observed. Notwithstanding the lichen's failure to digest its substrate to any marked degree, its tendency to trap and leach fine wind-blown rock fragments may result in the chemical weathering of significant quantities of material that would otherwise be transported and buried without being chemically weathered. Schwartzman and Volk ( 1989 ) have suggested that such a process might have allowed primitive land biota to restrain atmospheric CO2 from reaching excessive values before the rise of vascular plants, and indeed it may. The scale of such lichen-mediated weathering, however, does not approach that observed adjacent to the fine roots of vascular plants on these Hawaiian flows. Younger flows beneath plants such as the common Ohia tree (Metrosideros polymorpha) exhibit widening of primary microfractures into open channels and preferential congruent dissolution of plagioclase, olivine and volcanic glass. On older flows up to

What do dissolution experiments tell us about natural weathering?

Much information about the earliest stages of weathering comes from laboratory experiments where the mineral surface chemistry is rigorously controlled and the solution composition is maintained far from equilibrium with the solid. The experiments show that the pathways for dissolution are similar to those for ligand exchange around dissolved metal complexes. One difference is that the rates of mineral dissolution are controlled by the concentrations of surface species while rates of ligand exchange are proportional to bulk concentration. Nevertheless, the correlation is so strong that rates of olivine dissolution are predictable from the rates of solvent exchange around the corresponding divalent metal in solution. However, this level of resolution is achieved by maintaining unnaturally high fluid/mineral ratios and by taking great care to avoid precipitation of secondary minerals. Natural olivine weathers in intimate contact with secondary minerals. Reactions commonly proceed in clay-filled channels only a few tens of Ångströms in diameter and secondary minerals are topotactic with olivine. Primary and secondary minerals are so intimately intergrown in these channels that the olivine surface chemistry is undoubtably influenced by overlapping electrostatic double layers. Smectite growth may have proceeded at near-saturation with the fluid, but the solution is in gross disequilibrium with the olivine. There nevertheless remains some qualitative consistency between weathering in the laboratory and in the field. The ligand-exchange model for bond cleavage, for example, predicts that the presence of ferric iron at the mineral surface will retard weathering rates. Correspondingly, defect planes of oxidized olivine (to produce Fe 3+ and a vacancy) weather at a much slower rate than unoxidized material. Thus, the appropriate level of comparison of field and laboratory weathering is at the scale of reactivity trends; it is unreasonable to expect quantitative similarity in reaction rates.

オリビン／水反応界面のキャラクタリゼーション

Experimental studies on olivine dissolution and ion exchange reaction were performed to understand reaction mechanism and kinetics. In the dissolution experiments, olivine crystals (Fo91) were water treated at 25°C under different pH conditions. X-ray photoelectron spectroscopy and resonant nuclear reaction analysis revealed the different characteristic features of the surface layer with the thickness of 10 to 20 nm depending on pH. The rate controlling step seems to be Si detachment under low pH conditions and Mg detachment under high pH conditions respectively. The ion exchange experiments between olivine (Fo91) and FeCl2 solution were carried out at 600 to 800°C, 1 kb. Scanning electron microscopic observation and chemical analysis showed that the reaction front advances with precipitation/dissolution mechanism.

Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas

A model is developed for the origin of ultrapotassic melts by melting of veined lithosphere; the veins are rich in clinopyroxene and mica, whereas the wall-rocks consist principally of peridotites. The veins originate by solidification of low-degree melts which are themselves the results of earlier, deeper, multistage processes ultimately due to the presence of a transition zone between large-scale channelled and porous flow regimes. The melting event producing the ultrapotassic magma begins in the veins due to the concentration of hydrous phases and incompatible elements, but spreads to include the surrounding wall-rocks by a combination of two mechanisms. The alkaline magma composition is thus a hybrid of vein (V) and wall-rock (W) components. The melt hybridization mechanisms are: (i) Solid-solution melting: Minerals which from extensive solid-solutions are abundant in the vein assemblages (Cr/Al spinel, F/OH mica, amphibole and apatite). The breakdown of these phases take place over a temperature range between the solidus of the vein assemblage and the elimination of the more refractory end-members. This process bridges the temperature gap between the solid of vein and wall-rock, so that a melt component from the wall-rock is added to that from the vein before elimination of all vein minerals. Phlogopite forms the most effective of these sliding reactions, resulting in its stability at near-liquids temperatures in experiments. (ii) Dissolution of wall-rock minerals: The initial melt fraction in the vein infiltrates the surrounding wall-rock due to the dominance of surface energy minimization on melt flow at the intergranular scale. Following infiltration, dissolution of wall-rock minerals occurs at temperatures lower than their melting temperatures, thus imparting a refractory wall-rock component to the melt composition. Dissolution of olivine and/or orthopyroxene occurs preferentially, since these minerals are furthest from equilibrium with the strongly alkaline, vein-derived melt. Remobilisation of several generations of veins explains the occurrence within a restricted space and time of rocks bearing chemical characteristics which are generally thought to indicate contrasting tectonic settings (e.g. central Italy). The ultrapotassic rocks are explained as being dominanyly vein-derived (i.e. high V/W ratio): further dilution of the V-component by wall-rock, supplemented by asthenospheric melt in advanced cases, leads to the production of more voluminous basaltic rocks bearing incompatible element signatures reminiscent of those of ultrapotassic rocks.

Geochemistry of dendritic anorthosites and associated pegmatites in the Skaergaard Intrusion, East Greenland: Evidence for metasomatism by a chlorine-rich fluid

In the Upper Zone (UZa) of the Skaergaard Intrusion dendritic anorthosites and pegmatites are found in a zone of deformed gabbro along the flank of a trough-banded structure. Apatite, biotite (some having over 2 wt.% C1) and amphibole have higher chlorine contents than those found in the surrounding rocks and indicate the presence of a high-temperature aqueous fluid having a high HCl/HF fugacity ratio. The fluid must have been derived locally by exsolution of magmatic volatiles because hydrous minerals in nearby hydrothermal veins have low chlorine contents and the structures are far from the margin of the intrusion. Apatite and quartz in the pegmatites have abundant fluid inclusions containing cubic halide crystals, giving evidence for a separate aqueous fluid coexisting with an evolved residual melt. The pegmatites are also characterized by mafic and felsic clots with compositions similar to immiscible silicate liquids. Dendritic anorthosites have equal or higher abundances of Zr, Hf, Ta, and rare-earth elements than the surrounding gabbros yet they contain little pyroxene or Fe-Ti oxides. The unusual trace-element enrichment patterns and the associated rare assemblage of zirconolite, zircon, baddeleyite and allanite suggest that the chlorine-rich fluid was enriched in trace elements derived from the residual melt and the reacted mafic minerals. Deformation of the magma at a crystallinity below the critical melt fraction resulted in localized exsolution and channeling of a magmatic aqueous fluid and an evolved residual melt. Dissolution of pyroxene, olivine, and Fe-Ti oxides allowed for further deformation and expulsion of the mafic melt leaving a plagioclase-rich residue. These structures are the product of enhanced reaction locally of an aqueous fluid which may have moved upward as a nearly planar reactive front throughout the lower crystallization zone. In addition to the transport of trace elements, migration of a high-temperature fluid through the zone of crystallization could aid in the compaction of igneous cumulates and the development of layering. This study presents the first evidence for a high-temperature chlorine-rich aqueous fluid in the Layered Series of the Skaergaard Intrusion and illustrates the important effect of such fluids on the physical and chemical evolution of layered intrusions.

Olivine dissolution kinetics at near-surface conditions

Fluidized bed dissolution experiments have been conducted as a function of pH at 25°C with fayalitic olivine (Fo 6) in HCl solutions and with forsteritic olivine (Fo 91) in solutions containing the organic ligand potassium hydrogen phthalate (KHP). At 25°C the dissolution rate ( R) of fayalite as a function of pH is: R(mol cm −2s −))=1.1·10 −10a H+ 0.69 +3.2·10 −14+1.2·10 −16a H+ −0.31 where a H + is the activity of H + in solution. The dissolution rate at 25°C of Fo 6 at a given pH is a factor of 6 greater than that of forsteritic olivine. The assumption that the rates increase on a molar basis with Fe content allows calculation of dissolution rates of Fe-Mg solid solution olivines of intermediate compositions. Batch-type dissolution experiments were completed with Fo 91 at 65°C in solutions at pH 1.8, 6.0 and 9.8. The rate equation obtained from these experiments is: R (mol cm −2s −1)=3.5·10 −10a H+ 0.5+1.0·10 −13a H + 0.5 When combined with previously published data for Fo 91 at 25°C, the 65°C experiments indicate that the activation energy of the olivine dissolution reaction in acidic, organic-free solutions is ∼19 ± 2.5 kcal. mol −1. Dissolution experiments with forsteritic olivine in solutions containing KHP at 25°C demonstrate that the rate of dissolution is increased in these solutions relative to rates measured in KHP-absent HCl-H 2O solutions. Apparently, the increase in rate is caused by Mg complexation at the olivine surface. Ligand-promoted dissolution is thought to occur in parallel with proton-promoted dissolution. Therefore, the net rate, R net , is the sum of the two rates: R net (mol cm −2s −1)=0.8·10 −12[L P] 0.45+R H + where [ L P] denotes the concentration of KHP; and R H + denotes the proton-promoted dissolution rate.

Olivine dissolution at 25°C: Effects of pH, CO 2, and organic acids

The dissolution rates of Fo 100 and Fo 91 in aqueous solutions in the pH range 2–12.4 at 25°C have been measured using fluidized bed and batch reactors. Rates depend upon the pH, the partial pressure of CO 2, and the presence of organic ligands. At low P CO 2 (≤10 −4.5 atm) with no organic ligands present, the rate of olivine dissolution, R, is given by R ( mol cm −2 s −1 ) = 9.07 × 10 −12a H + 0.54 + 5.25 × 10 −15 + 2.33 × 10 −17a H + −0.31 , where a H + is the activity of H + in solution. However in basic solutions, when the partial pressure of CO 2 is equal to atmospheric levels ( P CO 2 = 10 −3.5 atm), olivine dissolution rates are nearly pH independent throughout the pH range 6–12 and are about equal to the minimum rate of dissolution under CO 2 purged conditions. At pH 11 the presence of atmospheric levels of CO 2 reduces the dissolution rate by over an order of magnitude (to 10 −14.1 mol cm −2 s −1). Apparently, positive charge on the olivine surface can be neutralized by increasing P CO 2 . In contrast, experiments conducted in the acidic and near neutral pH ranges indicate that organic ligands chelate surface Mg causing an increase in the olivine dissolution rate when present. Organic ligand effects are greatest in the near neutral pH domain. For example, at pH 4 dissolution rates are increased by 0.75 log units (to 10 −12.25 mol cm −2 s −1) in solutions of 10 −3 molar ascorbic acid or 0.05 molar potassium acid phthalate over rates measured in organic free solutions. The chelation effect becomes less important as pH decreases. Rates at pH 2 in the presence of these organic ligands are indistinguishable from those measured without organics.

Matrix mineralogy of the Lancé CO3 carbonaceous chondrite: A transmission electron microscope study

The Lancé CO3 carbonaceous chondrite (CC) is less altered than the CI and CM chondrites and so provides a view of the mineralogy and textures resulting from the earliest stages of aqueous alteration of CCs. Matrix olivine in Lancé has been partly altered to fine-grained, Fe-bearing serpentine and poorly crystalline Fe 3+ oxide, a process that required both hydration and oxidation. Serpentine occurs as discrete packets separated from the olivine surfaces by the Fe 3+ oxide. The Fe released during the dissolution of olivine was partly incorporated into the serpentine; the remainder was oxidized to form Fe 3+ oxide. Matrix metal was also altered to produce Fe oxides, leaving the residual metal enriched in Ni. Olivine grains in Lancé matrix contain channels along their [100]and [001]directions. The formation and convergence of such channels resulted in a grain-size reduction of the olivine. The alteration was pervasive but incomplete, suggesting a limited availability of fluid. A brief study of two other CO chondrites, Kainsaz and Warrenton, shows that these meteorites do not contain phyllosilicates in their matrices, although both contain Fe 3+ oxide between olivine grains. Prior to its alteration, Lancé probably resembled Kainsaz, an unaltered CO3 chondrite. The alteration assemblage in Lancé is only slightly different from that in Mokoia and essentially the same as that in C3 xenoliths from Murchison. Alteration products in Lancé show greater similarities to CI than to CM chondrites.

Neutralization of industrial waste acids with olivine — The dissolution of forsteritic olivine at 40–70°C

A process for neutralizing inorganic waste acids with forsteritic olivine has been developed. An advantage of the process compared with traditional ways of neutralizing and/or recycling is that it produces secondary products of potential value. The experimental work presented here consisted of following the dissolution kinetics of forsteritic olivine in the temperature range of 40–70°C in sulfuric and hydrochloric acid (pH 1.0, 2.0, 3.0). Dissolution kinetics are apparently first order and controlled by rates of surface reactions. The existence of Mg-depleted layers and etch pits on different surfaces of the same grains indicates, however, that small-scale diffusion-controlled dissolution mechanisms may take place as well. The formation of a solid intermediate reaction product, which is interpreted as a hydrolyzed olivine, is further evidence of two processes occurring simultaneously, dissolution control by surface reactions being the more important mechanism. Activation energies of dissolution are among the lowest reported for silicates, varying between 25 ± 2 kJ mol −1 in hydrochloric acid and 30 ± 2 kJ mol −1 in sulfuric acid.

Diffusive crystal dissolution

Crystal dissolution may include three component processes: interface reaction, diffusion and complications due to convection. We report here a theoretical and experimental study of crystal dissolution in silicate melt without convection. A reaction-diffusion equation is developed and numerically solved. The results show that during non-convective crystal dissolution in silicate melt, the interface melt composition reaches a constant or stationary “saturation” composition in less than a second, hence interface reaction is not the rate-determining step and crystal dissolution in silicate melt is usually diffusion-controlled. Crystal dissolution experiments (designed to suppress convection) show that the concentration profiles of all components propagate into the melt according to the square root of run duration, and that the dissolution distance is also proportional to the square root of run duration. Thus our experiments confirm that the dissolution is diffusion controlled, which is consistent with our numerical calculations. For some principal equilibrium-determining components, concentration profiles conform approximately to the analytical solution of the diffusion equation with a constant effective binary diffusion coefficient. Diffusive dissolution rates (which are inversely proportional to square root of time) can thus be predicted from the phase equilibria and the effective binary diffusion coefficients. To predict steady-state convective dissolution rates, the thickness of the boundary layer must be known. If the convective compositional boundary layer thickness around a dissolving crystal aggregate or near the wall of a magma chamber during convection is about 2 cm or larger, then convective dissolution would rarely result in any significant alteration of original melt. Our dissolution experiments also illustrate the complexity of the diffusion process. Uphill diffusion is common, especially during olivine dissolution into andesitic melt where a majority of the components show the effect of diffusion up their own concentration gradients. Uphill diffusion has implications to the understanding of crystal zoning, and suggests caution is required in applying least squares mass balance analysis to magmatic rocks affected by processes involving diffusion.

Behavior of Li, Rb and Cs during basalt glass and olivine dissolution and chlorite, smectite and zeolite precipitation from seawater: Experimental investigations and modelization between 50° and 300°C

This study investigates experimentally the behavior of Li, Rb and Cs during the hydrothermal alteration of a basalt glass and of forsterite. The behavior of trace alkalis during the dissolution of a basalt glass and forsterite has been followed between 150° and 300°C: Rb and Cs are preferentially leached from the glass while Li exhibits a stoichiometric release with respect to silica. On the other hand, incongruent loss of Li relative to Si during forsterite dissolution reflects a crystallographic (site energy) control of the release of trace elements from crystalline silicates. The partitioning of Li, Rb and Cs between hydrothermal solutions and chlorites, smectites and zeolites has been experimentally investigated between 50° and 260°C. The alteration phases were synthesized by altering silicate glasses in solutions containing the trace alkalis. The lower the temperature, the higher the exchange capacity of the mineral or the lower the ionic hydration energy, then the more the element is incorporated into the solid phases. Computer simulations generated with the EQ3/6 software package and our experimental data on distribution coefficients account for the behavior of alkali trace elements during the alteration of basalt glasses by seawater at mid-ocean ridges.

Adcumulus dunite growth in a laboratory thermal gradient

Laboratory experiments near 1450° C at 1 bar (QFM) on komatiite bulk composition show olivine and liquid in cumulus textures which evolve with experiment duration. Orthocumulus texture with settled olivine crystals separated by liquid matrix is developed within a day. Experiments quenched after a few days to a week show a progression of textures which include development of columns of olivine crystals separated by channels of liquid. Olivine grain sizes increase with the cube root of time suggesting that dissolution and reprecipitation of olivine may be involved in the organization into columns and channels. Experiments quenched after two weeks have well developed adcumulus texture. The basal polycrystalline granular olivine aggregate forms from the decay of the olivine columns. Melt expulsion from the aggregate can be virtually complete, leaving 1% or less of the melt originally present.

The rates of dissolution of olivine, plagioclase, and quartz in a basalt melt

AbstractThe dissolution rates of spheres of two magnesian olivines, two plagioclases, and quartz in tholeiitic basalt have been determined at three super-liquidus temperatures and one-atmosphere pressure. There are considerable differences in the rates among the minerals, e.g. at 1210°, 12° above the liquidus temperature of the basalt, labradorite dissolves at 86 µm/h. and the magnesian olivines at 9 and 14 µm/h. The rates are not time dependent and this, coupled with the existence of concentration gradients in the composition of quenched melt adjacent to partially dissolved crystals, indicates that the dissolution rates are dictated by a combination of diffusion and convection of components to and from the crystal-liquid interface. Values for the activation enthalpy of dissolution are small for quartz and plagioclase (40–50 kcal mol−1) but large for olivine 73–118 kcal mol−1). Dissolution of plagioclase in rock melts seems to be a much more rapid process than crystal growth, whereas olivines apparently dissolve and grow at similar rates. Crystal dissolution is sufficiently slow that ascending, crystal-bearing magma may become superheated and yet fail to dissolve the crystal fraction before quenching; this may be the reason that olivine phenocrysts are often rounded.

Changes in surface area and morphology and the mechanism of forsterite dissolution

Scanning electron microscope (SEM) photographs of olivine grains shown that dissolution of olivine may occur more rapidly on some surfaces than on others and that initial dissolution of freshly crushed grains occurs primarily along lattice imperfections such as dislocations or cleavage planes. The SEM photographs generally do not show the presence of thick or continuous residual or precipitated layers which might render the dissolution reaction diffusion controlled. The specific surface area of olivine grains increased greatly during initial dissolution due to formation of etch features. However, despite the increased surface area the rate of dissolution decreased during the experiments. These observations suggest that some assumptions underlying derivation of the diffusion controlled models are invalid for olivine dissolution and suggest that dissolution is controlled by rates of surface reactions.

The Dissolution of Olivine in Aqueous Solutions of Inorganic Acids

Abstract The dissolution of olivine was studied in aqueous solutions of inorganic acids at temperatures of 35, 50, and 65°C in order to obtain basic information about the alteration and leaching processes in acid hot spring regions. The rates of dissolution have been determined by measuring the change in the concentrations of silica, magnesium, and the total iron leached in the solutions as a function of the time. Olivine dissolved in stirred aqueous solutions of 0.05–2n perchloric, hydrochloric, and sulphuric acids without any selective dissolution of components in the mineral. The rate at which olivine dissolved was found to be two-thirds of the reaction order with respect to the sample-powder (10–20 μ in diameter) weight (i.e., first order with respect to the surface area) and to decrease in the order of H2SO4>>HCl>HClO4. HSO4− enchanced the rate remarkably. The complexing affinity of the anions did not play an important part in the reaction with olivine powder.