Chlorite variations, structural

The variations in Fe and Mg contents of the 14 A Fe-chlorite-14 A Mg-chlorite solid solution are considered here. However, structural formulae for chlorite are not as simple as those considered here. As mentioned by Walshe and Solomon (1981), Stoesell (1984), Cathelineau and Nieva (1985) and Walshe (1986), chlorite solid solution may be represented by six components, and accurate thermochemical data on each end-member component at the hydrothermal conditions of concern are necessary to provide a far more rigorous calculation of the equilibrium between chlorite and hydrothermal solution. However, the above argument demonstrates that the composition of chlorite is a highly useful indicator of physicochemical conditions of hydrothermal solution and extent of water-rock interaction. 

Vermiculites exist in various stages of dehydration. Because of the similar dimensions of the water-cation layer in vermiculite and the brucitelike layer in chlorite, vermiculites can be confused with the chlorites. The common substitutions of Fe" or Fe for Mg (in either the water or octahedral sheet of vermiculites), and AF for Si (in the tetrahedral sheets), as well as the hydration variations, present enormous potential for structural distortion in these types of minerals. Fibrous vermiculite was described by Weiss and Hofmann (1952). 

The most important character of all of the berthierine compositions is their low silica content. The variation of compositions found for pellets from the recent sediments appears to be the result of the crystallization of a chlorite structure with full octahedral occupancy. The meta-berthierines fall within the limits deduced for synthetic magnesian 7 X chlorites, limits which are also near full octahedral occupancy. 

While these minerals are not common in soils, they can be used as prototypes for the many variations on the 2 1 structure that occur in soils. Common ones that will be described here are (a) smectite, (b) vermiculite, (c) illite, and (d) chlorite. Each of these mineral names represents a clay group and a reasonably well-defined range of chemical compositions. 

Despite the discrepancies noted above, the use of (/(OOl) graphs or regression equations will give reasonable tetrahedral compositions for most trioctahedral chlorites. This is attested by the agreement noted above for the four test chlorites and also by the fit of the points to the empirical lines in the figures. The examples cited serve as a warning that other sorts of structural variations may have noticeable effects on (/(OOl) and that agreement better than 7-10% between observed and calculated AF values should not be expected unless these structural variations can be taken into account. 

Tetrahedral A1 is estimated best by /(OOl) Equation (1) (Figure 19) from Brindley [1961] or Equation (5) from Kepezhinskas [1965]. An estimate of Al" + Al + Cr can be obtained with equal accuracy from Equation (3) (Figure 20) from Albee [1962]. Some caution must be used in such estimates because structural factors and octahedral composition may affect f/(001) quite independently of tetrahedral A1 content. For example, the four test chlorites do not show a linear variation of heavy atom content. The corundophilite and ripidolite specimens have comparable Al values, yet iron content of the former. 

Lapham, D. M., 1958. Structural and chemical variation in chromium chlorite. Am. Mineral. 43 921-956. 

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