Isomorphous substitution chlorites

Figure 4.10. Schematic structure of chlorite showing the various isomorphous substitutions and d-spacing (top from Jackson, 1964, with permission).

The primary mineral, chlorite, which occurs in rocks as large crystals, possesses an interlayer sheet composed largely of Mg(OH)2. Since the mineral brucite is composed of magnesia sheets with the same basic structure, the single interlayer sheet in chlorite is termed the brucite layer. Isomorphous substitution of part of the Mg by produces a positively chained hydroxide sheet (see formula above) that props the 2 1 layers apart at a c-spacing of 14 A. This rigid interlayer contrasts with the hydrated interlayer of vermiculite, and even though chlorite and vermiculite have similar c-... 

The tetrahedral sheet of chlorites contains isomorphic substitution by Al(III), and sometimes Fe(III) or Cr(III) is found (Kohut and Warren 2002). The octahedral sheet normally contains Mg(II), Al(III), Fe(II), and Fe(III), but some substitution by Cr(III), Ni(II), Mn(II), or other metals occurs as well. Chlorites, in contrast with vermiculites and smectites, are hardly expandable, because the hydroxide cations are not easily exchangeable, nor is water easily incorporated to the intermediate octahedral sheet. 

Brown, G., 1955. The effect of isomorphous substitutions on the intensities of (00/) reflections of mica-and chlorite-type structures. Mineral Mag. 30 657-665. 

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