Physics
Scientific paper
Jul 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979gecoa..43..951r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 43, Issue 7, pp.951-955
Physics
4
Scientific paper
The instability of transition metal dolomites [CaR 2+ (CO 3 ) 2 where R 2+ is Fe, Co, Ni, Cu, or Zn] and the limited substitution of transition metal cations for Mg in the dolomite structure can be accounted for by the effect of octahedral distortion. For example, trigonal elongation of the Fe octahedron, due to the Jahn-Teller effect, observed in siderite and ankerite, results in elongation of the Ca octahedron which is sensitive to distortion because the radius of Ca 2+ is close to the upper limit for octahedral coordination. Co, Ni, Cu, Zn octahedra are also thought to be deformed, relative to Mg octahedra, in carbonates. The free energy of formation ( G o f ) of R 2+ CO 3 becomes more positive with increasing octahedral distortion. Estimated G o f(dolomite) as well as stabilities and solubility limits of R 2+ in natural and synthetic dolomites suggest a series in order of decreasing stability: Mg > Mn > Zn > Fe > Co > Ni > Cu . G o f(est.) for the terminal Fe-dolomite solid solution [72 mol% CaFe(CO 3 ) 2 ] in the system CaCO 3 -MgCO 3 -FeCO 3 may represent an empirical threshold value for dolomite stability which lies between G o f for Mn- and Zn-dolomites. While Zn-dolomite is probably not a stable phase, very extensive solid solution toward CaZn(CO 3 ) 2 is to be expected in the system CaCO 3 -MgCO 3 -ZnCO 3 . The tendency for transition metal dolomites to contain excess CaCO 3 can also be accounted for in terms of octahedral distortion and AG o f .
Foit Franklin F. Jr.
Rosenberg Philip E.
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