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Role of clay minerals in the transportation of iron

January 1, 1958

The clay minerals have iron associated with them in several ways:

1.

(1) as an essential constituent

2.

(2) as a minor constituent within the crystal lattice where it is in isomorphous substitution and

3.

(3) as iron oxide on the surface of the mineral platelets. Nontronite, “hydromica,” some chlorites, vermiculite, glauconite and chamosite contain iron as an essential constituent. Kaolinite and halloysite have no site within the lattice for iron, but in certain environments iron oxide (goethite or hematite) is intimately associated as a coating on the micelles. Analyses of clay minerals show that the content of Fe2O3 varies: 29 per cent (nontronite), 7·3 per cent (griffithite), 4.5 per cent (“hydromica”), 5.5 per cent (chlorite), 4 per cent (vermiculite) and 18 per cent (glauconite). The FeO content is: 40 per cent (chamosite), 7.8 per cent (griffithite), 1–2 per cent (“hydromica”), 3 per cent (glauconite) and 2 per cent (chlorite).

The iron associated with the clay minerals remains stable in the environment in which the minerals occur, but if either pH or Eh or both are changed the iron may be affected. Change of environment will cause:

1.

(1) removal of iron by reduction of Fe3+ to Fe2+;

2.

(2) ion-exchange reactions;

3.

(3) instability of the crystal lattice.

Experiments using bacterial activity to produce reducing conditions with kaolinite and halloysite coated with iron oxides and with nontronite in which ferric iron is in the octahedral position within the lattice showed that ferric oxide is removed at Eh +0·215 in fresh water and at Eh +0.098 in sea water. Hematite, goethite, and indefinite iron oxides were removed at different rates. Red ferric oxides were changed to black indefinite noncrystalline ferrous sulphide at Eh −0.020 but reverted to ferric oxide under oxidizing conditions. Nontronite turned bright green under reducing conditions and some of the ferrous iron remained within the lattice on a return to oxidizing conditions. Bacterial activity seems to be necessary for maintaining reducing conditions in the environments studied.