porosity evolution and crystallization driven fragmentation during weathering of andesite 1 a 10 m thick andesitic sill intrusion from the neuquen basin argentina shows spectacular examples of spheroidal weathering and liesegang banding the liesegang patterns demonstrate how andesite blocks initially cut out by a preweathering joint set are subdivided by fractures formed during the spheroidal weathering process the stresses that cause fracturing originate from the growth of ferrihydrite and calcite in the pore space of the andesite partly at the expense of original ilmenite amphibole and plagioclase the porosity evolution and fracture formation during progressive weathering was characterized by scanning electron microscopy studies x ray computed tomography and he and hg porosimetry fresh andesite has a porosity of approximately 8 and a major fraction 80 of the pore volume is composed of pores less than 10 m in diameter the extent of pore filling during weathering increases with pore size pores more than 100 m are almost completely filled by an intimate intergrowth of calcite and ferrihydrite whereas pores less than 10 m are filled less than 50 the fracturing associated with spheroidal weathering is caused by mineral growth in the largest pores which account for 10 20 of the total porosity the periodic precipitation of the weathering product to form liesegang bands indicates a significant supersaturation threshold before nucleation commences the increase in the weathering product growth rate with increasing size is therefore most likely due to higher nucleation probabilities in larger pores

microstructure and porosity evolution during experimental carbonation of a natural peridotite abstract carbonation experiments t 100 200 c pco2 14 6 19 4 mpa were performed under closed system conditions on a natural peridotite from almklovdalen w norway to investigate the microstructure and porosity evolution as a function of reaction progress the unreacted samples cubes of 2 5 3 cm3 were composed mainly of forsteritic olivine 80 90 vol clinochlore orthopyroxene amphibole and chromite initial porosities were on the order of 6 8 due to the presence of abundant inter and intragranular microfractures microstructural observations after 7 42 days of experimental treatment showed that olivine was by far the most reactive phase after 7 days at 100 c no carbonate products were formed but longer reaction times produced small amounts of magnesite on the surface and within the fracture network of the peridotite cubes increasing temperatures led to faster reactions and pore space filling by magnesite in addition replacement rims up to 50 m thick composed of magnesite and amorphous silica formed around individual olivine grains the replacement began with the formation of an amorphous silica layer that retained the shapes of the initial olivine grains this layer remained permeable during the reaction as it allowed further dissolution of the underlying olivine increasing extents of carbonation were accompanied by decreasing porosities and enhanced cementation of grain boundaries about 50 of the initial porosity was closed when carbonation reached about 10 the reacted samples showed no evidence of reaction induced cracking indicating that the carbonation reaction was self limiting in our experiments these results may have important implications for the efficiency of in situ co2 sequestration in peridotites

subarctic physicochemical weathering of serpentinized peridotite abstract frost weathering is effective in arctic and subarctic climate zones where chemical reactions are limited by the reduced availability of liquid water and the prevailing low temperature however small scale mineral dissolution reactions are nevertheless important for the generation of porosity by allowing infiltration of surface water with subsequent fracturing due to growth of ice and carbonate minerals here we combine textural and mineralogical observations in natural samples of partly serpentinized ultramafic rocks with a discrete element model describing the fracture mechanics of a solid when subject to pressure from the growth of ice and carbonate minerals in surface near fractures the mechanical model is coupled with a reaction diffusion model that describes an initial stage of brucite dissolution as observed during weathering of serpentinized harzburgites and dunites from the feragen ultramafic body fub se norway olivine and serpentine are effectively inert at relevant conditions and time scales whereas brucite dissolution produces well defined cm to dm thick weathering rinds with elevated porosity that allows influx of water brucite dissolution also increases the water saturation state with respect to hydrous mg carbonate minerals which are commonly found as infill in fractures in the fresh rock this suggests that fracture propagation is at least partly driven by carbonate precipitation dissolution of secondary carbonate minerals during favorable climatic conditions provides open space available for ice crystallization that drives fracturing during winter our model reproduces the observed cm scale meandering fractures that propagate into the fresh part of the rock as well as dm scale fractures that initiate the breakup of larger domains rock disintegration increases the reactive surface area and hence the rate of chemical weathering enhances transport of dissolved and particulate matter in the weathering fluid and facilitates co2 uptake by carbonate precipitation our observations have implications for element cycling and co2 sequestration in natural gravel and mine tailings

in situ carbonation of peridotite for co2 storage the rate of natural carbonation of tectonically exposed mantle peridotite during weathering and low temperature alteration can be enhanced to develop a significant sink for atmospheric co2 natural carbonation of peridotite in the samail ophiolite an uplifted slice of oceanic crust and upper mantle in the sultanate of oman is surprisingly rapid carbonate veins in mantle peridotite in oman have an average 14c age of 26 000 years and are not 30 95 million years old as previously believed these data and reconnaissance mapping show that 104 to 105 tons per year of atmospheric co2 are converted to solid carbonate minerals via peridotite weathering in oman peridotite carbonation can be accelerated via drilling hydraulic fracture input of purified co2 at elevated pressure and in particular increased temperature at depth after an initial heating step co2 pumped at 25 or 30 c can be heated by exothermic carbonation reactions that sustain high temperature and rapid reaction rates at depth with little expenditure of energy in situ carbonation of peridotite could consume 1 billion tons of co2 per year in oman alone affording a low cost safe and permanent method to capture and store atmospheric co2

intense fracturing and fracture sealing induced by mineral growth in porous rocks article i nfo mineral precipitation in the pores of a rock may exert a force which is called crystallization pressure this process has been studied experimentally and results bring a new look on the way fractures may develop and seal in natural systems cylindrical core samples of porous limestone and sandstone were left for several weeks in contact with an aqueous solution saturated with sodium chloride at 30 or 45 c under axial normal stress in the range 0 02 0 26 mpa the fluid was allowed to rise in the core samples by capillary forces up to a controlled height where evaporation and precipitation occurred the uniaxial deformation of the samples was measured using high resolution displacement sensors the samples were characterized using computed x ray tomography allowing therefore imaging in 3d the intensity and localization of the damage two kinds of damage could be observed firstly small rock fragments were peeled from the sample surface secondly

carbonation of ca bearing silicates the case of wollastonite experimental investigations and kinetic modeling abstract carbonation reactions of wollastonite have been investigated in experiments conducted at conditions relevant to geologic co 2 sequestration in subsurface environments t 90 c and pco 2 25 mpa experiments were performed in batch reactors on single mineral powders of selected grain size either in supercritical co 2 or in aqueous solutions with different alkalinities the main reaction products were calcite and amorphous silica calcite occurred either as a compact continuous coating of small crystals in the case of supercritical co 2 and in circum neutral ph aqueous solutions or as isolated larger crystals in acidic aqueous solutions measured extents of carbonation were faster in aqueous solutions than in supercritical co 2 extents of carbonation in aqueous acidic solutions were modelled using a geochemical code incorporating kinetic parameters for wollastonite dissolution and calcite precipitation taken from the literature comparison with experimental results shows that in aqueous solutions wollastonite dissolution is the rate limiting step of the carbonation reaction and that silica layers have only a minor passivation effect on the overall reaction rate in acidic solutions secondary calcite coating has only a minor effect on carbonation extent whereas the experimental data in circum neutral ph aqueous solutions are consistent with dissolution rates inhibited by the observed dense calcite coating a geochemical code incorporating a modified reactive surface model successfully accounted for this passivation effect the more efficient carbonation in acidic than in circum neutral ph solutions is thus ascribed to differences in microstructures of the secondary calcite layers due to differences in supersaturation states of calcite when its precipitation begins inhibitory effects of dense calcite coating are also invoked for limiting carbonation extents of wollastonite in supercritical co 2 secondary calcite precipitations are thus shown to play a major role both as coatings of reactive surfaces and by maintaining large undersaturation with respect to wollastonite dissolution

control of serpentinisation rate by reaction induced cracking abstract serpentinisation of mantle rocks requires the generation and maintenance of transport pathways for water the solid volume increase during serpentinisation can lead to stress build up and trigger cracking which ease fluid penetration into the rock the quantitative effect of this reaction induced cracking mechanism on reactive surface generation is poorly constrained thus hampering our ability to predict serpentinisation rate in geological environments here we use a combined approach with numerical modelling and observations in natural samples to provide estimates of serpentinisation rate at mid ocean ridges we develop a micromechanical model to quantify the propagation of serpentinisation induced cracks in olivine the maximum crystallisation pressure deduced from thermodynamic calculations reaches several hundreds of megapascals but does not necessary lead to crack propagation if the olivine grain is subjected to high compressive stresses the micromechanical model is then coupled to a simple geometrical model to predict reactive surface area formation during grain splitting and thus bulk reaction rate our model reproduces quantitatively experimental kinetic data and the typical mesh texture formed during serpentinisation we also compare the model results with olivine grain size distribution data obtained on natural serpentinised peridotites from the marum ophiolite and the papuan ultramafic belt papua new guinea the natural serpentinised peridotites show an increase of the number of olivine grains for a decrease of the mean grain size by one order of magnitude as reaction progresses from 5 to 40 these results are in agreement with our model predictions suggesting that reaction induced cracking controls the serpentinisation rate we use our model to estimate that at mid ocean ridges serpentinisation occurs up to 12 km depth and reaction induced cracking reduces the characteristic time of serpentinisation by one order of magnitude down to values comprised between 10 and 1000 yr the increase of effective pressure with depth also prevents cracking which positions the peak in serpentinisation rate at shallower depths 4 km above previous predictions

the role of silicate surfaces on calcite precipitation kinetics abstract the aim of this study is to illuminate how calcite precipitation depends on the identity and structure of the growth substrate calcite was precipitated at 25 c from supersaturated aqueous solutions in the presence of seeds of either calcite or one of six silicate materials augite enstatite labradorite olivine basaltic glass and peridotite rock calcite saturation was achieved by mixing a cacl2 rich aqueous solution with a nahco3 na2co3 aqueous buffer in mixed flow reactors containing 0 5 2 g of mineral rock or glass seeds this led to an inlet fluid calcite saturation index of 0 6 and a ph equal to 9 1 although the inlet fluid composition flow rate and temperature were identical for all experiments the onset of calcite precipitation depended on the identity of the seeds present in the reactor calcite precipitated instantaneously and at a constant rate in the presence of calcite grains calcite precipitated relatively rapidly on labradorite olivine enstatite and peridotite mainly composed of mg olivine surfaces but more slowly on augite and basaltic glass calcite precipitation rates however became independent of substrate identity and mass over time and all rates approach 10 9 68 0 08 mol s for 10 day long experiments and 10 9 21 0 2 mol s for 70 day long experiments scanning electron microscope images showed olivine enstatite and peridotite surfaces to be covered extensively with calcite coatings at the end of the experiments less calcite was found on labradorite and augite and the least on basaltic glass in all cases calcite precipitation occurs on the mineral rock or glass surfaces calcite precipitation on these surfaces however negligibly affects the dissolution rates of the silicate grains these results support ultramafic and basalt carbonation as a long term carbon storage strategy as calcite readily precipitates on the surfaces of minerals contained in these rocks without inhibiting their dissolution

enhanced co2 mineral sequestration by cyclic hydraulic fracturing and si rich fluid infiltration into serpentinites at malentrata tuscany italy abstract the magnesite deposits of malentrata tuscany italy were derived from serpentinite silicification carbonation of the ligurian ophiolite and represent a natural analogue of in situ co 2 mineral sequestration carbonation of magnesium silicate minerals e g serpentine olivine at temperatures below 200 c is an exothermal process involving incorporation of carbon dioxide into stable carbonates e g magnesite dolomite serpentinites at malentrata were transformed to a brownish friable rock characterized by the occurrence of opal chromian montmorillonite fe rich magnesite and minor iron sulfides and oxides widespread lenses of cohesive rocks occur within the altered serpentines resulting from the complete silicification of the protolith the pervasive alteration of serpentinite was accompanied by the formation of a network of magnesite and dolomite veinlets and large magnesite dolomite veins along major structures field observations petrography and mineral chemistry define the following crystallization sequence in the major veins i early microgranular fe poor magnesite ii comb textured fe rich magnesite and dolomite cementing the early brecciated magnesite vein infill and iii late quartz chalcedony and rare opal in the cavities the mineral assemblage observed both in veins and in host rock is indicative of low temperature hydrothermal alteration driven by si and co 2 rich fluids under relatively low ph conditions pervasive and cyclic hydraulic fracturing maintained a high structural permeability during the whole hydrothermal event creating conduits for the input and output fluids the concomitant presence of co 2 and silica in the fluid enhanced the congruent dissolution of serpentine and hampered immediate carbonate deposition the mg bearing solutions were focused out of the reactor zone and promoted massive carbonate veining along the main structural discontinuities this two steps reaction dissolution of serpentinite followed by carbonate precipitation outside the reacting volume seems to be very efficient and provide new insights on in situ and ex situ induced mineral carbonation and its industrial applications

reaction induced fracturing during replacement processes replacement processes are common transformation mechanisms in minerals and rocks at a variety of conditions and scales the underlying mechanisms are in general poorly understood but both mechanical and chemical processes are thought to be important replacement of leucite kalsi2o6 by analcime naalsi2o6 h2o is common in silica poor igneous rocks a 10 increase in volume is associated with the replacement process and this generates stresses that eventually cause fracturing of the reacting leucite experimentally reacted leucite samples display characteristic fracturing patterns that include both spalling of concentric onion skin like layers near the reacting interface and the formation of cross cutting often hierarchically arranged sets of fractures that divide the remaining leucite into progressively smaller domains these structures may explain the patchy alteration patterns observed in natural leucite samples and similar so called mesh textures associated with the serpentinization of olivine grains during hydration of mafic or ultramafic rocks they are also strikingly similar to larger scale patterns formed during spheroidal weathering processes a simple discrete element model illustrates the mechanics that control the formation of such systems and shows how these replacement processes may be accelerated due to the generation of new reactive surface area by hierarchical fracturing processes

spatially variable carbonation reactions in polycrystalline olivine abstract carbon dioxide co 2 injection into olivine rich mafic and ultramafic rocks is expected to result in the precipitation of divalent metal carbonate minerals permanently storing the co 2 underground previous experiments that used unconsolidated forsterite mg 2 sio 4 particles in experimental investigations of reactions with water and carbon dioxide have been useful for determining the identity rates of formation and spatial location of carbonate mineral reaction products however there remains a need for information regarding the influence of the internal pore structure and grain boundary surfaces on the extent and locations of these reactions in dense aggregates we conducted several experiments at 100 c and 100 bar co 2 using sintered san carlos olivine fo 90 and pure forsterite fo 100 cylinders and we documented the type and spatial distribution of the reaction products timing of carbonation was measured using in situ 13 c nmr spectroscopy without removing the sample from the reactor ex situ solid state nmr spectroscopy raman spectroscopy and electron microscopy were used to examine reacted samples and precipitates within 15 days magnesite is observed only on the surface of fo 90 after 53 and 102 days of reaction magnesite and amorphous silica are observed as a crust around the entire fo 100 cylinder and as isolated layers within the sample the spatial transition from an amorphous silica layer to the host fo 100 indicates that the development of amorphous silica did not impede further forsterite dissolution while earlier studies documented localized reactions at the grain scale the development of distinct zones of magnesite and amorphous silica suggest that divalent metal cations are mobile during carbonation of olivine grain boundaries pore structure and geochemical gradients strongly influence the locations of silicate mineral dissolution and carbonate mineral precipitation even in the absence of advective transport or confinement the clear spatial distribution of precipitates observed in this study indicates that carbonation in heterogeneous reservoirs may be strongly affected by grain scale microstructure

co2 sequestration and extreme mg depletion in serpentinized peridotite clasts from the devonian solund basin sw norway the conglomerates of the solund devonian basin of sw norway contain numerous locally up to 20 vol peridotitic clasts with concentric mm to 10 cm thick zones of varying red to black color the peridotite clasts show a clear alteration related textural evolution the least altered rocks are partly serpentinized peridotites showing a typical mesh texture with veins of serpentine magnesite and ni rich magnetite surrounding olivine fo91 relicts and its mg depleted clay like alteration product deweylite assemblage in the more advanced ophicarbonate stage the mesh cells contain calcite silica and are surrounded by talc in the final stage quartz calcite and hematite dominate the mineralogy and occur together with minor amounts of chromite talc cr chlorite and cr hydroandradite in tandem with this textural evolution is a decrease in mgo from 40 to 2 5 wt and a cao increase from 1 to 35 wt all peridotite clasts are characterized by high cr and ni concentrations the chemistry and the textural evolution show that the clasts formed by an extreme mg mobilization from the peridotite with development of secondary porosity and subsequent precipitation of calcite mgo removed from the clasts after burial is in part consumed by replacement reactions in the sediment matrix around the clasts where mg free minerals e g almandine are replaced by mg bearing minerals e g talc calculated apparent 87 sr 86 sr ratios of the clasts at 385 ma 0 7124 0 7139 corresponding to the inferred age of sediment deposition and incipient clast alteration indicate interaction with diagenetic basinal fluids we explain the reaction history as a three stage process involving a partial serpentinization of olivine in an oceanic environment b breakdown of olivine relicts to the deweylite assemblage resulting in mobilization of mgo under near surface conditions in a tropical devonian climate and c further mg mobilization and replacement of the deweylite assemblage by calcite and quartz after diagenesis sedimentary basins with abundant weathered peridotite represent potential sites for a permanent co2 storage by formation of calcite in a low temperature environment 2010 elsevier ltd all rights reserved

reaction enhanced permeability during retrogressive metamorphism abstract retrogressive metamorphism is in most cases associated with influx of external fluids into igneous or metamorphic rocks with low to negligible initial permeabilities thus fluid migration is associated with some process that creates the necessary channels for migration one such mechanism is associated with the volume changes resulting from volatilization reactions petrographic studies of a partly serpentinized troctolite from the duluth complex in minnesota reveal how the increase in solid volume associated with hydration of olivine phenocrystals generates an extensive network of microfractures in the surrounding plagioclase dominated matrix this microfracture network allows hydrous fluids to penetrate further into the rock a 2 d spring network model illustrates how expanding polygons or non spherical grains in an elastically deforming matrix cause dilatant microfracturing and porosity generation even if the overall stresses are compressive the model emphasises how fluid infiltration may be promoted by stress heterogeneities set up by metamorphic reactions

controls on rock weathering rates by reaction induced hierarchical fracturing abstract weathering of rocks takes place through complex processes involving an intimate coupling between mechanical disintegration of the rocks and interactions between the exposed rock surface the hydrosphere and the biosphere it is still poorly understood how this coupling affects weathering rates and patterns produced by weathering here we show that the observed spheroidal weathering patterns in basaltic intrusions dolerites in the karoo basin in south africa are created through volume expanding reactions which set up stresses that not only lead to spalling of spheroidal layers of rock but also drive a large scale hierarchical fracturing process by which the dolerite is continuously undergoing domain division the fracture processes generates new reactive surface area in a self accelerating manner thus providing a first order control on the total weathering rate

forsterite dissolution and magnesite precipitation at conditions relevant for deep saline aquifer storage and sequestration of carbon dioxide the products of forsterite dissolution and the conditions favorable for magnesite precipitation have been investigated in experiments conducted at temperature and pressure conditions relevant to geologic carbon sequestration in deep saline aquifers although forsterite is not a common mineral in deep saline aquifers the experiments offer insights into the effects of relevant temperatures and pco2 levels on silicate mineral dissolution and subsequent carbonate precipitation mineral suspensions and aqueous solutions were reacted at 30 c and 95 c in batch reactors and at each temperature experiments were conducted with headspaces containing fixed pco2 values of 1 and 100 bar reaction products and progress were determined by elemental analysis of the dissolved phase geochemical modeling and analysis of the solid phase using scanning electron microscopy infrared spectroscopy and x ray diffraction the extent of forsterite dissolution increased with both increasing temperature and pco2 the release of mg and si from forsterite was stoichiometric but the si concentration was ultimately controlled by the solubility of amorphous silica during forsterite dissolution initiated in deionized water the aqueous solution reached supersaturated conditions with respect to magnesite however magnesite precipitation was not observed for reaction times of nearly four weeks magnesite precipitation was observed in a series of experiments with initial solution compositions that simulated extensive forsterite dissolution the precipitation of magnesite appears to be limited by the process of nucleation and nucleation requires a critical saturation index between 0 25 and 1 14 at 95 c and 100 bar pco2 magnesite precipitation is fastest in the presence of an initial magnesite seed although magnesite precipitates do form on the surfaces of forsterite particles the presence of the forsterite surface does not significantly accelerate magnesite precipitation relative to solid free systems

reaction assisted hierarchical fracturing during serpentinization abstract hierarchical fracture patterns are the result of a fracturing process that successively divides rocks into smaller domains in a 2d outcrop section such fracture patterns are characterized by four sided domains and t junctions where new fractures stop growing at right angles to pre existing fractures here we describe fracturing of mm to dm thick serpentinized orthopyroxenite dykes in a dunite matrix from the leka ophiolite complex in nord trondelag norway the fracturing process is driven by volume changes associated with the serpentinization of the dunites during this process the orthopyroxenite dykes are extended by the expanding dunite and undergo considerable fracturing the resulting fracture patterns are hierarchical the cumulative distributions of fracture lengths show a scaling behavior that lies between a log normal and power law fractal distribution this is consistent with a simple fragmentation model in which domains are divided according to a top hat distribution of new fracture positions within unfractured domains reaction assisted hierarchical fracturing is also likely to be responsible for other structures commonly observed in serpentinized ultramafic rocks including the mesh textures observed in individual olivine grains and the high abundance of rectangular domains at a wide range of scales this mechanism may provide first order controls on fluid migration rates in a wide variety of geological settings

the interface scale mechanism of reaction induced fracturing during serpentinization peridotite serpentinization has first order effects on geochemical and petrophysical processes in the lithosphere this process induces intensive fracturing generating fluid pathways to facilitate the hydration of vast amounts of originally impermeable rocks but the mechanism linking interfacial reaction processes with fracture propagation has not been understood by combining microstructural characteristics of olivine lizardite serpentinization with fundamental aspects of interface coupled dissolution precipitation and crack growth theory we propose a microstructurally consistent self propagating fracturing mechanism fracturing is driven by stress generated from the growth and transformation of a metastable amorphous proto serpentine phase where stress is localized within surface perturbations etch pits and coalesced etch pits that originate from the anisotropic dissolution of olivine water migration into fractures reiterates the process resulting in hierarchical olivine grain segmentation our results indicate that the advancement of serpentinization at the grain scale is independent of solid state diffusion and does not rely on external forces

dissolution of olivine during natural weathering abstract naturally weathered olivine occurring as phenocrysts in hawai ian volcanic rocks from several volcanic centers and regolith outcrop settings and as tectonized olivines from several metadunite bodies in the southern appalachian blue ridge are all similarly corroded by natural weathering conical funnel shaped etch pits occur as individual pits base to base pairs of cone shaped pits or en echelon arrays etch pit shapes and orientations in the smallest etch pit arrays visible in conventional scanning electron microscopy resemble even smaller features previously reported from transmission electron microscope investigations of olivine weathering etch pits occur in samples with chemical and or mineralogical evidence of weathering and or are associated with or proximal or directly connected to fractures or exposed outcrop surface and therefore are formed by weathering and not inherited from pre weathering aqueous alteration e g serpentinization iddingsitization of these parent rocks many etch pits are devoid of weathering products natural weathering of olivine is surface reaction limited similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization recrystallization history eruption weathering exposure ages of the olivine s host rock and the local regolith history

phase transitions in the mgco2h2o system and the thermal decomposition of dypingite mg5 co3 4 oh 2 5h2o implications for geosequestration of carbon dioxide abstract the application of hydrated mg carbonates as co2 sequestering media is a pressing environmental challenge which requires a deep knowledge of the phase transitions occurring in the mg co2 h2o system as well as the thermal and structural stability of these phases in this paper we investigate the phase transition of nesquehonite mgco3 3h2o to dypingite mg5 co3 4 oh 2 5h2o occurring after an incubation of months and years in solution at ambient conditions however as the kinetics of this process resulted to be slow the phase transition of dypingite to hydromagnesite mg5 co3 4 oh 2 4h2o was investigated at non ambient conditions using in situ real time high resolution x ray powder diffraction moreover the thermal behaviour of dypingite and its decomposition have been also investigated with the aim to explore the appropriateness of this carbonate and the products of its decomposition as sinks of anthropogenic carbon dioxide the results suggest that the dypingite structure remains unaffected up to 438 k at temperature above this threshold dypingite transforms into hydromagnesite a further increase in temperature converts the well ordered hydromagnesite into a collapsed form at 528 k the heating of dypingite does not produce a loss of co2 as the intermediate phases have the same co2 mg molar ratio the final product of the heating is periclase mgo its nucleation occurs at temperature ranging from 573 to 663 k and it becomes the only phase in the temperature range 633 678 k these results highlighted that dypingite assures a stable storage of co2 in the conditions that prevail at the earth s surface moreover the transformation of dypingite in more thermodynamically stable hydromagnesite occurring without release of co2 enhances the safety of carbon dioxide disposal in solid form furthermore the observed volume changes during phase transitions which in turn could affect the porosity and permeability of the geological reservoir have been evaluated in order to improve the prediction of the safe and permanent storage of co2 in underground

olivine dissolution rates a critical review abstract the dissolution rates of olivine have been considered by a plethora of studies in part due to its potential to aid in carbon storage and the ease in obtaining pure samples for laboratory experiments due to the relative simplicity of its dissolution mechanism most of these studies provide mutually consistent results such that a comparison of their rates can provide insight into the reactivity of silicate minerals as a whole olivine dissolution is controlled by the breaking of octahedral m2 oxygen bonds at or near the surface liberating adjoining sio44 tetrahedra to the aqueous fluid aqueous species that adsorb to these bonds apparently accelerate their destruction for example the absorption of h h2o and at some conditions selected aqueous organic species will increase olivine dissolution rates nevertheless other factors can slow olivine dissolution rates notably olivine dissolution rates are slowed by lowering the surface area exposed to the reactive aqueous fluid by for example the presence and or growth on these surfaces of either microbes or secondary phases the degree to which secondary phases decelerate rates depends on their ability to limit access of the reactive aqueous fluids to the olivine surface it seems likely that these surface area limiting processes are very significant in natural systems lowering olivine surface reactivity in many environments compared to rates measured on cleaned surfaces in the laboratory a survey of the literature suggests that the major factors influencing forsteritic olivine dissolution rates are 1 ph 2 water activity 3 temperature and 4 mineral fluid interfacial surface area evidence suggests that the effects of aqueous inorganic and organic species are relatively limited and may be attributed at least in part to their influence on aqueous solution ph moreover the observed decrease in rates due to the presence of secondary mineral coatings and or the presence of microbes can be attributed to their ability to decrease olivine surface area directly exposed to the reactive aqueous fluid as the reactivity of forsterite surfaces are spatially heterogeneous its surface area normalized rates will tend to decrease as it dissolves even in the absence of a mineral or bacterial coating each of these factors limits and or influences the application of forsterite dissolution to 1 enhanced weathering efforts 2 mineral carbonation and 3 the low temperature generation of hydrogen or hydrocarbons via the oxidation of its divalent iron

influence of amorphous silica layer formation on the dissolution rate of olivine at 90 c and elevated pco2 for mitigating against rising levels of atmospheric co2 carbonation of m2 bearing silicates has been proposed as a possible option for sequestering co2 over long time spans due to its rapid far from equilibrium dissolution rate and its widespread occurrence in mafic and ultramafic rocks olivine has been suggested as a potentially good candidate for achieving this goal although the efficacy of the carbonation reaction still needs to be assessed with this as a goal the present study aims at measuring the carbonation rate of san carlos olivine in batch experiments at 90 c and pco2 of 20 and 25 mpa when the reaction was initiated in pure water the kinetics of olivine dissolution was controlled by the degree of saturation of the bulk solution with respect to amorphous silica this yet unrecognized effect for olivine was responsible for a decrease of the dissolution rate by over two orders of magnitude in long term 45 days carbonation experiments with a high surface area to solution volume ratio sa v 24 600 m 1 the final composition of the solution was close to equilibrium with respect to sio2 am independent of the initial concentration of dissolved salts nacl and naclo4 ranging between 0 and 1 m and with an aqueous mg si ratio close to that of olivine no secondary phase other than a ubiquitous thin 40 nm si rich amorphous layer was observed these results are at odds with classic kinetic modeling of the process due to experimental uncertainties it was not possible to determine precisely the dissolution rate of olivine after 45 days but the long term alteration of olivine was indirectly estimated to be at least 4 orders of magnitude slower than predicted taken together these results suggest that the formation of amorphous silica layers plays an important role in controlling the rate of olivine dissolution by passivating the surface of olivine an effect which has yet to be quantified and incorporated into standard reactive transport codes

current induced scour along a breakwater at ventura harbor ca experimental study this paper describes a 1 25 scale movable bed physical model study of scour resulting from longshore current on the leeside of the detached breakwater at ventura harbor ca usa the current induced scour developed as an unanticipated result of structural modifications that reduced chronic shoaling of the main navigation channel the physical model was calibrated by adjusting the total flow discharge to achieve equilibrium scour development that matched the scour hole measured at ventura harbor the calibrated model was then used to predict future scour potential and to optimize the design for remedial toe protection intended to prevent leeside armor layer damage on the detached breakwater the optimized toe protection design cross section used 25 less stone than the initial remediation design and the new design prevented an estimated us 475 000 in additional breakwater damage that would have occurred in the absence of adequate toe scour protection in addition the model study identified and eliminated several construction problems that could have caused significant unforeseen expenditures

experimental study of the carbonation of partially serpentinized and weathered peridotites abstract carbonation of partially serpentinized and weathered peridotites was studied experimentally under hydrothermal conditions t 200 c p co 2 130 180 bars experiments were performed in a closed system using whole rock drill core samples height 1 cm diameter 1 cm as starting material the initial samples were composed mainly of meshwork serpentine relicts of primary olivine and an olivine weathering product deweylite assemblage two types of solutions each with a total salt content corresponding to that of average seawater 35 g l dissolved salts were used 1 a na ca cl solution 12 5 g l cacl2 22 5 g l nacl and 2 a nacl solution 35 g l nacl after 15 25 days of experimental treatment the samples were partly covered with carbonates in addition noticeable carbonation reactions had occurred below the sample surfaces within zones with thicknesses up to 250 m in the na ca cl solution both the olivine relicts and the deweylite assemblage were partly replaced by calcite along the surrounding serpentine veins however the extent of calcitization was found to be considerably larger for the deweylite assemblage than for the olivine bulk fluid analyses show an increase in the mg and si concentrations with reaction time in the nacl solution the deweylite assemblage was partly dissolved resulting in large voids within the reaction zone in contrast the olivine was replaced by magnesite under the conditions of our experiments the meshwork serpentine was not reactive but aided fluid infiltration into the rock samples the experimentally produced microtextures closely resemble those found in natural examples our study elucidates the mechanisms by which carbonates form in ultramafic rocks under relatively high p co 2 t conditions and particularly in the presence of ca bearing aqueous solutions the existence of a serpentine meshtexture and the presence of weathering products formed from primary mg silicates may have significant beneficial effects on in situ co2 mineral sequestration in ultramafic rocks

in situ sequestration of atmospheric co2 at low temperature and surface cracking of serpentinized peridotite in mine shafts abstract the investigation of carbonate formation at low temperature during weathering of ultramafic rocks has become increasingly important as it represents an analog study for a cost efficient carbon disposal strategy here we present new insight into carbonate formation under subarctic surface conditions obtained from extensively carbonated chromite mine shafts in the feragen ultramafic body e norway carbonation proceeded by formation of mm to cm thick carbonate surface coatings on the serpentinized peridotite host rock the surface coatings consist in most cases of pure lansfordite mgco 3 5h 2 o and sometimes nesquehonite mgco 3 3h 2 o heavy o and c isotopes are strongly enriched in the carbonate coatings relative to drip water samples from the mines and water samples from subaerial ponds the major element composition of the water samples reveal that infiltrating rainwater reacts rapidly with the peridotite mainly due to brucite dissolution within the mines discharging alkaline water forms thin films on walls and ceilings that evaporate upon circulation of cold dry air in mines with only one opening the carbonation is limited to the entrance area in mines with a more efficient air circulation due to multiple entrances carbonation is found throughout the mine evaporation of the mine water results in lansfordite saturation and precipitation and is accompanied by a rayleigh type distillation effect causing enrichment of the heavy c and o isotopes in the liquid from which the carbonates precipitate formation of the carbonate coatings involved fragmentation of the underlying serpentinite substrate fragmentation proceeded by subcritical cracking i e by a combination of substrate weakening due to brucite dissolution and stress build up due to lansfordite precipitation in fractures close to the surface reaction induced fracturing represents a positive feedback mechanism as it creates additional reactive surface area during the carbonation mining operations in the area ceased in the mid 1920s indicating that the described carbonation occurs on a time scale relevant for the disposal of anthropogenic co 2

experimental study of mg rich silicates carbonation at 400 and 500 c and 1 kbar the reactivity of olivines orthopyroxenes and serpentines chrysotile with co2 rich fluids was experimentally studied at 400 500 c and 1 kbar nacl levels of formation of solid carbonate phases were measured both by analysing the recovered gas phase and by step heating of the solid samples carbonation levels of several percents per hour from 3 to 57 in 4 h depending upon experimental conditions were measured with increasing efficiency in the order orthopyroxene chrysotile olivine in the case of olivine a positive impact of water fugacity and salinity on carbonation levels was evidenced microstructures of the samples showed that the carbonation reaction proceeded by dissolution precipitation a coupling between solid carbonate production mostly as magnesite and olivine serpentinisation was demonstrated isotopic compositions of carbon in gas phases and carbonates were measured they were found to be consistent with mass balance calculations additionally suggesting the existence of an accessory carbon bearing phase carrying negative 13c signatures this phase was identified by transmission electron microscopy as an ill ordered graphite phase which would account for about 15 of carbon mineral under these conditions

reaction induced porosity and onset of low temperature carbonation in abyssal peridotites insights from 3d high resolution microtomography abstract in a drillcore sample of serpentinized harzburgite from the uppermost oceanic crust mid atlantic ridge odp leg 209 site 1270 we demonstrate using high resolution 3d microtomography that micron sized open cavities are present the development of porosity is interpreted to result from dissolution of brucite and or olivine petrographic observations indicate that voids are integrated in a network of carbonate veins the formation of which is linked to changing alkalinity in conjunction with dissolution reactions partial carbonate filling of pore spaces indicates that under static conditions low temperature carbonation leads to clogging of fluid pathways and thus to a reduction in permeability electron microprobe analyses show that the inner walls of open voids are lined with fe rich precipitates we propose that the iron in those phases was released by brucite or olivine dissolution and was subsequently oxidized and precipitated as ferric hydroxide thermodynamic computations show that this process may be a potential source of catabolic energy for microorganisms inhabiting serpentinites the proposed carbonation mechanism implies that carbonate precipitation may start soon after exposure of the abyssal peridotites when dissolution of brucite and weathering of olivine begin and continue until the phases become inaccessible to seawater predicting carbonation rates of abyssal peridotites will hence require understanding of permeability reactions

ground coupled acoustic airwaves from mount st helens provide constraints on the may 18 1980 eruption abstract the may 18 1980 mount st helens eruption perturbed the atmosphere and generated atmosphere to ground coupled airwaves which were recorded on at least 35 seismometers operated by the pacific northwest seismograph network pnsn from 102 distinct travel time picks we identify coherent airwaves crossing washington state primarily to the north and east of the volcano the travel time curves provide evidence for both stratospheric refractions at 200 to 300 km from the volcano as well as probable thermospheric refractions at 100 to 350 km the very few first hand reports of audible volcano sounds within about 80 km of the volcano coincide with a general absence of ground coupled acoustic arrivals registered within about 100 km and are attributed to upward refraction of sound waves from the coherent refracted airwave arrivals we identify at least four distinct sources which we infer to originate 10 s 114 s 180 s and 319 s after the onset of an 8 32 11 pdt landslide the first of these sources is attributed to resultant depressurization and explosion of the cryptodome most of the subsequent arrivals also appear to be coincident with a source located at or near the presumed volcanic conduit but at least one of the later arrivals suggests an epicenter displaced about 9 km to the northwest of the vent this dislocation is compatible with the direction of the sector collapse and lateral blast we speculate that this concussion corresponds to a northern explosion event associated with hot cryptodome entering the toutle river valley

fracture initiation during volume increasing reactions in rocks and applications for co2 sequestration abstract volume increasing processes in rocks can induce various types of fractures such as hierarchical domain division of the unreacted material and spalling of the reacted material the mechanism responsible for the selection of a particular fracture type is generally not known we study such deformations and fracture mechanisms in a simplified system with a range of numerical techniques and find that sharpness of reaction fronts and magnitude of volume increase are the main parameters controlling which fracture type forms spalling dominates when there is sharp reaction fronts and large magnitude of volume change otherwise we get mainly domain dividing fractures increasing reaction front sharpness and magnitude of volume change also increases the fracture density and we argue that the change in fracture density can be important for the viability of in situ co 2 sequestration in ultramafic rock