Synthetic minerals chlorites

It is clear from the figures that these chlorites are usually richer in silica, than berthierines or magnesian 7 X synthetic minerals. The compositions are quite variable (1 5 ionic percent Si or Al), but nevertheless the compositions determined from grain to grain in one sample do not exceed the limits defined for synthetic 14 X chlorites (Velde, 1973). 

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. 

Stubican and Roy [1961] have made an infrared study of Mg-Al synthetic chlorites and aluminian serpentines (called septechlorites). They note that a band at 1080 cm found in some natural chlorites is not present in the synthetic products, perhaps due to Si,Al disorder in the latter. The spectra of the 14 A and 7 A forms are similar, but with some distinctions that permit identification. There is generally no resolved band at 910 cm in true chlorites, and the two bands in the 750 to 850 cm frequency region are not as well resolved as in the 7 A minerals. The position of the strongest Si-0 band in the 665 to 685 cm region can be used for quantitative determination of octahedral and tetrahedral A1 in both the chlorites and 7 A serpentines. 

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).

Differential thermal curves for some common chlorites are reproduced in Figure 28, from which it will be observed that particle size can markedly affect the appearance of the curve. The variability among different species is also noteworthy, and it is impossible to define a mean curve for chlorite. Curves for some pseudochlorites (Figure 29), the synthetic ones with incomplete brucite layers, show some similarities to the curves for the chlorite minerals themselves. Some clay-sized chlorites in soils do not, however, give pronounced peaks (Mackenzie [1956]), apparently because of oxidation of Fe to Fe in the lattice (Bain [1972]). 

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