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Coprecipitation of Sodium, Magnesium, and Silicon with Calcium Carbonate
Author(s) -
Levy D. B.,
Amrhein C.,
Anderson M. A.,
Daoud A. M.
Publication year - 1995
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1995.03615995005900050008x
Subject(s) - calcite , chemistry , dissolution , solubility , carbonate , precipitation , mineralogy , inorganic chemistry , nuclear chemistry , physics , organic chemistry , meteorology
Calcite oversaturation in soil solutions is often attributed to overestimations of Ca 2+ (aq) and CO 2− 3 (aq) activities, organic matter mineralization, and the presence of metastable forms of CaCO 3 . Earlier research, however, has indicated that these explanations are often insufficient to explain the levels of calcite oversaturation observed in soil suspensions. This study was conducted to evaluate the solubilities of precipitates formed during evapoconcentration of synthetic irrigation waters. Solids precipitated from waters of various Mg and Si concentrations were redissolved in deionized water under controlled CO 2 conditions. Ion activities were calculated from the solution composition using the geochemical speciation model MINTEQA2. X‐ray diffraction (XRD) and infrared absorption spectroscopy provided evidence for precipitation of poorly crystalline Mg silicates and hydrous Na‐Mg silicates (sepiolite [Mg 2 Si 3 O 7.5 (OH)·3H 2 O], loughlinite [Na 2 Mg 3 Si 6 (OH) 16 ·8H 2 O], stevensite [Na 0.33 Mg 2.87 Si 4 O 10 (OH) 2 ], or magadiite [Na Si 7 O 13 (OH) 3 ·3H 2 O]). Calcite, aragonite, gypsum, and amorphous SiO 2 were also identified. The solubility of the solid phase was proportional to the amount of Si and Mg in the solid, and dissolution of the solids in deionized water under atmospheric CO 2 yielded an average CaCO 3 ion activity product value of 10 −8.0 . The maximum mole fraction of Mg ( X Mg ) in the calcite determined by XRD was <0.03, while X Mg values calculated from mass balance were up to 0.53. The high solubility of the solid phases and the high Ca 2+ (aq) and CO 2− 3 (aq) activities are attributed to poorly crystalline Mg silicates and Na silicates similar in composition to sepiolite, loughlinite, stevensite, and magadiite.