Why Mineral Interfaces Matter Putnis, Andrew
Science (American Association for the Advancement of Science),
03/2014, Volume:
343, Issue:
6178
Journal Article
Peer reviewed
Reactions at mineral-fluid interfaces play a key role in processes ranging from the deep Earth to materials synthesis and nuclear waste storage.
Throughout Earth, rocks respond to changing physical ...and chemical conditions by converting one rock type to another. These conversions have conventionally been described in terms of solid-state mechanisms, in which new minerals nucleate and grow through exchange of elements by diffusion. The slow rates of solid-state diffusion suggested geological time scales for these processes. However, rocks in Earth's crust are not dry (
1
), and even very low concentrations of aqueous solutions can increase reaction rates substantially (
2
). In the presence of a fluid phase, mineral conversions turn out to proceed not via solid-state diffusion but through dissolution and recrystallization at the mineral-fluid interface (
3
). Well beyond mineralogy, these insights may prove useful in developing new methods of materials synthesis, for carbon removal from the atmosphere, and for safe nuclear waste storage.
The preservation of morphology (pseudomorphism) and crystal structure during the transformation of one solid phase to another is regularly used as a criterion for a solid-state mechanism, even when ...there is a fluid phase present. However, a coupled dissolution–reprecipitation mechanism also preserves the morphology and transfers crystallographic information from parent to product by epitaxial nucleation. The generation of porosity in the product phase is a necessary condition for such a mechanism as it allows fluid to maintain contact with a reaction interface which moves through the parent phase from the original surface. We propose that interface-coupled dissolution–reprecipitation is a general mechanism for reequilibration of solids in the presence of a fluid phase.
A single crystal of KBr is transformed to a porous single crystal of KCl by immersion in saturated KCl solution. The image shows partial transformation of a crystal of KBr (core) to KCl (porous, milky rim) by an interface coupled dissolution–reprecipitation mechanism. The external dimensions and crystallographic orientation of the original crystal are preserved, while a reaction interface moves through the crystal.
The dissolution and carbonation of portlandite (Ca(OH)2) single crystals was studied by a combination of in situ Atomic Force Microscopy, Scanning Electron Microscopy, and two-dimensional X-ray ...diffraction. The dissolution of portlandite {0001} surfaces in water proceeds by the formation and expansion of pseudohexagonal etch pits, with edges parallel to ⟨100⟩ directions. Etch pits on {010} surfaces are elongated along ⟨001⟩, with edges parallel to ⟨101⟩. The interaction between carbonate-bearing solutions and portlandite results in the dissolution of the substrate coupled with the precipitation of thick islands of CaCO3 that appear oriented on the portlandite substrate. Ex situ carbonation of portlandite in contact with air results in the formation of pseudomorphs that fully preserve the external shape of the original portlandite single crystals. Our observations suggest that portlandite carbonation in contact with air and carbonate-bearing solutions occurs by a similar mechanism, i.e. coupled dissolution-precipitation. Calcite grows epitaxially on {0001} portlandite surfaces with the following orientation: ⟨001⟩Cc∥ ⟨001⟩Port. Apparently, no porosity is generated during the reaction, which progresses through the formation of fractures. Our results are of relevance to many processes in which the carbonation of portlandite takes place, such as CO2 capture and storage or the carbonation of cementitious materials.
The chemical composition of the continental crust cannot be adequately explained by current models for its formation, because it is too rich in Ni and Cr compared to that which can be generated by ...any of the proposed mechanisms. Estimates of the crust composition are derived from average sediment, while crustal growth is ascribed to amalgamation of differentiated magmatic rocks at continental margins. Here we show that chemical weathering of Ni- and Cr-rich, undifferentiated ultramafic rock equivalent to ~1.3 wt% of today's continental crust compensates for low Ni and Cr in formation models of the continental crust. Ultramafic rock weathering produces a residual that is enriched in Ni and also silica. In the light of potentially large volumes of ultramafic rock and high atmospheric CO
concentrations during the Archean, chemical weathering must therefore have played a major role in forming compositionally evolved components of the early Earth's crust.
Apatite (Ca5(PO4)3(OH, F, Cl)) is one of the main host of halogens in magmatic and metamorphic rocks and plays a unique role during fluid–rock interaction as it incorporates halogens (i.e. F, Cl, Br, ...I) and OH from hydrothermal fluids to form a ternary solid solution of the endmembers F-apatite, Cl-apatite and OH-apatite. Here, we present an experimental study to investigate the processes during interaction of Cl-apatite with different aqueous solutions (KOH, NaCl, NaF of different concentration also doped with NaBr, NaI) at crustal conditions (400–700°C and 0.2GPa) leading to the formation of new apatite. We use the experimental results to calculate partition coefficients of halogens between apatite and fluid. Due to a coupled dissolution–reprecipitation mechanism new apatite is always formed as a pseudomorphic replacement of Cl-apatite. Additionally, some experiments produce new apatite also as an epitaxial overgrowth. The composition of new apatite is mainly governed by complex characteristics of the fluid phase from which it is precipitating and depends on composition of the fluid, temperature and fluid to mineral ratio. Furthermore, replaced apatite shows a compositional zonation, which is attributed to a compositional evolution of the coexisting fluid in local equilibrium with the newly formed apatite. Apatite/fluid partition coefficients for F depend on the concentration of F in the fluid and increase from 75 at high concentrations (460μg/g F) to 300 at low concentrations (46μg/g F) indicating a high compatibility of F in apatite. A correlation of Cl-concentration in apatite with Cl− concentration of fluid is not observed for experiments with highly saline solutions, composition of new apatite is rather governed by OH− concentration of the hydrothermal fluid. Low partition coefficients were measured for the larger halogens Br and I and vary between 0.7*10−3–152*10−3 for Br and 0.3*10−3–17*10−3 for I, respectively. Br seems to have D values of about one order of magnitude higher than I. These data allow an estimation of the D values for the other halogens based on a lattice strain model which displays a sequence with DF of ∼120, DOH of ∼100, DCl of ∼2.3 DBr ∼0.045, and DI ∼0.0025. Results from this experimental study help to better understand fluid–rock interaction of an evolving fluid, as it enables the composition of hydrothermally derived apatite to be used as a fluid probe for halogens at crustal conditions. It further shows the importance of mineral replacement as one of the key reactions to generate apatite of different composition.
Unraveling the kinetics of calcium orthophosphate (Ca-P) precipitation and dissolution is important for our understanding of the transformation and mobility of dissolved phosphate species in soils. ...Here we use an in situ atomic force microscopy (AFM) coupled with a fluid reaction cell to study the interaction of phosphate-bearing solutions with calcite surfaces. We observe that the mineral surface-induced formation of Ca-P phases is initiated with the aggregation of clusters leading to the nucleation and subsequent growth of Ca-P phases on calcite, at various pH values and ionic strengths relevant to soil solution conditions. A significant decrease in the dissolved phosphate concentration occurs due to the promoted nucleation of Ca-P phases on calcite surfaces at elevated phosphate concentrations and more significantly at high salt concentrations. Also, kinetic data analyses show that low concentrations of citrate caused an increase in the nucleation rate of Ca-P phases. However, at higher concentrations of citrate, nucleation acceleration was reversed with much longer induction times to form Ca-P nuclei. These results demonstrate that the nucleation-modifying properties of small organic molecules may be scaled up to analyze Ca-P dissolution-precipitation processes that are mediated by a more complex soil environment. This in situ observation, albeit preliminary, may contribute to an improved understanding of the fate of dissolved phosphate species in diverse soil systems.
The replacement of a natural carbonate rock (Carrara marble) by apatite was used as a model to study the role of fluid chemistry in replacement reactions, focusing on the mineralogy, chemical ...composition, and porosity of the replacement product. Carrara marble was reacted with diammonium phosphate solutions ((NH4)2HPO4), in the presence and absence of four salt solutions (NH4Cl, NaCl, NH4F, and NaF) at different ionic strengths, at 200°C and autogenous pressure. The replacement products were analyzed using powder X-ray diffraction, Scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and Raman spectroscopy. The reaction in all samples resulted in pseudomorphic replacements and shared the characteristics of an interface-coupled dissolution–precipitation mechanism. Increasing the ionic strength of the phosphate fluid increased the replacement rates. With a fixed concentration of phosphate, replacement rates were reduced with the addition of NH4Cl and NaCl and increased significantly with the addition of NaF and NH4F. The addition of different salts resulted in specific porosity structures resulting from the formation of different phosphate phases. Chloride-containing fluids showed a higher degree of fluid percolation through grain boundaries. This study illustrates the significant impact that small differences in solvent composition can have in the progress of replacement reactions, the nature of the products and the resultant porosity.
•The hydrothermal replacement of Carrara marble (calcite) by apatite for different fluid compositions is presented•The replacement is pseudomorphic•The replacement rates decreased in the presence of high Cl concentrations whilst in the presence of F increased•Small fluid compositional changes can change the nature and the microstructure of rocks•Mass transfer through grain boundaries becomes more significant for the case of lower replacement rates
PDF
