Chlorite polymorphs

Although the information available from synthetic studies strongly indicates a P-T control of the chlorite polymorph, natural minerals appear to exhibit both polymorph, 7 and 14 8, at low temperature. Most diagenetic chlorites correspond to a 7 8 polymorph. However, there are occurrences especially in deep ocean sediments of a 14 8 phase. The contradiction cannot be resolved with the information available at present. It is probably reasonable to assume that the 7 8 polymorph stable for all chlorite compositions (i.e., various Fe +, Mg, A1 ratios) and that the 14 8 forms are metastable at low temperatures. However, this is certainly not definitive. 

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.

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.

Microprobe analyses of some berthierine pellets from sedimentary rocks (Velde, e t al., 1974, and new data, Figure 30) indicate that these minerals have compositions close to those of 7 8 chlorites delimited by synthesis studies and here there is a more restricted range of silica substitution in the structure than is found in 14 8 chlorites. Thus the two polymorphs have at least different limits in tetrahedral substitutions. 

First we will consider the chlorite known as berthierine. These are the pelletal ocean bottom 7 8 chlorites. Those reported in the literature (Velde, et al,, 1974 Leone, et al., 1975) and several new analyses from Alpine samples indicate a homogeneous composition throughout the pellet. These samples have undergone metamorphism and the minerals now have a 14 8 polymorph. This may well explain their relative compositional homogeneity. The analyses done on many single grains (with microprobe) show some scatter of compositions but they all lie within the 7 8 chlorite... 

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. 

Steinfink, H., 1958a. The crystal structure of chlorite, 1, A monoelinic polymorph. Acta Crvst., 11 191-195. 

Brigatti MF, Poppi L (1993) Crystal chemistry of Ba-rich trioctahedral micas-IM Etrr J Mineral 5 857-871 Brigatti MF, Lalonde AE, Medici L (1997) Crystal chemistry of Fe3+-rich phlogopites A combined singlecrystal X-ray and Mossbauer study. Proc. 11 Int. Clay Conf. Ottawa, Canada, 317-327 Brindley GW, Oughton BM, Robinson K (1950) Polymorphism of the chlorites. 1. Ordered stmctrues. Acta Crystallogr 3 408-416... 

Baer AJ (1981) Geotherms - evolution of the lithosphere and plate tectonics. Tectonophysics 72 203-227 Bailey SW (1963) Polymorphism of kaolin minerals. Am Mineralogist 48 1196-1209 Bailey SW, Brown BE (1962) Chlorite polytypism. I. Regular and semirandom one-layer structures. Am Mineralogist 47 819-850... 

Figure 26. Two possible phase equilibrium diagrams for synthetic chlorites. (From Nelson and Roy [1958].) (a) Version with 7 A aluminian serpentines as metastable phases (b) version with 7 A aluminian serpentines as stable, low-temperature polymorphs of chlorite. From Am, Mineral. 43 718 (1958).

Beryl M. Oughton, and K. Robinson, 1950. Polymorphism of the chlorites. I. Ordered... 

Zvyagin, B. B., 1963. Theory of the polymorphism of chlorites. Soviet Phys.—Crystallogr. 8 (Eng. trans.) 23-27. 

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