Structural celadonite

The main method used to distinguish the relative quantities of neoformed illite is by the polymorph or structure of the material. Using the criteria that 2M and 3T polymorphs of dioctahedral potassic mica are high temperature forms (Velde, 1965a), the determination of the relative quantities of lMd, and 1M vs. 2M, 3T polymorphs permits a semi-quantitative estimation of the proportion of neo-formed or low temperature illite present in a specimen. A method commonly used is a determination of relative intensities of X-ray diffraction peaks of non-oriented mica (Velde and Hower, 1963 Maxwell and Hower, 1967). Usually only 2M and lMd polymorphs are present in illite specimens which simplifies the problem. The 1M polymorph is typical of ferric illites and celadonite-glauconites, the more tetrasilicic types. 

Experimental work in the systems K-Mg-Si-Al-Fe- O concerning celadonites has also produced expandable minerals (Velde, 1972 Velde, unpublished). In both the muscovite-MgAl celadonite and MgFe-MgAl celadonite compositional series, fully expandable phases were produced below 300°C at 2Kb pressure. These expandable phases can coexist with a potassic feldspar (Figure 23). Their (060) reflection near 1.50 X indicates a dioctahedral structure which can apparently be intimately... 

It can be surmised that even though X-ray data indicated only expandable material, there must be significant interlayering with illite or other non-expandable mica-like phases such as glauconite-celadonite in order to give such a high structural charge imbalance. If not, one wonders why illites, with a similar chemical formula, are not expandable as well. 

The converse is true of the Mg ion. It is more abundant in the octahedral sheets of the low-temperature 2 1 dioctahedral minerals, attaining an average value of 3.55% in the montmorillonites and even higher values in glauconite and celadonite. Mg in the octahedral position increases the size of the octahedral sheet and decreases structural strain. 

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 . 

Chemical analyses and structural formulas of some celadonites (After Wise and Eugster, 1964)... 

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

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. 

The Fe2+ content of celadonites (continental origin) and glauconites (marine origin) is identical suggesting that its abundance is not controlled by environmental conditions. Structural control is more likely. Apparently, layer strain is less and the structure more stable when there are 0.20 large Fe2+ ions in the octahedral sheet. 

Most of the celadonite samples lie in the area where some mixed-layering is to be expected. Although celadonite is commonly considered to be non-mixed, the literature suggests that little effort has been made to establish this. Of the 15 analyses examined by Wise and Eugster (1964) six reported adsorbed water and in the others it was not determined. In any event, the sheet structure of the celadonite is distinctly different from that of the other 2 1 dioctahedral clays (Radoslovich,1963a). It has a very thick octahedral sheet all three octahedral positions are of equal size (in the other 2 1 dioctahedral clay the two filled positions are smaller than the vacant position) and the interlayer separation is larger than in other contracted 2 1 dioctahedral micas (Radoslovich,1963a). 

Zussman, J., 1954. Investigation of the crystal structure of antigorite. Mineral Mag., 30 498-512. Zvyagin, B.B., 1957. Determination of the structure of celadonite by electron diffraction. Kristal-laografiya, 2 388-394. 

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. 

The formula Dim is that of an idealized Al-celadonite, Bio is that of an idealized phlogopite, and Chi is that of an idealized clinochlore. (Dim + tk) is, of course, the idealized muscovite formula (Bio + tk) is that of an idealized eastonite, and (Chi + tk) is that of an idealized corundophyllite. The last two are taken as the extreme tk+ limits on the basis of the aluminum avoidance rule. The choice of the clinochlore formula as the tk- limit makes good crystallo-chemical sense for 14-Aigstrdn chlorites, the ones found in pelitic schists. The original chlorite structure described by Pauling (1930) was, in fact, that of a clinochlore found in a blackwall skarn in close proximity to a lizardite-bearing serpentinite near Chester, Vermont, U.S.A. The idealized lizardite formula may be taken... 

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