Person: Sánchez Pastor, Nuria
Loading...
First Name
Nuria
Last Name
Sánchez Pastor
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Department
Mineralogía y Petrología
Area
Cristalografía y Mineralogía
Identifiers
2 results
Search Results
Now showing 1 - 2 of 2
Item Epitaxial growth of celestite on barite (001) face at a molecular scale(Surface science, 2005) Sánchez Pastor, Nuria; Pina Martínez, Carlos Manuel; Astilleros García-Monge, José Manuel; Fernández Díaz, María Lourdes; Putnis, AndrewIn situ AFM experiments have been conducted in order to obtain information about kinetics of celestite epitaxial growth on barite. Growth has been promoted by passing aqueous solutions supersaturated with respect to celestite over freshly cleaved barite (001) surfaces. Solution supersaturation, bcelestite, was varied from 1 to 45.7 (βcelestite = a(Sr2+) . a(SO2-4)/Kspcel). At supersaturations below 10 neither two-dimensional nucleation neither step advancement are observed on barite (001) surfaces. However, once the two-dimensional nucleation barrier is overcome (bcelestite > 10), nuclei preferentially form on cleavage steps parallel to [100], [110] and [120] directions and more scarcely on terraces. The subsequent growth of two-dimensional nuclei leads to the development of celestite ‘‘islands’’. Their morphology is defined by (001) face and {210} and {100} forms and can be explained on the basis of PBCtheory. The coalescence of such islands results in the formation of a homogeneous SrSO4 layer. Growth rates along [001] direction have been measured for the whole supersaturation range. The growth rate equation for ‘Birth and Spread’’ crystal growth mechanism has been successfully fitted to our experimental data. The fitting process has provided basic growth parameters in a good agreement with theoretical ones. Both the high transitional supersaturation required for twodimensional nucleation and the high interfacial energy value obtained from the fitting of the ‘‘Birth and Spread’’ equation (σcel–bar 001 = 0:137 J/m2) indicate low affinity of SrSO4 growth units for barite (001) faces. This is consistent with the relative high mismatch between celestite and barite structure. The behaviour of the epitaxial growth described in this work can help to interpret the oscillatory zoning frequently occurring in both natural and synthetic crystals of the BaxSr1-xSO4 solid solution.Item The effect of CO2-3 on the growth of barite {001} and {210} surfaces: An AFM study(Surface science, 2006) Sánchez Pastor, Nuria; Pina Martínez, Carlos Manuel; Fernández Díaz, María Lourdes; Astilleros García-Monge, José ManuelThe growth of barite {001} and {210} faces from aqueous solutions moderately supersaturated with respect to barite (βbarite ≈ 12 for experiments on {001} surfaces and βbarite ≈ 7 for experiments on {210} surfaces) and bearing different concentrations of carbonate has been studied in situ using an atomic force microscope (AFM). Nanoscopic observations show that, above a certain carbonate concentration threshold in the aqueous solution, the advancement of monolayers (~3.5Å in height) on barite {001} and {210} surfaces is strongly inhibited. However, inhibition never affects the growth of the first monolayer, whose growth rate increases in the presence of carbonate. In contrast, the second monolayer growth rate decreases as the concentration of carbonate in the solution increases. For high carbonate concentrations in the solution, growth stops after the formation of the first monolayer. While on barite {001} faces, the formation of a second monolayer does not occur for carbonate concentrations higher than 0.2 mM, on barite {210} faces the complete inhibition of the second monolayer is observed for carbonate concentrations higher than 0.05 mM. Once growth on {001} or {210} faces is completely inhibited, i.e. such surfaces are in the ‘‘dead zone’’, growth can be recovered by increasing supersaturation. In order to study the recovery behaviour of barite {001} and {210} faces from the ‘‘dead zone’’, an additional series of AFM experiments have been conducted. In these experiments, carbonate-free aqueous solutions with increasing supersaturations with respect to barite were passed over {001} and {210} surfaces previously ‘‘poisoned’’ with carbonate. Our experimental results show that the recovery of growth on barite {001} faces requires an important increase of the solution supersaturation. In contrast, the recovery of barite {210} surface growth does not require any supersaturation increase, but spontaneously occurs in a few minutes. Our observations of inhibition and growth recovery on barite surfaces at a nano-scale are discussed and compared with the descriptions given by the classical crystal growth inhibition models.