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Title: Fluorapatite crystal growth from modified seawater solutions

Abstract

Seeded precipitation experiments were conducted in a pH/fluoride-stat system to study the crystal growth of fluorapatite (FAP) in carbonate-free NaCl-CaCl{sub 2}-NaF-Na{sub 2}HPO{sub 4} solutions, at seawater calcium concentration, chlorinity, and pH. With increasing supersaturation, the dependence of the growth rate on the relative supersaturation changes from parabolic to exponential. This is interpreted as reflecting a transition in the crystal growth mechanism from growth at dislocation-induced surface steps to surface nucleation-controlled growth. The analysis of the kinetic data leads to a mineral-aqueous solution interfacial tension for FAP of 289 mJ/M{sup 2}. The Arrhenius activation energy of the growth reaction in the temperature range 12 to 35C is 47 kJ/mol. The effect of pH on FAP growth was tested for pH values from 7 to 8.5. In this range, growth of FAP is catalyzed by hydrogen ions. The apparent growth rate constant is proportional to (a{sub H{sup +}}){sup m} where m, the rate order with respect to H{sup +}, is a non-integral number which depends on pH. At identical degrees of supersaturation, the growth rate of FAP at pH = 7 is nearly twice that at pH = 8. When corrected for bottom water temperatures, pore water pH, and the retardation ofmore » Mg{sup 2+}, the experimental growth rates predict that during burial in modern phosphatic sediments, apatite particles grow to sizes on the order of 0.1-10{mu}m. The relatively slow growth kinetics of FAP are consistent with the observed small particle sizes of marine sedimentary apatite.« less

Authors:
;  [1]
  1. (Yale Univ., New Haven, CT (United States))
Publication Date:
OSTI Identifier:
5891161
Resource Type:
Journal Article
Journal Name:
Geochimica et Cosmochimica Acta; (United States)
Additional Journal Information:
Journal Volume: 55:5; Journal ID: ISSN 0016-7037
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; APATITES; CRYSTAL GROWTH; NUCLEATION; PRECIPITATION; ACTIVATION ENERGY; BENCH-SCALE EXPERIMENTS; BRINES; CHEMICAL REACTION KINETICS; GEOCHEMISTRY; PH VALUE; SEAWATER; SEDIMENTS; SUPERSATURATION; CHEMISTRY; ENERGY; HYDROGEN COMPOUNDS; KINETICS; MINERALS; OXYGEN COMPOUNDS; PHOSPHATE MINERALS; REACTION KINETICS; SATURATION; SEPARATION PROCESSES; WATER; 580000* - Geosciences

Citation Formats

Van Cappellen, P., and Berner, R.A. Fluorapatite crystal growth from modified seawater solutions. United States: N. p., 1991. Web. doi:10.1016/0016-7037(91)90302-L.
Van Cappellen, P., & Berner, R.A. Fluorapatite crystal growth from modified seawater solutions. United States. doi:10.1016/0016-7037(91)90302-L.
Van Cappellen, P., and Berner, R.A. Wed . "Fluorapatite crystal growth from modified seawater solutions". United States. doi:10.1016/0016-7037(91)90302-L.
@article{osti_5891161,
title = {Fluorapatite crystal growth from modified seawater solutions},
author = {Van Cappellen, P. and Berner, R.A.},
abstractNote = {Seeded precipitation experiments were conducted in a pH/fluoride-stat system to study the crystal growth of fluorapatite (FAP) in carbonate-free NaCl-CaCl{sub 2}-NaF-Na{sub 2}HPO{sub 4} solutions, at seawater calcium concentration, chlorinity, and pH. With increasing supersaturation, the dependence of the growth rate on the relative supersaturation changes from parabolic to exponential. This is interpreted as reflecting a transition in the crystal growth mechanism from growth at dislocation-induced surface steps to surface nucleation-controlled growth. The analysis of the kinetic data leads to a mineral-aqueous solution interfacial tension for FAP of 289 mJ/M{sup 2}. The Arrhenius activation energy of the growth reaction in the temperature range 12 to 35C is 47 kJ/mol. The effect of pH on FAP growth was tested for pH values from 7 to 8.5. In this range, growth of FAP is catalyzed by hydrogen ions. The apparent growth rate constant is proportional to (a{sub H{sup +}}){sup m} where m, the rate order with respect to H{sup +}, is a non-integral number which depends on pH. At identical degrees of supersaturation, the growth rate of FAP at pH = 7 is nearly twice that at pH = 8. When corrected for bottom water temperatures, pore water pH, and the retardation of Mg{sup 2+}, the experimental growth rates predict that during burial in modern phosphatic sediments, apatite particles grow to sizes on the order of 0.1-10{mu}m. The relatively slow growth kinetics of FAP are consistent with the observed small particle sizes of marine sedimentary apatite.},
doi = {10.1016/0016-7037(91)90302-L},
journal = {Geochimica et Cosmochimica Acta; (United States)},
issn = {0016-7037},
number = ,
volume = 55:5,
place = {United States},
year = {1991},
month = {5}
}