An integrated program has been developed to explore the reactivity of 2:1 phyllosilicates (biotit... more An integrated program has been developed to explore the reactivity of 2:1 phyllosilicates (biotite and the clays montmorillonite, hectorite, and nontronite) with respect to acid dissolution using in situ atomic force microscopy (AFM). Three techniques are described which make it possible to fix these minerals and other small particles to a suitable substrate for examination in the fluid cell of the atomic force microscope. A suite of macros has also been developed for the Image SXM image analysis environment which make possible the accurate and consistent measurement of the dimensions of clay particles in a series of AFM images, so that dissolution rates can be measured during a fluid cell experiment. Particles of biotite and montmorillonite were dissolved, and their dissolution rates normalized to their reactive surface area, which corresponds to the area of their edge surfaces (Ae). The Ae-normalized rates for these minerals between pH 1-2 are all ~10 -8 mol/m•s, and compare very well to other Ae-normalized dissolution rates in the literature. Differences between the Ae-normalized rates for biotite and the BET-normalized rates (derived from solution chemical studies) found in the literature can be easily explained in terms of the proportion of edge surface area and the formation of leached layers. However, the differences between the Ae-normalized montmorillonite rates and the literature values cannot be explained in the same way. Rather, it is demonstrated that rates derived from solution studies of montmorillonite dissolution have been affected by the colloidal behavior of the mineral particles. Finally, the dissolution behavior of hectorite (a trioctahedral smectite) and nontronite (a dioctahedral smectite) were compared. Based on the differential reactivity of their crystal faces, a model of their surface atomic structures is formulated using Hartman-Perdock crystal growth theory, which explains the observed data if it is assumed that the rate-determining step of the dissolution mechanism is the breaking of connecting bonds between the octahedral and tetrahedral sheets of the mineral structure.
The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was predicted using pseudop... more The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was predicted using pseudopotential planewave density functional theory calculations. Ideal (110) and (010) type neutral edge surfaces of pyrophyllite and ferripyrophyllite were considered. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial edge surface terminations. The edge surfaces were protonated using various schemes to neutralize the surface charge, and total energy comparisons were used to determine which schemes are the most energetically favorable. The calculations also show that significant surface relaxation should occur on the (110) type faces, as well as in response to different protonation schemes. This result is consistent with atomic force microscopy observations of phyllosilicate dissolution behavior. Bond lengths from calculated structures can also be used to predict intrinsic acidity constants for acid base reactions involving the surface functional groups present on (110) and (010) type edge surfaces, using bond valence methods. However, the issue of surface relaxation poses problems for current bond valence methods, so an alternative method is tentatively proposed that takes into account bond relaxation, and accounts for the energetics of various protonation schemes on phyllosilicate edges.
The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudo... more The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudopotential planewave density functional theory. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial terminations for ideal (110)- and (010)-type edge surfaces. The edge surfaces were protonated using various schemes to neutralize the surface charge,
An integrated program has been developed to explore the reactivity of 2:1 phyllosilicates (biotit... more An integrated program has been developed to explore the reactivity of 2:1 phyllosilicates (biotite and the clays montmorillonite, hectorite, and nontronite) with respect to acid dissolution using in situ atomic force microscopy (AFM). Three techniques are described which make it possible to fix these minerals and other small particles to a suitable substrate for examination in the fluid cell of the atomic force microscope. A suite of macros has also been developed for the Image SXM image analysis environment which make possible the accurate and consistent measurement of the dimensions of clay particles in a series of AFM images, so that dissolution rates can be measured during a fluid cell experiment. Particles of biotite and montmorillonite were dissolved, and their dissolution rates normalized to their reactive surface area, which corresponds to the area of their edge surfaces (Ae). The Ae-normalized rates for these minerals between pH 1-2 are all ~10 -8 mol/m•s, and compare very well to other Ae-normalized dissolution rates in the literature. Differences between the Ae-normalized rates for biotite and the BET-normalized rates (derived from solution chemical studies) found in the literature can be easily explained in terms of the proportion of edge surface area and the formation of leached layers. However, the differences between the Ae-normalized montmorillonite rates and the literature values cannot be explained in the same way. Rather, it is demonstrated that rates derived from solution studies of montmorillonite dissolution have been affected by the colloidal behavior of the mineral particles. Finally, the dissolution behavior of hectorite (a trioctahedral smectite) and nontronite (a dioctahedral smectite) were compared. Based on the differential reactivity of their crystal faces, a model of their surface atomic structures is formulated using Hartman-Perdock crystal growth theory, which explains the observed data if it is assumed that the rate-determining step of the dissolution mechanism is the breaking of connecting bonds between the octahedral and tetrahedral sheets of the mineral structure.
The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was predicted using pseudop... more The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was predicted using pseudopotential planewave density functional theory calculations. Ideal (110) and (010) type neutral edge surfaces of pyrophyllite and ferripyrophyllite were considered. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial edge surface terminations. The edge surfaces were protonated using various schemes to neutralize the surface charge, and total energy comparisons were used to determine which schemes are the most energetically favorable. The calculations also show that significant surface relaxation should occur on the (110) type faces, as well as in response to different protonation schemes. This result is consistent with atomic force microscopy observations of phyllosilicate dissolution behavior. Bond lengths from calculated structures can also be used to predict intrinsic acidity constants for acid base reactions involving the surface functional groups present on (110) and (010) type edge surfaces, using bond valence methods. However, the issue of surface relaxation poses problems for current bond valence methods, so an alternative method is tentatively proposed that takes into account bond relaxation, and accounts for the energetics of various protonation schemes on phyllosilicate edges.
The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudo... more The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudopotential planewave density functional theory. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial terminations for ideal (110)- and (010)-type edge surfaces. The edge surfaces were protonated using various schemes to neutralize the surface charge,
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