Auger spectroscopy analysis of magnesian calcite overgrowths precipitated from seawater and solutions of similar composition

Journal Article · Mon Apr 01 04:00:00 EST 1985 · Am. J. Sci.; (United States)

DOI:https://doi.org/10.2475/ajs.285.4.289· OSTI ID:6399734

Mucci, A; Morse, J W; Kaminsky, M S

Novel procedures were developed to determine the composition of magnesian calcite overgrowths precipitated on calcite crystals from seawater solutions. A surface sensitive analytical technique, Scanning Auger Microanalysis (SAM), was used to determine the composition of very thin overgrowths (< 0.5 ..mu..m) which could not be analyzed by more conventional techniques. Thin magnesian calcite overgrowths were precipitated by exposing Iceland spar calcite crystals to slightly supersaturated (Omega similarly ordered 1.2) synthetic seawater solutions of various Mg to Ca ratios, at 25/sup 0/C, for various lengths of time. The SAM analysis of these thin overgrowths (30-300 A) reveals that their composition is identical to the composition of much thicker overgrowths (approx. = 4000 A) which were precipitated from highly supersaturated solutions (3 less than or equal to Omega less than or equal to 17) on reagent grade calcite powder. These results strongly suggest that the magnesian calcite overgrowth is in exchange equilibrium with the solution from which it precipitates and that it is representative of the solubility controlling phase. A magnesian calcite overgrowth containing 8 +/- 1 mol percent magnesium carbonate will precipitate from standard composition seawater ((Mg/sup 2 +/)/(Ca/sup 2 +/) = 5.13) under their experimental conditions. The SAM analysis of the surface-most adsorbed layer (approx. = 10 A) on the overgrowths precipitated from seawater solutions of various magnesium-to-calcium concentration ratios reveals that the magnesium-to-calcium concentration ratio in the surface-most region follows a typical Langmuir adsorption isotherm profile. Since it appears that the composition of the surface-most adsorbed layer of a magnesian calcite does not play an active role in determining the composition of the precipitating solid, they speculate that the adsorbed layers on the overgrowth may be viewed as a porous region within which the growth process takes place.

• Research Organization: Texas A and M Univ., College Station
• DOE Contract Number: W-31109-ENG-38
• OSTI ID: 6399734
• Journal Information: Am. J. Sci.; (United States), Journal Name: Am. J. Sci.; (United States) Vol. 285:4; ISSN AJSCA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580500* -- Oceanography-- (1980-1989)
ALKALINE EARTH METAL COMPOUNDS
AUGER ELECTRON SPECTROSCOPY
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATES
CHEMICAL COMPOSITION
DOLOMITE
ELECTRON SPECTROSCOPY
HYDROGEN COMPOUNDS
MAGNESIUM CARBONATES
MAGNESIUM COMPOUNDS
MINERALS
OXYGEN COMPOUNDS
SEAWATER
SPECTROSCOPY
WATER