Oxidation Of Olivine Research Articles

Olivine nanoparticles for Fast Atmospheric CO2 capture at Ambient Conditions

AbstractTo mitigate climate change, CO2 sequestration from the atmosphere is being considered as a method to reduce its greenhouse effect and subsequently lower the Earth's surface temperature. A promising approach is the storage of CO2 in minerals, of which Olivine is a promising candidate due to its Earth abundance and high CO2 absorption capacity, which is of the order of 50 wt.%. A bottleneck for Olivine carbonation is the slow reaction rate at ambient conditions, which previously resulted in supplying CO2 at extreme pressures and temperatures to force carbonation. In this study, nanoscale Olivine particles are fabricated, which due to their high surface‐to‐volume ratio, reach a very high carbonation conversion at a time scale of minutes at ambient conditions. The carbonation is measured by X‐ray photoelectron spectroscopy (XPS), which yielded both the presence of carbonates as well as information on the Olivine oxidation state, in agreement with electron diffraction analysis. This work forms the basis for employing Olivine nanoparticles, as fabricated by the relatively simple method of magnetron sputtering, to capture CO2 from the atmosphere at economic conditions.

Epitactic magnetite growth in fluid inclusions as driving force for olivine oxidation coupled with hydrogen production at high pressure

We disclose the crystallisation evolution of magnetite-bearing multiphase inclusions hosted in metamorphic olivine of harzburgites from the Cerro de Almirez (Betic Cordillera, Spain), which have been interpreted as final products of the trapping of the aqueous fluid produced by the subduction-zone dehydration of former serpentinites. The chemical exchange between inclusion fluid and olivine started soon after entrapment, at peak P-T conditions of 1.6–1.9 GPa and 650–700 °C, and continued during cooling along the retrograde path, with the coexistence of olivine and magnetite with orthopyroxene, chlorite, talc, antigorite and the destabilisation of olivine and antigorite into brucite and low-temperature chrysotile serpentine, as recognised by Raman analyses. Thermodynamic modelling and mass balance calculations demonstrate that the water component of fluid trapped in the inclusions of metamorphic olivine is expected to trigger the oxidation of the fayalite component in olivine, producing a mineral assemblage made of magnetite + orthopyroxene and molecular hydrogen, where the elemental redox processes are Fe2+ of olivine that oxidizes to Fe3+ and H+ of water that reduces to H2. Probable H2 trapped in the olivine host close to the inclusion wall has been detected by Raman spectroscopy. To corroborate its presence, we performed quantitative mass spectrometry analyses of the fluid phase trapped in the multiphase inclusions and of the olivine crystals hosting the inclusions, revealing that 1 kg of olivine matrix contains 6.2 ± 0.1 mmol of H2. We identify two synergistic driving forces of the whole process, which has the peculiarity to produce molecular hydrogen at apparently oxidising conditions: i) the building up of an epitaxial interface between olivine and magnetite, and ii) the olivine ability to trap H2 at high pressures. The olivine + H2O system of these natural microreactors simulates a process of oxidation of the mantle olivine by water, with production of H2 at pressure and fO2 conditions (FMQ + 2) at which water reduction is considered an unlike mechanism.

High-Temperature Oxidation of Magnesium- and Iron-Rich Olivine under a CO2 Atmosphere: Implications for Venus

Understanding crust–atmosphere interactions on Venus is fundamentally important to interpretations of Venus’ surface spectroscopic data. Olivine, in basaltic crust, is oxidized under a heated CO2 atmosphere. However, the oxidation rates, product assemblages and spectral characteristics of olivine samples with different Fa# values remain largely unclear. Herein, we investigated the oxidation of olivine with different Fa# values (Fa09, Fa29 and Fa71) under CO2 atmosphere at 470 °C and 900 °C and characterized the oxidation products (both microscopically and macroscopically), conversion rates and VNIR spectra. The results showed that the oxidation of olivine produced magnesioferrite, magnetite, laihunite, hematite and maghemite at 470 °C and hematite, magnetite, magnesioferrite and amorphous SiO2 at 900 °C. Both high temperature and high Fa# values accelerated the oxidation rates. The production of oxide coatings on olivine grains (74 μm in size) was estimated to be completed within tens to hundreds of years at 470 °C in natural settings, with even shorter periods under higher temperatures. Thus, CO2 oxidation would quickly eliminate olivine spectral characteristics, and spectral parameters at 850 and 1020 nm, as well as other relevant spectral windows (considering shifts induced by the elevated temperature), could be used to trace olivine oxidation processes. This work presented a case study connecting microscopic features to spectral characteristics for Venus’ surface–atmosphere interactions. Further studies considering more realistic Venus’ surface–atmosphere conditions will be essential to better interpret the measured spectroscopic data and determine the origins of the high emissivity detected on elevated terrain on Venus.

Grain-size-sensitive creep of olivine induced by oxidation of olivine in the Earth's deep upper mantle: Implications for weakening of the subduction interface

Creep behavior of olivine has been studied at pressures of 9–16 GPa and temperatures of 960–1450 K, which is equivalent to the conditions in/around the slab subducted into the deep upper mantle, using a combination of synchrotron radiation and a deformation-DIA apparatus. At pressures above 12 GPa, the creep strength of olivine was primarily controlled by Peierls creep, and nucleation of wadsleyite caused softening by a contribution of grain-size-sensitive creep to deformation. Similarly, oxidation of deforming olivine caused a significant softening due to another grain-size-sensitive creep presumed to be dislocation-accommodated grain boundary sliding (dislGBS) of olivine with a low activation enthalpy (~260 kJ/mol) at pressures of 9–12 GPa. Extrapolation of the obtained flow law of dislGBS to mantle conditions predicts a formation of a weak layer along the slab-asthenosphere interface in the deep upper mantle. The viscosity contrast between the weak layer and the surrounding asthenosphere is estimated to be up to 3 orders of magnitude, resulting in a partial slab-asthenosphere decoupling in the deeper part of the subduction zones. The weak layer would allow for the occurrence of trench-parallel flow above/beneath the slab.

Solubility of gold in the reduced carbonic fluid

The first experimental data on the gold solubility in the fluid with composition CO-CO2 and C-O-S with small water content at the pressure 200 MPa and temperature 950°С are reported. Solubility in the fluid C-O-S is about 27 ppm. Estimation of the solubility of gold in fluid CO-CO2 with 10–15 mol.% CO is less accurate: its value is at least 2–3 ppm, but it probably can reach 200–300 ppm. The high solubility of gold found in this work, and previously Pt (Simakin et al., 2016), in the reduced carbon dioxide can explain the formation of these noble metal ore occurrence in the Gluli intrusion (Polar Siberia) by their fluid extraction and re-deposition at temperatures below the solidus. The reducing of a substantially oxidized carbon dioxide fluid, established by mineralogical sensors, was probably caused by subsolidus oxidation of olivine.

The effect of oxidation on the mineralogy and magnetic properties of olivine

Although nucleation of magnetite and/or hematite along dislocations upon oxidation of olivine has been observed by many workers, the effect of oxidation on the magnetic properties of the sample with specific mineralogical alterations has not been studied. Therefore, we investigate this problem using a set of time series 1 bar oxidation experiments at 600 and 900 °C. Results show rapid olivine oxidation and alteration at both 600 and 900 °C, forming magnetite and hematite associated with a change from paramagnetic to ferromagnetic behavior after oxidation. Magnetite and hematite nucleate along dislocations and impurities in the crystal structure, along with surface coatings and within cracks in the crystals. Fresh, unaltered mantle xenoliths containing magnetite have been interpreted as having formed in cold tectonic regimes in the mantle, rather than through oxidation during or after ascent. Mantle xenoliths rapidly ascend through the mantle with estimates of the ascent of up to 90 km/h (3 GPa/h) based on the diffusion profile of water in mantle olivine. The rates correspond to xenoliths ascending through the mantle over hours and not days or weeks. Our results show that olivine oxidation and alteration can occur in days to weeks at 600 °C and within minutes at 900 °C. Therefore, if the xenolithic material is transported to the surface in a cold magma (at temperatures ≤600 °C), then the timescale of ascent is likely not long enough for oxidation to cause magnetite formation or a ferromagnetic signature to occur. However, if the material is transported in a hot oxidized basaltic magma (with temperatures ≥900 °C), then oxidation can cause magnetite formation and a ferromagnetic signature.

Gold Solubility in Reduced Carbon-Bearing Fluid

The paper presents first experimental data on gold solubility in CO–CO2 and C–O–S fluids with a low H2O concentration under reduced conditions, a pressure of 200 MPa, and a temperature of 950°C. Gold solubility in C–O–S fluid is approximately 27 ppm. The estimate of gold solubility in CO–CO2 fluid with 10–15 mol % CO is less accurate, and the solubility is no lower than 2–3 ppm but may reach 200–300 ppm. The high gold solubility in reduced CO2 fluid determined in the course of this research, and our earlier estimates of high platinum solubility (Simakin et al., 2016), may explain the deposition of mineralization of these noble metals in the Guli intrusion, polar Siberia, as a result of fluid extraction of these metals and their redeposition at a temperature slightly below the solidus. The reduction of the largely oxidized CO2 fluid, which was determined using mineralogical sensors, was likely related to the subsolidus oxidation of olivine.

The fate of olivine in the lower crust: Pseudomorphs after olivine in coronitic metagabbro from the Grenville Orogen, Ontario

Orthopyroxene-oxide symplectites after olivine are among the most enigmatic features of corona assemblages in metagabbros. Two coronitic metagabbro bodies from the Algonquin suite in the Grenville Orogen, Ontario, contain exceptionally well preserved orthopyroxene+Fe–Ti oxide symplectite formed during prograde Ottawan (ca. 1060Ma) granulite-facies metamorphism. Based on textural evidence, we propose a new hypothesis for the formation of these symplectites. Under oxidising conditions associated with fluid infiltration, magmatic olivine and ilmenite underwent a coupled reaction whereby magnetite produced by oxidation of olivine replaced adjacent igneous ilmenite. Ilmenite was re-precipitated as a fine-grained intergrowth with orthopyroxene and some magnetite in the former olivine sites. This hypothesis is supported by textural evidence showing partial replacement of magmatic ilmenite by magnetite and a close spatial association between magmatic oxides and orthopyroxene+Fe–Ti oxide symplectite, which locally radiates from ilmenite into olivine. Measured orthopyroxene/oxide ratios in the symplectite (20–35% oxides) agree with the ratio predicted from the proposed reaction (ca. 30%). Coronas and pseudomorphs formed during high-grade metamorphism, with increasing fO2 interpreted to result from fluid infiltration at near-peak conditions of ca. 13kbar, 800°C. The same samples contain red-brown fine-grained aggregates interpreted as iddingsite pseudomorphs after olivine. Raman spectroscopy suggests that the iddingsite consists largely of amorphous silica and Fe-hydroxide; textural evidence indicates that it formed by late-stage oxidation and hydration of olivine that survived earlier metamorphism. The unusual co-occurrence of granulite-facies pseudomorphs after olivine with an alteration product formed at near-surface conditions indicates that some olivine may survive protracted high-grade metamorphism in environments where fluid access is limited.

A combined electron microprobe (EMP) and Raman spectroscopic study of the alteration products in Martian meteorite MIL 03346

AbstractWe examine the secondary alteration products in MIL 03346 using Raman spectroscopic and electron microprobe traverses. Discussion focuses on the single olivine in ,177 supplemented with observations from ,168 and ,169. Traverses start at the rim and progress into the interior. Dark brown, nearly opaque, laihunite [Fe2+Fe3+2(SiO4)2] is present as overgrowths, and 20–50 µm veins of reddish‐brown stilpnomelane [(K,Na,Ca)4(Ti0.1,Al2.3,Fe3+35.5,Mn0.8,Mg9.3) (Si63Al9)(O,OH)206∗n(H2O)] occur inside the olivine. Stilpnomelane crosscuts and postdates the laihunite; veins are in sharp contact with the host olivine but lined by ~5 µm films of jarosite [KFe3+3(SO4)2(OH)6] from a later generation of alteration. An interstitial laihunite also hosts stilpnomelane. The most recent secondary phases are gypsum and bassanite in our X‐ray maps of ,168 and ,169. Ca‐sulfates were not observed in X‐ray maps of ,177 but were detected in our Raman point count. All sulfates are believed to be Martian. The groundmass of MIL indicates rapid cooling from elevated temperatures with fO2 near QFM. Reports of laihunite synthesis by olivine oxidation at elevated temperatures (100–800°C) suggest the overgrowths formed during consolidation. In terrestrial rocks, stilpnomelane is a product of late diagenesis to garnet‐grade metamorphism. In MIL, stilpnomelane appears to be a secondary phase formed at the lower end of this stability range, at conditions akin to diagenesis. Raman spectra indicate that the stilpnomelane, jarosite, and Ca‐sulfates are hydrated. The stilpnomelane contains Cl− and was followed by jarosite, a product of acid alteration, and the deposition of Ca‐sulfates and halide salts from more neutral chloride solutions.

Magnetite-orthopyroxene symplectites in gabbros of the Urals: A structural track of olivine oxidation

The chemical compositions of magnetite-orthopyroxene symplectites (MOS) and rock-forming minerals—olivine (Ol), clinopyroxene (Cpx), and magnetite(Mt)—have been studied in 20 samples of olivine-bearing rocks in the Urals, including troctolite, olivine gabbro, and gabbronorite. MOS are orthopyroxene (Opx) monocrystals up to 500 μm in size containing myrmekite-like magnetite intergrowths up to 20–30 μm in width. According to the microprobe examination, the dark-colored minerals are characterized by a high Fe mole fraction F = Fe/(Fe + Mg) = 0.20–0.50, whereas F = 0.33–0.65 is typical of the bulk rock compositions. The plagioclase varies in composition from An90 to An50. No significant compositional difference has been established between the MOS and rock-forming minerals. The F opx and F Ol are closely correlated (linear trend, r = 0.97); F Ol/F Opx is ∼1.2. Similarly, a positive correlation between F Opx and F Cpx is noted (linear trend, r = 0.90); F Opx/F Cpx is ∼1.2. The crystallization temperature of the Ol-Opx-Cpx assemblage is roughly estimated at 700–800°C. A high positive correlation (r = 0.95) is established between the TiO2 contents in the magnetites from the MOS (Mt1) and host rock (Mt2). The Mt1/Mt2 ratio reaches ∼0.8, implying that Mt1 contains somewhat less TiO2 than Mt2. Hence, the rock-forming and MOS minerals make up an equilibrium assemblage. As follows from the structural pattern, symplectites were formed as products of the reaction between olivine and oxygen in the solid state with the entire volume of the rock involved in the oxidation; i.e., the distance of the diffusion was significant. Free oxygen appeared as a product of the dissociation of the water penetrating into the hot gabbro and ultramafic rocks at the initial stage of the tectonic extension and high-temperature hydration. According to the redox state of dunite coexisting with gabbro, the oxygen fugacity is estimated at +2.7 log units of fO2 relative to the QFM buffer. The structure and products of the olivine oxidation were eventually obliterated in the course of the hydration.

Oriented chromite–diopside symplectic inclusions in olivine from lunar regolith delivered by “Luna-24” mission

Calcium–chromium rich lamellae in olivine grain No. 1611 from the Luna-24 regolith were studied with FEG-EMPA and TEM. The lamellae consist of a worm-like intergrowth of FeCr2O4 chromite (Chr) and CaMgSi2O6 diopside (Di), with a Chr:Di modal proportion of 1:3. The linear extension of the lamellae and crystallographic orientation relationships among the symplectite phases and the olivine suggest that the lamellae nucleated at deformation defects in the olivine host. Calcium depletion haloes surrounding the lamellae amount to about 75μm and indicate that the chromite+diopside lamellae were formed by segregation of calcium and chromium from the host olivine into the lamellae without addition of calcium and/or chromium from outside the olivine. The segregation of calcium and chromium and, consequently, the growth of the symplectic lamellae were diffusion-controlled.The segregation of a calcium–chromium component from the host olivine was associated with oxidation of divalent to trivalent chromium. Oxidation was facilitated by dehydrogenation, which was driven by decompression and/or a change in redox potential. Hydrogen point defects in the original olivine with H+ substituting for divalent cations on the M-sites provided the necessary electron acceptors for the oxidation of chromium and after electron transfer left olivine as molecular H2. The internal microstructure of the lamellae suggests that exsolution of the calcium–chromium rich lamellae from the host olivine and formation of the chromite–diopside symplectic intergrowth occurred simultaneously. The time scale derived from diffusion modeling of the calcium depletion haloes around the lamellae indicates a thermal event on the order of several months to several hundred years at most.Symplectic inclusions found in olivine from lunar, martian and terrestrial rocks are similar with respect to their shape, crystallographic orientation relationships, and internal microstructure of the spinel–clinopyroxene intergrowth. These features are inconsistent with “dry” olivine oxidation that is by the uptake of oxygen. Dehydrogenation of OH-bearing precursor olivine is suggested as a general mechanism of oxidation accompanied by chemical segregation and phase separation resulting in symplectite formation.

Olivine-Respiring Bacteria Isolated from the Rock-Ice Interface in a Lava-Tube Cave, a Mars Analog Environment

The boundary between ice and basalt on Earth is an analogue for some near-surface environments of Mars. We investigated neutrophilic iron-oxidizing microorganisms from the basalt-ice interface in a lava tube from the Oregon Cascades with perennial ice. One of the isolates (Pseudomonas sp. HerB) can use ferrous iron Fe(II) from the igneous mineral olivine as an electron donor and O(2) as an electron acceptor. The optimum growth temperature is ∼12-14°C, but growth also occurs at 5°C. Bicarbonate is a facultative source of carbon. Growth of Pseudomonas sp. HerB as a chemolithotrophic iron oxidizer with olivine as the source of energy is favored in low O(2) conditions (e.g., 1.6% O(2)). Most likely, microbial oxidation of olivine near pH 7 requires low O(2) to offset the abiotic oxidation of iron. The metabolic capabilities of this bacterium would allow it to live in near-surface, icy, volcanic environments of Mars in the present or recent geological past and make this type of physiology a prime candidate in the search for life on Mars.

Lamellar pyroxene-spinel symplectites in lunar olivine from the Luna 24 regolith

Cr-Ca lamellae in a magnesian olivine grain (section 1611) from the Luna 24 regolith were investigated in detail by electron microprobe analysis (EMPA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the lamellae are parallel to the (100) plane of oxygen closest packing in olivine and consist of regular vermicular intergrowths of two phases, diopside (Di) and chromite (Chr), in the volume proportion Di: Chr ≈ 3: 1. The bulk chemical composition of the lamellae is approximated as Ca2Mg2Fe2+(Cr3+)2Si4O16. They are identical in phase composition to type A, F, and E symplectites from Apollo lunar samples [9]. Based on morphology and phase composition, the lamellar aggregates in the olivine grain from the Luna 24 regolith were classified as pyroxene (Px)-spinel (Spl) symplectites of a lamellar type, the formation of which was related to olivine oxidation at IW ≤ logfO2 ≤ QFM. The obtained data indicate a solid-phase mechanism of lamella formation and the existence of a lamellar precursor phase, which transformed subsequently into the Px-Spl symplectite. It was supposed that uvarovite-knorringite garnet produced by the oxidation of olivine at high pressures and t > 800°C could be the transitional phase during symplectite formation. The subsequent conversion of the garnet into the low-pressure assemblage of Px-Spl symplectites could occur via cellular decomposition in accordance with the reaction Ca2MgCr2Si3O12 + (Mg,Fe)2SiO4 = 2CaMgSi2O6 + FeCr2O4. The reported results are the first data of a detailed nanomineralogical investigation of lamellar Px-Spl symplectites in lunar olivine.

Fe mineralogy of rocks from the Vredefort impact structure investigated with Mössbauer spectroscopy

The Vredefort impact structure in South Africa is the largest and oldest remnant impact structure on Earth. Observations from above the crater reveal lower than average magnetic field intensities, but the rocks in the crater have been shown to possess much higher magnetic intensities than the regional average that varies on a centimeter scale. Various mechanisms, including the presence of single domain magnetite structures, have been proposed for this anomaly. Mossbauer spectroscopy has been applied to study the Fe-mineralogy of samples from near the centre of the Vredefort crater. Transmission Mossbauer measurements on bulk and microtomics sections of samples showed that the magnetic minerals were magnetite and hematite, and we suggest that oxidation of olivine during the impact is responsible for the magnetic properties of the rocks.

Hematite and magnetite precipitates in olivine from the Sulu peridotite: A result of dehydrogenation-oxidation reaction of mantle olivine?

Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3–6.6 GPa; 853–957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101] Mt //[001] Ol , [110] Mt //[011] Ol , and [011] Mt //[011] Ol ; and [0001] Hm //[100] Ol and [1010] Hm //[001] Ol . These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe 2+ to Fe 3+ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6–11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

Magnetic properties of olivine basalt: Application to Mars

The natural remanent magnetisation (NRM) of basalt lava containing (oxidised) magnetic phases is usually assumed to be proportional to the weight percent of magnetic phases. It is shown here that olivine basalt has a different behaviour. The NRM intensity increases at an increasing rate with the amount of magnetic phases. This is attributed to the oxidation of olivine during cooling of the basalt that leads to the exsolution of magnetite in a single domain state. In this way, olivine basalt is found to become an order of magnitude more magnetic than basalt that does not contain olivine. A simple explanation for the magnetic anomalies on Mars is offered, based upon these findings and Mössbauer spectroscopy data from Gusev crater on Mars.

Phase transformations and reaction kinetics during the temperature-induced oxidation of natural olivine

This work describes the sequence of transformations and their reaction kinetics during the oxidation of olivine in air at high temperature. A natural olivine sample from the layered series of the Ivrea-Verbano igneous complex (Western Alps, Italy) was heated in the temperature range 25-1300 °C in air and investigated by in situ, real time powder X-ray diffraction (PXRD). The evolution of the peaks (measured integrated intensities) was followed in non-isothermal conditions using variable heating rates (b = 20, 22, 25, 27, and 30 °C/min). The total time of the experiments ranged from about 256 (b = 30 °C/min) to 277 (b = 20 °C/min) min including the time for the data collections. An additional isothermal run was performed at 800 °C. The analysis of the kinetic data was attempted with the use of different equations including the classical Avrami theory for solid-state reactions. The kinetic results were confirmed by independent experimental data from electron microscopy (SEM, TEM). In the transformation sequence, hematite appears at about 600 °C producing amorphous segregations. of silica that later recombine with forsterite to form pyroxene Hematite is stable up to 1130 °C where it is transformed into magnetite. The rate limiting step for the formation of hematite is a twodimensional diffusion with constant or decelerating nucleation rate and apparent activation energy of 15 kcal/mol. The concentration of Fe 3+ in Fe-rich regions favors the heterogeneous nucleation of hematite, which may take place on existing defects or at the grain boundaries with impurity phases such as serpentine. At 1130 °C, magnetite is formed at the expenses of hematite, with a contracting volume interface-controlled reaction in two or three dimensions with an apparent activation energy of 30-31 kcal/mol. The hematite to magnetite transformation is direct, without a metastable amorphous intermediate. It is described by the “shrinking core model,” with the formation of a magnetite outer layer at the surface of the hematite particles that proceeds toward the core of the reacting hematite by diffusion of the oxygen throughout the newly formed magnetite layer. Its rate is limited by the advancement of the reaction front. The rate-limiting step for the formation of pyroxene is two-dimensional diffusion with decelerating nucleation rate with an apparent activation energy of 29 kcal/mol.

Conversion Electron Mössbauer Spectroscopic Study of Subsolidus Oxidation of Olivine

Single crystal samples of natural olivine, Mg1.82Fe0.17Ni0.01SiO4, have been oxidized in air at temperatures of 600 and 800°C, respectively, and studied by conversion electron and transmission Mossbauer spectroscopy. Oxidation of olivine was found to result in ferriolivine and spinel ferrite formation; the corresponding kinetic data also were discussed.

Segregation structures in vapor-differentiated basaltic flows

Vesicle cylinders represent a spectacular kind of segregation structure involving residual liquids formed in situ during the cooling of lava flows. These vertical pipes, commonly found within basalt flows typically 2–10 m thick, are interpreted as the product of a vapor-driven differentiation process. The olivine phenocrysts and the earliest generation of groundmass olivines found in cylinder-bearing basalts appear to have been generally affected by magmatic oxidation, resulting in high-temperature iddingsite (HTI) alteration. This feature is also observed within cylinder-free basalt flows which exhibit other kinds of vesicular segregation structures, such as vesicle-rich pegmatoid segregation sheets and/or segregation vesicles. Detailed textural, petrological, and geochemical characteristics of two types of cylinders, three types of vesicle sheets, and five types of segregation vesicles are described, based on the study of 12 occurrences of HTI-bearing basalt flows from oceanic shield volcanoes or continental basalt plateaus. We propose a general classification of these segregation structures likely to derive from vapor differentiation. Flow thickness is probably the main factor influencing their morphology. Finally, we suggest that the concomitant occurrence of olivine oxidation and vapor-differentiation effects results from the late persistence of water oversaturation after eruption, perhaps due to a high rate of magma ascent.

Symplectic Reaction in Olivine and the Controls of Intergrowth Spacing in Symplectites

Symplectite growth is analysed in terms of non-equilibrium therin some reactions, many other textures are more inmodynamics and maximum rate of energy dissipation. For a given fluenced by diffusion. Here we develop a general theory reaction, the spacing k of lamellae or rods is predicted to be for a common texture of that type, termed symplectic. proportional to the cube root of Ld/v, where v is reaction rate and Symplectites are mineral intergrowths produced by solidL is the Onsager diffusion coefficient of a reference element in the state reaction (Sederholm, 1916), the grains being elongreaction front of width d. The result is comparable with, but not ated perpendicular to the reaction front at which the identical to, metallurgical theory for discontinuous precipitation in product minerals grew from a single reactant mineral. alloys. It is reasoned that concentration-gradient constraints place The fine-scale, approximately periodic spacing of ina lower limit on k, which depends on grain-boundary energy c. An tergrowth reflects difficult diffusion of some chemical upper limit c > 0·3 J/m is thus estimated using literature elements. Close analogies have been noted with disdata from experimental oxidation of olivine. Combined with new continuous precipitation (also known as cellular preobservations on exsolution symplectites in olivine from the Lilloise cipitation) in alloys (Voll, 1982; Boland & van Roermund, intrusion, Greenland, this suggests that the exsolution reaction took 1983; Mongkoltip & Ashworth, 1983; Boland & Otten, place above 800°C. Using previous modelling of a corona with a 1985). An example of a rod symplectite, with the minor symplectic layer, c > 1 J/m is estimated for hornblende–spinel mineral as rods in the major (host) mineral, is myrmekite symplectite. The energy driving diffusion plus grain-boundary pro(quartz rods in plagioclase). Alternatively, the minor duction in the reaction front was a small proportion of the overall mineral can be lamellar. Many symplectites occur in affinity of the corona reaction. The theory explains symplectite metamorphic coronas (Sederholm, 1916), which are regrowth over a wide range of igneous and metamorphic temperatures action rims produced between two reactant minerals. and timescales. The finer the intergrowth, the shorter the diffusion range but also the more grain-boundary area in the symplectite. The scale must be determined by a balance involving diffusive energy dissipation and grain-boundary energy.