Berthierine composition

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. 

Figure 30. Comparison range of 7-14 8 chlorite compositions in the Mg-Si-A1 system (Velde, 1973). Dots show composition of berthierine pellets in these coordinates.

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... 

Si-Al-R coordinates and Figure 31a shows the compositional range in Fe-Al-Mg coordinates for grains in ten rock samples. It is evident that these berthierines are iron and alumina-rich minerals. 

It is possible that the mechanism of berthierine formation in this example is one of accretion, i.e., the grain would accumulate material at the exterior and this is eventually transformed into a chlorite composition. None of the grains was noted to have the form of a shell test as is often noted glauconite pellets. However the meta-berthierine pellets reported by Velde, et al.. (1974), were often found inside foraminifera tests. 

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). 

Compositions. Berthierines are ferrous almost to the total exclusion of Fe (Brindley, 1982). Diagenetic and metamorphic chlorites contain... 

If we combine the features of the two simulations, we have a composite spectrum with bands at 7.2 A, 7.5 A, and > 15 A. We can see by using these simulations that the XRD results for odinite are probably due to a mineral which is a mixture of a ferrous 7 A chlorite (berthierine) and a ferric smectite/ferrous 7 A chlorite mixed layer mineral. Different proportions of each structural element will give different bulk compositions. 

The relations of verdine and berthierine mineral chemistry can be shown diagrammatically as in Figure 9, where the mineral compositional zones... 

Berthierine, as shown by Brindley (1982) is essentially a trioctahedral mineral, following the line of trioctahedral chlorites in Figure 7. In our simulations of the XRD spectra of odinite, we use a ferrous serpentine and a ferric dioctahedral smectite component. Translated into constituent ions of a mineral structure, this mineral combination will give a bulk average composition between nontronite (ferric, dioctahedral smectite) and berthierine (trioctahedral chlorite). 

Figure 9 Diagram indicating the chemical evolution of ferric smectite material towards berthierine (Be). The arrow follows chemical compositions of verdine (Ve) materials. Si-R +-R coordinates where R = Fe, Al and R = Fe, Mg. Sm = smectite, kaol = kaolinite.

In general, the minerals now identified as chamosite are found in iron ore bodies of sedimentary origin (e.g., Maynard, 1986 Fernandez and Moro, 1998 Wiewora et al, 1998 Kim and Lee, 2000). Chamosite associated with iron oxides appears to follow a compositional trend from iron oxides plus kaolinite to chlorite, as indicated in Figure 8, using the data of Velde (1989). The recombination of iron oxide in the presence of kaolinite gives an aluminous, ferrous mineral, chamosite. This mineral is formed under burial conditions where ferric iron oxide is reduced to feiTous iron which is rapidly incorporated into a 7 A chlorite mineral. Both chamosite and berthierine result from the reduction of ferric iron to ferrous iron. 

Figure 11 Representation of the evolution of clay pellets in shallow shelf sediment areas according to the oxido-reduction conditions locally present. Lower arrow shows berthierine formation through reduction of iron, shifting the pellet composition from the ferric (R = Fe ) pole to the ferrous pole (R = Fe ). This reaction passes through a chemical evolution by the formation of a berthierine/smectite mixed layer mineral (chi in the figure). The arrow towards glauconite indicates the change in composition with increase in potassium and some reduction of ferric iron. The diagram represents feldspar, dioctahedral clays, and trioctahedral clays, respectively. R + = Fe +, R = Al, Fe. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite) and end-member celadonite (Ce) are indicated.

Brindley G. W. (1982) Chemical composition of berthierine a review. Clays Clay Min. 30, 152—155. 

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