Polymorphism kaolinite

Kaolinite, ideally Al2Si205(OH)4, consists of 1 1 layers, alternating sequences of silicate and hydrated Al-octahedra (dioctahedral) sheets. There is potential for disorder in the specificity of the site occupied by Al and in the stacking of the sheets and layers, which give rise to the polymorphs dickite and halloysite. 

Figure 1. Representation of the ideal compositions of some major phyllosilicate phases in the MR - 2R - 3R coordinates. M = muscovite, paragonite P - phlogopite Py = pyrophyllite Kaol = kaolinite S serpentine T = talc Chlor = chlorite, 14 8 or aluminous 7 8 polymorphs Ce = celadonite F = feldspar.

Studies of hydrothermal alteration products associated with ore mineralization in acidic rocks have established the general propensity for the original minerals to be replaced by illite, sericite or hydromica in the innermost zone near the source of hydrothermal fluids and by kaolinite or expandable minerals further from the vein or center of fluid emanation. The newly-formed "mica" can be 2M, 1M, or lMd in polymorph and range compositionally from muscovite to a low potassium, silicic species which can be assimilated in the term illite (Lowell and Guilbert, 1970 Schoen and White, 1966, 1965 Kelly and Kerr, 1957 Bonorino, 1959 Tomita, e al., 1969 Yoder and Eugster, 1955 Meyer and Hemley, 1959, among many authors). 

Figure 29. Possible general phase relations for illite and associated phyllosilicates as a function of varying P-T conditions. Ill = illite, either predominantly IMd or 2M in polymorph I = illite, 2M mica ID = k layer ordered mixed layered phase MLSS = mixed layered 3 or 2 layer ordering giving a superstructure reflection ML0 = mixed layered, ordered structure with no superstructure MLr = mixed layered non-ordered M, = fully expandable montmorillonite Chi = chlorite Kaol = kaolinite Exp 3 " expanding chlorite and/or corrensite.

Kaolins. The kaolin minerals include kaolinite. dickite. and nacrite which all have composition AUOi 2 SiO 2 FLO halloysite (7 At. AGO, 2 SiO, 2 H.O and halfoysile (10 A). Al 0, 2 SiO 4 H 0 The struclural formulas for kaolinite and halloysite tIO At. which are shown in Figure I, are AbSiaOiotOH)). and AljSi.iO, i(OH) -4 H,(). respectively. The so-called fire day mineral is a h-axis disordered kaolinite halloysite (7 A) and halloysite (10 A) are disordered along both the a- and h-Mcs Indeed, most variations in the kaolin group originate as structural polymorphs, related to variations in layer slacks. 

From the same compositions, but at lower temperatures (below 400—500°C), a 7 A structure of the kaolinite-type is developed. Nelson and Roy (1958) called these materials septechlorites. It was not established whether the 7 A phase was metastable or not, but able to persist for long periods as a stable low-temperature polymorphic form of chlorite. They did not synthesize any ferroan chlorites but believed that similar polymorphic relations should exist in the iron chlorites. Tumock and Eugster (1958) synthesized a 7 A chlorite, daphnite, at a temperature of 400°C with a composition of (Fes AlXSi3 A1)0, o(OH)8 but were not able to convert it completely to the 14 A polymorph. 

Nacrite—more regular layers and larger in size (a polymorph of kaolinite)... 

Because of their alternating tetrahedral (T) and octahedral (O) layers, the two-layer phyllosil-icates are said to have T 0 structures. Listed in Table 9.1 are other T 0 minerals, including the kaolinite polymorphs nacrite, dickite, and halloysite, and the trioctahedral serpentine minerals lizardite, antigorite, and chrysotile, in which brucite layers alternate with layers of silica tetrahedra. 

In the model systems C through I (Table II) different sources of calcium and sodium were mixed with clay (kaolinite) and heated to observe their role in forming the aluminosilicates typical of ash fouling deposits. In systems C and D calcium acetate and sodium acetate were mixed with kaolinite in equal molar ratios. X-ray diffractometer patterns showed that both systems were, for the most part, amorphous at 750°C. However, in system E where sodium and calcium acetate were present in equal molar ratios carnegietite was formed at 750°C. Carnegietite is a polymorph of nepheline. In carnegietite the sodium cation is tetrahedrally coordinated whereas in nepheline the sodium cation is octahedrally coordinated. 

Quartz is present throughout, and is relatively unaffected. In the presence of quartz, kaolinite is the expected aluminosilicate phase. However, as mentioned above, gibbsite (Al(OH)3) or a gibbsite-like phase (such as one of its polymorphs, or an amorphous phase) is often found apparently to be controlling the dissolved A1 concentration. Very similar buffering stages would be observed in this case. 

Halloysite has the same stoichiometrical composition of kaolinite, except for its water content. Layered halloysite occurs mainly in two different polymorphs, the hydrated form (with interlayer spacing of 10 A) with the formula Al2Si20s(0H)4.2H20 and the anhydrous form (with interlayer spacing of 7 A) and kaolinite composition - Al2Si20 (0H)4. The intercalated water is weakly bound and can be readily and irreversibly removed 0oussein et at, 2005). 

This approach is reminiscent of Newnham s discussion [1961] of polymorphism in kaolin minerals, in which he pointed out that there should be two single-layer cells, kaolinite and its mirror image. These should, however, give identical X-ray patterns and would only be distinguishable by anomalous dispersion experiments, optical activity, or etch patterns, for example. 

The three minerals kaolinite, dickite, and nacrite differ not in the fundamental nature of their structural units, but in the ways in which these units are stacked one on top of another. The numerous forms of cronstedite investigated by Steadman and Nuttall [1963, 1964] and of chrysotile and other serpentine minerals investigated by Whittaker and Zussman in numerous publications also differ from one another only in the ways the structural units are stacked. Thus, the many minerals in this group can be considered as polymorphic variants of a few basic structures. 

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