Al Transport Research Articles

Fluid flow and Al transport during quartz‐kyanite vein formation, Unst, Shetland Islands, Scotland

AbstractQuartz‐kyanite veins, adjacent alteration selvages and surrounding ‘precursor’ wall rocks in the Dalradian Saxa Vord Pelite of Unst in the Shetland Islands (Scotland) were investigated to constrain the geochemical alteration and mobility of Al associated with channelized metamorphic fluid infiltration during the Caledonian Orogeny. Thirty‐eight samples of veins, selvages and precursors were collected, examined using the petrographic microscope and electron microprobe, and geochemically analysed. With increasing grade, typical precursor mineral assemblages include, but are not limited to, chlorite+chloritoid, chlorite+chloritoid+kyanite, chlorite+chloritoid+staurolite and garnet+staurolite+kyanite+chloritoid. These assemblages coexist with quartz, white mica (muscovite, paragonite, margarite), and Fe‐Ti oxides. The mineral assemblage of the selvages does not change noticeably with metamorphic grade, and consists of chloritoid, kyanite, chlorite, quartz, white mica and Fe‐Ti oxides. Pseudosections for selvage and precursor bulk compositions indicate that the observed mineral assemblages were stable at regional metamorphic conditions of 550–600 °C and 0.8–1.1 GPa. A mass balance analysis was performed to assess the nature and magnitude of geochemical alteration that produced the selvages adjacent to the veins. On average, selvages lost about −26% mass relative to precursors. Mass losses of Na, K, Ca, Rb, Sr, Cs, Ba and volatiles were −30 to −60% and resulted from the destruction of white mica. Si was depleted from most selvages and transported locally to adjacent veins; average selvage Si losses were about −50%. Y and rare earth elements were added due to the growth of monazite in cracks cutting apatite. The mass balance analysis also suggests some addition of Ti occurred, consistent with the presence of rutile and hematite‐ilmenite solid solutions in veins. No major losses of Al from selvages were observed, but Al was added in some cases. Consequently, the Al needed to precipitate vein kyanite was not derived locally from the selvages. Veins more than an order of magnitude thicker than those typically observed in the field would be necessary to accommodate the Na and K lost from the selvages during alteration. Therefore, regional transport of Na and K out of the local rock system is inferred. In addition, to account for the observed abundances of kyanite in the veins, large fluid‐rock ratios (102–103 m3fluid m−3rock) and time‐integrated fluid fluxes in excess of ∼104 m3fluid m−2rock are required owing to the small concentrations of Al in aqueous fluids. It is concluded that the quartz‐kyanite veins and their selvages were produced by regional‐scale advective mass transfer by means of focused fluid flow along a thrust fault zone. The results of this study provide field evidence for considerable Al mass transport at greenschist to amphibolite facies metamorphic conditions, possibly as a result of elevated concentrations of Al in metamorphic fluids due to alkali‐Al silicate complexing at high pressures.

Microstructure and High Temperature Oxidation of the Hot-Dipping Al-Si Coating on Low Carbon Steel in Ethanol, Water Vapor and Air at 700°C

Low carbon steel was coated by hot-dipping into a molten Al-10%Si bath. The high-temperature oxidation was performed at 700oC for 1 h to 49 h in air, air +100% H2O, and air + 30% ethanol under atmospheric pressure. An elemental composition distribution, morphologies of the aluminide layer and the oxide scale were characterized by OM, XRD, and SEM/EDS. After hot-dipping treatment, the coating layers consisted of Al, Si, FeAl3, τ5-Fe2Al8Si, and Fe2Al5. The results of high temperature oxidation tests showed the oxidation rate were parabolic law in three different atmospheres. The polyhedral τ1-(Al,Si)5Fe3 formed at a short time oxidation completely transformed to FeAl2 and FeAl due to the composition gradient and the chemical diffusion. The effect of water vapor on the oxidation resistance of the Al-Si coating may be attributed to increase in Al and Fe ions transport, leading to loss of protective aluminide layer by formation of iron oxide nodules on the coating surface and at interface between aluminide layer and the steel substrate.

Problems and progresses in speciation of Al in human serum: An overview

Aluminium (Al) is associated with many clinical disorders in renal patients. Al accumulation in brain has also been related to the neurodegenerative processes in Alzheimer's disease. In order to better understand Al transport in the human body, it is necessary to identify and quantify chemical species in which Al is present in body fluids and tissues. Among a variety of biological samples, Al speciation was the most frequently investigated in human serum. Improvements were made in the development of analytical techniques for the determination of the amount and composition of high molecular mass Al (HMM-Al) and low molecular mass Al (LMM-Al) species in human serum. However, due to the complex chemistry of Al in serum, its low total concentration and the high risk of contamination, speciation of Al in biological samples is still a difficult task for analytical chemists. In this work, problems related to speciation of Al in human serum are critically discussed. An overview of the progress that was made by the use of different analytical procedures, in order to propose analytical protocols for reliable speciation of Al in serum at low ng mL(-1) concentration range, is presented.

Microstructural investigation of the initial oxidation at 1450°C and 1500°C of a Mo(Si,Al)2-based composite

The initial oxidation behaviour of a Mo(Si,Al)2-based composite (Kanthal Super ER) was investigated. The microstructures of unexposed reference specimens and samples oxidized isothermally in air at 1450°C for 5 and 50 minutes and also at 1500°C for 5 minutes were evaluated.Thin foil specimens were prepared using a focused ion beam/scanning electron microscope and analysed in an SEM with scanning transmission electron microscopy detectors and an energy dispersive X-ray system for high-resolution imaging and chemical analysis.The samples formed relatively pure α-Al2O3 scales during the oxidation. The Al supply to the growing alumina scale is mainly provided by the Mo(Si,Al)2 phase in the bulk material. The outward Al transport is counterbalanced by inward diffusion of Si into Mo(Si,Al)2. Therefore an Al poor Mo5(Si,Al)3 phase forms directly below the scale. The Al concentration in the subjacent Mo(Si,Al)2 decreased to 20 at.%, corresponding to Mo(Si1.4,Al0.6).

First-principles investigation of migration barriers and point defect complexes in B2–NiAl

We perform a systematic first-principles investigation of atomic hop mechanisms in B2–NiAl and discover a low barrier collective hop that can mediate Al diffusion through the anti-structural bridge mechanism. We also find an alternative hop sequence for the migration of a triple defect than that proposed previously. To shed light on the dominant hop mechanisms that mediate diffusion in B2–NiAl, we study point defects and defect clusters in B2–NiAl at high temperature by combining a cluster expansion with Monte Carlo simulations. Going beyond the mean field approximation, we find that the inclusion of interactions among the various point defects is crucial to predict the concentration of defect complexes, such as the triple defects of B2–NiAl. Interactions among point defects also introduce an important degree of short-range order between Al antisite defects and Ni antisite defects. We find an increasing probability between pairs of Al antisite atoms and Ni vacancies as the alloy concentration of B2–NiAl becomes both Al rich and Ni rich, suggesting that the anti-structural bridge mechanism should play an important role in facilitating Al transport.

Temperature Estimation and Al Content Prediction Focusing on Microstructural Change in a Thermal Barrier Coating

A superalloy with a thermal barrier coating (TBC) simulating a gas turbine blade is exposed to a high-temperature environment to develop a method for predicting the local temperature and Al content in a bond coat (BC). The Al content decreases with an increase in the test time due to the Al transport induced by the oxidation of the BC and the interdiffusion between the BC and the substrate. This brings about Al-decreased layer (ADL) at the boundary between the BC and the top coat. The thickness of the ADL increases in proportion to the square root of the test time, and the temperature dependence of the growth rate shows an Arrhenius-type behavior. Based on this relation, the local temperature of an in-service blade can be estimated by measuring the ADL thickness when the operation time is known. The Al content decreases in proportion to the ADL thickness. The prediction method of the Al content based on the relation is also presented.

Formation of kyanite–quartz veins of the Alpe Sponda, Central Alps, Switzerland: implications for Al transport during regional metamorphism

In this study, we have investigated the formation of quartz–kyanite veins of the Alpe Sponda, Central Alps, Switzerland. We have integrated field observations, fluid inclusion and stable isotope data and combined this with numerical geochemical modeling to constrain the chemical processes of aluminum transport and deposition. The estimated P–T conditions of the quartz–kyanite veins, based on conventional geothermometry (garnet–biotite, white mica solvus and quartz–kyanite oxygen isotope thermometry) and fluid inclusion data, are 550 ± 30°C at 5.0 ± 0.5 kbar. Geochemical modeling involved construction of aqueous species predominance diagrams, calculation of kyanite and quartz solubility, and reaction–path simulations. The results of the modeling demonstrate that (1) for the given chemical composition of the vein-forming fluids mixed Al–Si aqueous species are dominant in transporting Al, and that (2) fluid cooling along a small temperature gradient coupled with a pH decrease is able to explain the precipitation of the quartz–kyanite assemblages in the proportions that are observed in the Alpe Sponda veins. We conclude that sufficient amounts of Al can be transported in typical medium- to high-grade regional metamorphic fluids and that immobile behavior of Al is not very likely in advection–dominanted fluid–rock systems in the upper and middle crust.

Rock-eating mycorrhizas: their role in plant nutrition and biogeochemical cycles

A decade ago, tunnels inside mineral grains were found that were likely formed by hyphae of ectomycorrhizal (EcM) fungi. This observation implied that EcM fungi can dissolve mineral grains. The observation raised several questions on the ecology of these “rock-eating” fungi. This review addresses the roles of these rock-eating EcM associations in plant nutrition, biogeochemical cycles and pedogenesis. Research approaches ranged from molecular to ecosystem level scales. Nutrient deficiencies change EcM seedling exudation patterns of organic anions and thus their potential to mobilise base cations from minerals. This response was fungal species-specific. Some EcM fungi accelerated mineral weathering. While mineral weathering could also increase the concentrations of phytotoxic aluminium in the soil solution, some EcM fungi increase Al tolerance through an enhanced exudation of oxalate. Through their contribution to Al transport, EcM hyphae could be agents in pedogenesis, especially podzolisation. A modelling study indicated that mineral tunnelling is less important than surface weathering by EcM fungi. With both processes taken together, the contribution of EcM fungi to weathering may be significant. In the field vertical niche differentiation of EcM fungi was shown for EcM root tips and extraradical mycelium. In the field EcM fungi and tunnel densities were correlated. Our results support a role of rock-eating EcM fungi in plant nutrition and biogeochemical cycles. EcM fungal species-specific differences indicate the need for further research with regard to this variation in functional traits.

Al-toxicity studies in yeast using gallium as an aluminum analogue

Aluminum (Al) is normally present in soils as the insoluble, harmless Al2O3. The highly toxic Al3+ and AlOH2+ monomeric cations are formed in acid soils but there is little consensus on the physiological basis of Al toxicity in plants. A major factor that has retarded progress in understanding aluminum toxicity in vascular plants is the lack of a convenient radioisotope for Al. Yeast and vascular plants share similar membrane transport mechanisms and so yeast (Saccharomyces cerevisiae) provides a convenient model system for studies of Al-toxicity. Al and gallium (Ga) have closely similar toxic effects on the yeast cells (Ki approximately 100 mmol m-3) and Ga3+ and Al3+, respond similarly to pH and are both reversible by a chelation agent (citric acid). We tested the feasibility of using 67Ga radioisotope as a tracer for Al transport with the view of using it to investigate the mechanism of Al uptake and toxicity in plants. The clinically available 67Ga citrate is unsuitable to use as an aluminum analogue because the chelated form is not toxic. Arrangements need to be made for it to be supplied as 67GaCl3. Large amounts of 67Ga rapidly bind to the cell wall of yeasts with a t 1/2 of approximately 1 s. There is a very slow net uptake of 67Ga into a second phase, presumably the cytoplasm. Uptake into the slow phase has a Vmax of only approximately 16 +/- 4 pmol m(-2) s(-1) (n = 16). The Km of 67Ga uptake could not be precisely determined but is below 100 mmol m(-3) (45 +/- 42 mmol m(-3), n = 16).

Modeling the Transport of Metals with Rate-Limited EDTA-Promoted Extraction and Dissolution during EDTA-Flushing of Copper-Contaminated Soils

EDTA-flushing of artificially contaminated soils enhanced Cu extraction but also induced mineral dissolution simultaneously. The mobilization and transport of these metal-EDTA complexes was investigated with column experiments. A quantitative transport model was proposed for simulating the experimental breakthrough curves of Cu, Fe, Al, and Ca. The rate-limited EDTA-promoted extraction and dissolution could be described by respective second-order kinetic terms, which were necessary for explaining the time-dependent depletion of extractable metals (sorbed and indigenous) in soils with continuous EDTA-flushing. Simultaneous simulation of extraction of sorbed Cu and dissolution of soil Fe, Al, and Ca is more conceptually accurate than individual modeling of each metal because the latter approach tends to overestimate the concentration of free EDTA during transport and thus underestimate the rate coefficients of EDTA-promoted dissolution. The fitted rate coefficients of Cu were about an order of magnitude larger than those of Fe and Al; these values probably reflect Cu extraction from weakly sorbed fractions and Fe and Al dissolution from amorphous oxides. The apparent retardation of Fe, Al, and Ca transport had to be taken into account by empirical determination, which was attributed to the metal lability in soils and thermodynamics of surface complexation.

Is There a Role for First Responders in EMS Responses to Medical Facilities?

Objective: Emergency medical dispatch (EMD) protocols should match response resources with patient needs. We tested a protocol sending only a commercial ambulance, without fire department first responders (FR), to all non-cardiac-arrest EMS calls at a physician-staffed HMO facility. Study objectives were to determine how often FR provided patient care at such facilities andwhether EMD implementation could conserve FR resources without compromising patient care. Methods: All EMS dispatches to this facility in the 4 months before implementation of the EMD protocol and4 months after implementation were identified through dispatch records, andall FR andambulance patient care reports were reviewed. In the “after” phase, all cases needing ALS transport were reviewed to examine whether there would have been benefit to FR dispatch. Results: Of 242 dispatches in the “before” phase, BLS FR responded to 156 (64%), andALS FR to 117 (48%). BLS FR provided patient care in 2 cases, andALS FR in 17. Of 227 dispatches in the “after” phase, BLS FR responded to 10 (4%), andALS FR to 10 (4%); all but one were protocol violations. BLS FR provided care in one case, andALS FR in three. Review of the 93 “after” cases requiring ALS transport found none where FR presence would have been beneficial. Conclusions: First responders rarely provided patient care when responding to EMS calls at a physician-staffed medical facility. Implementation of an EMD protocol can safely reduce the number of FR responses to unscheduled ambulance calls at such a facility. Key words: emergency medical services; dispatch.

Transport mechanism for aluminum citrate at the blood–brain barrier: kinetic evidence implies involvement of system Xc − in immortalized rat brain endothelial cells

Although accumulation of aluminum (Al) in the brain is known to cause neurodegenerative disorders and to be regulated mainly by the blood–brain barrier (BBB), the mechanism responsible for Al transport at the BBB has not been clarified yet. In this study, we investigated what kind of transporter is involved in the transport of Al citrate, which is the major species of Al in the brain, at the BBB using a rat immortalized brain endothelial cell line (RBEC1), focusing on the glutamate transporter family. The uptake of Al citrate showed temperature- and concentration-dependency, and did not require an inwardly directed Na +-gradient as a driving force, ruling out the involvement of Na +-dependent glutamate transporters in its transport. By RT-PCR, in RBEC1, there were mRNAs for the components of a Na +-independent glutamate transporter, system Xc −. l-Glutamate and l-cystine, representative ligands for system Xc −, significantly inhibited the uptake of Al citrate, and loading of them into the cells resulted in stimulation of its uptake in RBEC1. These results demonstrated that Al citrate is taken up into RBEC1 via system Xc −, and that this system might play an important role in Al citrate transport at the BBB.

Aluminium absorption by intact roots of the Al-accumulating plant Camellia sinensis L.

Tea (Camellia sinensis L.) is an Al-accumulating plant, taking up a large amount of aluminium (Al). However, information about Al transport into the roots is limited. In the present study, the absorption of Al by intact roots of young tea plants as well as the effects of fluoride, and solution pH were investigated. The absorption of Al by intact tea roots can be described by two phases, a rapid phase in the first 60 min followed by a slower phase for at least 24 h, which is not different from those observed in non-Al-accumulating plants. This pattern remained more or less the same after citric acid desorption for 30 min, being non-linear until the end of the initial 60 min. Absorption of Al was significantly influenced by solution pH, increasing remarkably from pH 3.2 to 4.8. A combined supply of Al and F in the nutrient solution significantly reduced the uptake of Al.

Effect of aluminium supply on aluminium uptake, translocation and blueing of Hydrangea macrophylla (Thunb.) Ser. cultivars in a peatclay substrate

SummaryThe aluminium (Al) accumulator Hydrangea macrophylla (H. m.) is extremely Al-resistant compared with other plant species. Sufficient transport of Al into the flowers is essential to achieve the desired colour change from original pink to blue by complexation of the anthocyanin delphinidine with Al. To overcome wide-spread difficulties in producing blue H. m., an improved understanding of the relationship between Al supply, Al uptake, translocation, and blueing of sepals is required. A strategy was previously developed that allows estimation of the Al supply needed for adjustment of a target pH at the beginning of forcing. After treatment with 0, 3, 6, 12 g Al2(SO4)3 × 18 H2O (l substrate)–1, determination of substrate pH, Al content and colour of the inflorescence proved that the strategy was suitable to achieve satisfying blue inflorescences in eight cultivars differing considerably in their easiness of blueing. These differences between cultivars could be attributed to cultivar-specific modifications of the substrate pH, translocation of Al in the xylem from the roots to the shoots and the inflorescences, and the Al contents of the sepals necessary to produce the desired blue colour. Measurement of Al and organic acid concentrations in the xylem sap and calculations with the speciation programme GEOCHEM-PC revealed a dominant role of citrate for Al transport particularly in the easy-to-blue cultivar ‘Renate Steiniger’. The possible role of malonate and malate needs further experimental support.

An ab initio approach to electricaltransport in molecular devices

We present a theoretical study of scanning tunnelling microscopyof a C60 molecule adsorbed on Al and Au(111) surfaces andof electrical transport of short metallic nanotubes contactedin two ways to such surfaces. For the case of an adsorbedC60 molecule, the change of substrate metal is reflected in adifferent charge transfer, in a different alignment of theFermi level with respect to the molecular orbitals and, consequently, in a very different zero-bias transparency. Forthe case of metallic nanotubes with realistic contacts, theconductance turns out to be, at best, half the ideal value of4e/h2. We perform these studies within a recently developedab initio scheme suitable for studying electrical transport in generic single-molecule junctions.

Narrow nuclear resonance profiling of Al with subnanometric depth resolution

We report on the use of the narrow and isolated resonance at 404.9 keV in the cross-section curve of the 27 Al(p,γ) 28Si nuclear reaction for profiling Al in ultrathin aluminum oxide films on Si. The samples were characterized as-deposited and after thermal annealing, so that Al transport could be studied. An estimated depth resolution of approximately 0.4 nm near the surface of the films could be obtained owing to: (i) the very small resonance width; (ii) the high stopping power of Al 2O 3 for 404.9 keV protons; (iii) the high energy stability of the proton beam provided by the 500 kV HVEE ion implanter at Porto Alegre; and (iv) an apparent thickness magnification by a factor between 2.0 and 2.4 with the use of glancing incidence. This technique is compared to other methods for Al profiling like medium energy ion scattering and some sputtering-based techniques.

Aluminum speciation in crustal fluids revisited

Aluminum speciation in crustal fluids is assessed by means of standard thermodynamic properties at 25°C, 1 bar, and revised Helgeson-Kirkham-Flowers (HKF) (Tanger J. C. IV and Helgeson H. C., “Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Revised equations of state for the standard partial molal properties of ions and electrolytes,” Am. J. Sci. 288, 19–98, 1988) equations of state parameters for aqueous species in the system Al-O-H-Na-Si-Cl-F-SO4 derived from recent experimental data with the help of isocoulombic reactions and correlations among parameters in the HKF model. In acidic to neutral hydrothermal solutions and for fluorine concentrations in excess of 1 ppm, the fluoride complexes AlFn3−n dominate Al speciation at temperature (T) < 100°C, whereas the hydroxide fluoride species Al(OH)2F(aq)0 and AlOHF20(aq) are dominant up to ∼400°C. In high-temperature (T > 300°C) hydrothermal and metamorphic fluids, aluminum mobility is considerably enhanced by formation of NaAl(OH)3F(aq)0 and NaAl(OH)2F20(aq) ion paired mixed species. NaAl(OH)2F20(aq) controls Al transport in granite-derived fluids and during greisenization. At alkaline pH, Al(OH)4−, Al(OH)3H3SiO4−, and the NaAl(OH)40(aq) ion-pair are the dominant Al species. Thermodynamic calculations show that as a result of strong interactions of Al(aq) with NaOH, NaF, HF, and SiO2(aq) present in crustal fluids, the concentrations of aluminum in equilibrium with Al-bearing minerals can be several orders of magnitude higher than those calculated assuming that only Al hydroxyde complexes are formed. Interactions with these components are likely to be responsible for aluminum mobility during hydrothermal and metamorphic reactions.

Local origin of high pressure vein material in eclogite facies rocks of the Zermatt-Saas-Zone, Switzerland

Prograde dehydration reactions within metabasalts at the blueschist- eclogite facies transition produce kyanite veins, usually with quartz, sometimes with omphacite or chloritoid, of up to several meters length and 20 cm width. Bulk rock analyses show clear depletion haloes for Al2O3 and SiO2 near the veins. Mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings. These veins are interpreted as local segregation phenom- ena and not as fluid dehydration channels of the eo-Alpine subduction zone. Calcula- tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions (k) responsible for providing the H2O for Al and Si transport and the rate of diffusion (D) of these from wall rock to growing vein. The model is satisfied for quite a wide range of values for k/D and upper limits on the duration of diffusional vein formation of about 1 Ma are indicated.

Pediatric pre-hospital advanced life support care in an urban setting.

To describe pediatric advanced life support (PALS) in a single urban environment and clarify educational priorities for ALS pre-hospital providers and pediatric medical control physicians. Retrospective observational review of all pediatric pre-hospital PALS transport and medical control records of the two-tiered, unified, municipal emergency medical service of the City of Boston (catchment area 590,000) over a 1-year period. Of the 555 pediatric patients receiving ALS transport, 38% were for respiratory emergencies, 24% for nonrespiratory medical emergencies, 19% for traffic-related blunt trauma, and 10% for penetrating trauma. Two percent involved cardiac arrests. The most frequent procedures performed were intravenous (IV) cannulation (n = 184, 33%), bag-mask ventilation (n = 28, 5%) and intubation (n = 15, 3%). Intraosseous access was only performed in three patients (0.5%). Fifty ALS providers in the EMS system averaged pediatric IV cannulation 3.7 times, intubation 0.3 times, and intraosseous access 0.06 times per provider per year. On-line medical control was requested in 28 % of PALS transports. The chief complaints managed by medical control closely mirrored the distribution of all ALS transports. The most frequent medication ordered by on-line medical control was additional nebulized albuterol after standing orders (off-line medical control) had been exhausted. A limited number of chief complaints make up the majority of PALS transports. Initial and continuing education for ALS providers needs to reflect the importance of these critical entities. Education for urban pre-hospital providers should reflect that certain procedures will be only executed every few years (eg, pediatric intubation) or once in the career of an ALS pre-hospital provider (eg, intraosseous access). With a limited amount of pediatric teaching time, paramedic education will have to strike a careful balance between teaching about the chief complaints most frequently encountered and teaching rare, high-risk procedures that could provide maximal support for the uncommon critically ill child. On-line medical control physicians need to be prepared to direct and support the management by ALS pre-hospital providers for the chief complaints most frequently seen in pediatric patients.

Diffusion control of garnet growth, Harpswell Neck, Maine, USA

AbstractA detailed analysis of chemical zoning in two garnet crystals from Harpswell Neck, Maine, forms the basis of an interpretation of garnet nucleation and growth mechanisms. Garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation, giving rise to complex two‐dimensional zoning patterns depending on the orientation of the thin section cut. Contours of Ca zoning cross those of Mn, Fe and Mg, indicating a lack of equilibrium among these major garnet constituents. Zoning of Fe, Mg and Mn is interpreted to reflect equilibrium with the rock matrix, whereas Ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal.Image analysis reveals that the growth of garnet is more rapid along triple‐grain intersections than along double‐grain boundaries. Moreover, different minerals are replaced by garnet at different rates. The relative rate of replacement by garnet along double‐grain boundaries is ordered as muscovite &gt; chlorite &gt; plagioclase &gt; quartz. Flux calculations reveal that replacement is limited by diffusion of Si along double‐grain boundaries to or from the local reaction site. It is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet, with Si and Al transport being rate limiting in these samples.