Person: Sánchez Pastor, Nuria
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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
6 results
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Now showing 1 - 6 of 6
Item Growth of BaxSr1-xSO4 nano-steps on barite (001) face(Surface science, 2007) YuHang, Chen; Asenjo Barahona, Agustina; Sánchez Pastor, Nuria; Fernández Díaz, María Lourdes; Gómez Herrero, Julio; Pina Martínez, Carlos ManuelCrystal growth has been promoted in the fluid cell of an Atomic Force Microscope (AFM) by passing Ba–Sr–SO4 aqueous solutions over barite (001) cleavage surfaces. Steps advance in structural continuity with the original barite (001) surfaces and two-dimensional nucleation occurs preferentially on the newly-formed terraces. The terraces are, on average, 7.5% lower than pure barite terraces. Since the ionic radius of Sr2+ is smaller that the ionic radius of Ba2+, the reduction of terrace height is consistent with an extensive incorporation of Sr2+ into the barite structure. Therefore, it can be considered that the newly-formed terraces have compositions corresponding to terms of the BaxSr1xSO4 solid solution. A non-linear dependence of step rate on [SrSO4] concentration in the solution (and therefore on supersaturation) has been found. The growth behaviour has been discussed by considering both the physicochemical properties of the BaxSr1xSO4 solid solution–aqueous solution (SS–AS) system and a kinetic-based step growth model.Item HAFM observations of the growth of calcite in the presence of dissolved sulphate at 40ºC(Macla, 2009) Sánchez Pastor, Nuria; Jordan, Guntram; Fernández Díaz, María LourdesItem A combined in situ AFM and SEM study of the interaction between celestite (001) surfaces and carbonate-bearing aqueous solutions(Surface science, 2007) Sánchez Pastor, Nuria; Pina Martínez, Carlos Manuel; Fernández Díaz, María LourdesIn this paper, we present in situ atomic force microscopy (AFM) observations of the interaction between celestite (SrSO4) (001) surfaces and Na2CO3 aqueous solutions. The observations indicate that the interaction is characterized by a rapid alteration (carbonatation) and dissolution of the original surface, shortly followed by the formation of a new phase. EDX analyses indicate that the new phase is strontianite (SrCO3). Its crystallization involves the formation and spreading of islands of about 2.75 nm in height, which chiefly occurs on the step edges of the dissolving celestite substrate. The thickness of the islands remains almost constant during their spreading, which occurs mainly parallel to the celestite [010] direction. As a result of the progressive coalescence of the islands, a fairly homogeneous epitaxial layer forms on the celestite (001) face. At the initial stages, the formation of islands on the celestite (001) faces enhances dissolution, indicating the existence of a coupling between dissolution and crystallization reactions. Our measurements on series of FM images provided quantitative information about coupled dissolution-growth rates on a nanoscale. The effect of the coupled reactions on the celestite (001) surface on a microscopic scale was also studied by scanning electron microscopy (SEM).Item Relationships between crystal morphology and composition in the (Ba,Sr)SO4–H2O solid solution–aqueous solution system(Chemical geology, 2006) Sánchez Pastor, Nuria; Pina Martínez, Carlos Manuel; Fernández Díaz, María LourdesThis paper deals with the relationship between habit and compositional changes in crystals of the BaxSr1xSO4 solid solution at 25 8C and 1 atm. Crystals corresponding to the whole range of compositions of the BaxSr1xSO4 solid solution have been grown in a diffusing–reacting system. The physicochemical evolution resulting from counter-diffusion of SrCl2/BaCl2 and Na2SO4 reactants along a silica hydrogel column results in the development of spatio-temporal crystallization sequences. The external habit and the internal zoning reflecting both the morphological and compositional evolution of the crystals have been studied by scanning electron microscopy, EDAX and microprobe analysis. As composition varies from barite to celestite end-members, a clear evolution both in internal crystal morphologies and in final habit of crystals grown along the gel column has been observed. The main change observed is in the relative development of {001} and {011} forms, which is accompanied by a change in the crystals elongation. While tabular habits dominated by the {001} pinacoid correspond to crystal compositions close to pure barite, crystals with compositions closer to celestite show elongated morphologies dominated by the {011} rhombic prism. This variation is also evident in the internal morphologies of the crystals. The progressive predominance of {011} forms is crystals become richer in Sr has been interpreted on the basis of surface structure description by Hartman–erdok theory. Moreover, Atomic Force Microscopy investigations of the epitaxial growth of pure celestite on barite (001) surfaces under different degrees of supersaturation with respect to celestite have provided complementary quantitative information. Growth morphologies for different supersaturations with respect to celestite can be predicted using growth rates of crystal faces measured on a molecular scale. The results clearly show the effect of Sr incorporation into barite structure on the BaxSr1xSO4 crystal habit at a molecular scale. The results presented in this paper can help to develop morphological criteria relating morphology and composition of BaxSr1xSO4 crystals occurring in natural environments as a result of inorganic crystallization and biomineralization processes.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.