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Calcium Carbonate Supersaturation in Soil Solutions of Calciaquolls
Author(s) -
Inskeep William P.,
Bloom Paul R.
Publication year - 1986
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/sssaj1986.03615995005000060011x
Subject(s) - calcite , supersaturation , dissolution , calcium carbonate , solubility , mineralogy , precipitation , chemistry , carbonate , physics , organic chemistry , meteorology
We found that soil solutions of Calciaquolls in western Minnesota were up to 40‐fold oversaturated with respect to pure calcite. Selective dissolution of soil calcites and x‐ray diffraction showed that the Mg content of these calcites ranged from 1 to 10 mole % MgCO 3 . This amount of Mg substitution cannot explain the degree of oversaturation observed in the soil solutions. X‐ray peak broadening and scanning electron microscopy (SEM) was used to determine the size and shape of soil calcites. The mean size of soil calcite crystallites was approximately 0.04 to 0.09 µm. Surface energy relationships predict that soil calcites of this size would have solubility products (−log K sp ) near 8.3, not low enough to explain the observed supersaturation. Energy dispersive analysis of x‐rays (EDAX) and SEM observations of these samples showed that the larger soil calcites (0.5–20 µm) were irregularly shaped and did not exist as independent crystals, and were associated with other aluminosilicate solid phases. Soil suspensions of Calciaquolls prepared at 10‐fold oversaturation remained oversaturated for 40 d. Adding pure calcite seeds to soil suspensions and soil solution supernatants induced some precipitation, but the solutions remained oversaturated, attaining CaCO 3 ion activity products (−long IAP) ranging from 7.6 to 7.9. These results suggest that the metastable condition of supersaturation is a result of inhibition of the calcite precipitation process by water‐soluble organic components, which are adsorbed and block potential crystal growth sites.