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Relationships between crystal morphology and composition in the (Ba,Sr)SO4–H2O solid solution–aqueous solution system


Sánchez Pastor , Nuria and Pina Martínez, Carlos Manuel and Fernández Díaz, Lourdes (2006) Relationships between crystal morphology and composition in the (Ba,Sr)SO4–H2O solid solution–aqueous solution system. Chemical geology, 225 . pp. 266-277. ISSN 0009-2541

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This 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 as crystals become richer in Sr
has been interpreted on the basis of surface structure description by Hartman–Perdok 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 Type:Article
Uncontrolled Keywords:Barite; Celestite; Crystal morphology; Gel crystal growth method; AFM; Crystal growth rates
Subjects:Sciences > Geology > Crystallography
ID Code:14470
Deposited On:25 Jan 2012 13:30
Last Modified:25 Jan 2012 13:30

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