Celadonite cations

Fig.6. Variations of average interlayer cation total with average octahedral total for sixty-six analyzed glauconites and celadonites. Data divided into eight approximately equal groups. (After Tyler and Bailey, 1961.)...

Analysis No. 1 represents the only truly tetrasilic celadonite. The average value for Si in the 17 analyses is 3.83. Except for Nos. 17-19 variations are between 3.73 and 4.00. Octahedral occupancy is close to 2 for all celadonites, with an average of 2.05. Interlayer cations often number substantially less than 1.0 and average 0.81 with a low value of 0.38 (No. 14). This, of course, is typical also for glauconites and hydrous micas. ... 

Fig. 10. Histograms showing the distribution of the cations of twenty-one celadonite structural formulas.

When an octahedral sheet has appreciably more Fe3+ (i.e., glauconite) than is present in celadonite, less Mg is needed to provide the total number of large cations (Fe3++Mg). This results in an increase in the total trivalent ions in the octahedral positions. In glauconites the Al + Fe3+ total is larger than 1.3, dominating the octahedral sheet, and a plot indicates there is no relation between the Mg content and the Al/(A1+Fe3+) ratio. 

In general, when either Al or Fe3+ is the dominant (greater than 1.0) cation in the octahedral sheet of a 2 1 dioctahedral clay, the maximum Mg content the sheet can accommodate is 0.50-0.60 (0.5 if Fe3+ is dominant and 0.6 if Al is dominant). When the Mg content is larger than 0.6, as for most celadonites, seldom is any other cation present in amounts greater than 1.0. This suggests structural control of composition. 

Present data indicate that Fe3+-rich low-charge clays increase their layer charge by increasing the Mg and Fe2+ content of the octahedral sheet at the expense of Fe3 + more so than of Al. The average Al content of glauconite and celadonite is similar to that of nontronite, but the Fe3+ values are lower. With increased octahedral charge there is an increase anion-anion repulsion and the octahedral sheet increases relatively more in the c direction than the 6 direction, which also favors the large cations. Thus, relatively less tetrahedral Al is required to afford the sheet size differential to allow sufficient tetrahedral rotation to lock the K into place. 

With respect to trioctahedral micas, dioctahedral muscovite and celadonitic muscovite have smaller interlayer separations but similar a values. In dioctahedral micas, the proton position results in part from repulsion by the interlayer cation and the cations in the M(2) sites. Thus, the proton is located in that portion of the structure with minimal local positive-charge concentration, near the M(l) site (Radoslovich 1960 Guggenheim et al. 1987). The six-fold coordination of the interlayer cation with the basal inner O atom is distorted and elongated parallel to c. Both effects (i.e., the distorted coordination of the interlayer cation and the smaller H -K repulsion) thus control the interlayer separation. 

Drits VA, Dainyak LG, Muller F, Besson G, Manceau A (1997) Isomorphous cation distribution in celadonites, glauconites, and Fe-illites determined by Infrared, Mossbauer, and EXAFS spectroscopies. Clay Minerals 32 153-179... 

Doyle CS, Traina SJ, Ruppert H, Kandelewicz T, Rehr JJ, Brown GE Jr (1999) XANES studies at the A1 X-edge of aluminum-rich surface phases in the soil environment. J Synchrotron Rad 6 621-623 Drager G, Frahm R, Materlik G, Briimmer O (1988) On the multiplet character of the X-ray transitions in the pre-edge stmcture of Fe K absorption spectra. Phys Status Solidi 146 287-294 Drits VA, Dainyak LG, Muller F, Besson G, Manceau A (1997) Isomorphous cation distribution in celadonites, glauconites and Fe-illites determined by infrared, Mdssbauer and EXAFS spectroscopies. Clay Minerals 32 153-179... 

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