Chlorite tetrahedral sheet, composition

The hydrothermal dioctahedral chlorites have considerably less tetrahedral substitution than those formed in sediments. The former would appear to be a stable phase and the latter a metastable phase. The tetrahedral composition of the hydro-thermal specimens is similar to that for the other dioctahedral clays and represents a reasonable fit between the tetrahedral and octahedral sheets. 

The trioctahedral chlorite structure consists of 2 1 talclike layers of composition (R, R " )3(Si4 j.AyOio(OH)2 that alternate in the structure with octahedral brucitelike interlayer sheets of composition (R ", R )3(OH)6. The tetrahedral portion of each 2 1 layer has a negative charge x due to substitution of x ions of AP, or occasionally of Fe or Cr, for Si . The interlayer sheet has a positive charge due to substitution of R " " ions for R and serves to neutralize the negative charge on the 2 1 silicate layer. In most cases, it is not possible to determine if the tetrahedral charge is compensated entirely within the interlayer sheet or whether the octahedral portion of the 2 1 layer also acquires a positive charge. The main constituents of the two octahedral sheets are Mg, Fe, Al, and Fe , but with important substitutions of Cr, Ni, Mn, V, Cu, or Li in certain varieties. Any medium-sized cation will fit in the octahedral sites. 

It is likely that the amount of tetrahedral rotation is governed primarily by the misfit within the 2 1 layer, but that the interlayer sheet has a secondary and modifying influence on a and on b (obs.). This influence can be seen especially well in the dioctahedral chlorites. Chlorites with two dioctahedral sheets have b (obs.) values near 8.94 A, similar to b (obs.) for the dioctahedral kaolin minerals. Most chlorites, believed to have a dioctahedral 2 1 layer, but a larger and trioctahedral interlayer sheet, have b (obs.) near 9.05 A so that the interlayer sheet is probably stretching the 2 1 octahedral sheet and reducing the amount of tetrahedral rotation required. The interlayer sheet itself must be compressed within the (001) plane and thickened. This misfit of 2 1 layer and interlayer sheets would be minimized by restricting the amount of tetrahedral substitution, as is often observed. Cookeite also has di,trioctahedral sheets, but b (obs.) is smaller and quite similar to b (obs.) for the kaolin minerals. The reason for this similarity is that the composition of the interlayer sheet in cookeite is 2A1 -1- ILi. The Li is just about the right size to fit in the octahedral site that is normally vacant in a kaolin structure. 

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