Glauconite layer charge

Since this review was originally completed, Foster (1969) published a review in which similar conclusions are drawn about the glauconites and celadonites. The lack of correlation between iron and potassium content in glauconite is substantiated in her paper. Foster considered the process of glauconitization to be of two separate, unrelated processes, incorporation of iron into the crystal structure and fixation of potassium in interlayer positions, with incorporation of iron and development of negative layer charge preceding complete fixation of potassium . 

Electron micrographs show that glauconites have both a subequant and a lathshaped morphology. Limited data suggest that the lath-shaped glauconites have few expanded layers, high layer charge, and a relatively low octahedral population. 

Calculated layer charge is 0.88 with nearly 80% of this charge originating in the octahedral sheet compared with 55% for glauconite. 

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. 

The calculated tetrahedral rotation for glauconites with high octahedral R3 + values ranges from 8 to 10°. There appears to be no overlap of the illite values (12°—13.5°). As the amount of octahedral R3+ decreases, the octahedral sheet increases in size and charge and rotation values decrease. As the amount of tetrahedral Al decreases, the sheets become similar in size and the amount of rotation approaches zero (K cannot be locked in position to provide sufficient layer separation). As octa-... 

The maximum potassium content is less than 10 wt.% when the samples show no smectite component by XRD. According to Thompson and Hower (1975) this maximum potassium content represents less than one atom of potassium per unit cell of mica (in fact 0.65 atoms). XRD determinations show that the evolution of glauconite mixed-layer minerals and iUite/smectite are similar (Velde and Odin, 1975). However, the geological conditions under which these transformations occur are in general not the same. The glauconite micas show a significant substitution of low-charge ions in both the tetrahedral and octahedral sites, as do Ulites. 

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