The Numedal River basin (6000 km2) in central and east Norway contains 3 × 109 m3 of clayey material, corresponding to a clay loam sediment, which contains 2 million tons of absorbed rare earth elements.

The clay mineralogy varies systematically from the till clays to a marine facies. It is concluded that the phyllosilicates of the clay fraction and tills originated from phyllosilicate minerals of the crystalline bedrock through a degradation partly due to preQuaternary weathering which dissolved a great part of the rock-forming minerals.

A 25°C, 1 atm total pressure phase diagram for the system SiO2-AlOOH-Fe2O3-H2O, is determined using the Phase Rule and Shreinemaker’s technique as in Chesworth (1974). The waterpresent assemblages so defined are equivalent to a kaolinitic stage of the goethite facies of the weathering zone. As such they constitute the ideal stable assemblages of several earth-surface deposits and the stage can therefore be used to construct models of these deposits.

All MDO-polytypes (i.e. regular polytypes) of trioctahedral, dioctahedral and monoctahedral 1:1 phyllosilicates are derived from their OD-character using the results of our preceding paper. The terms ‘simple polytype’, ‘standard polytype’ and ‘regular polytype’ turn out to be synonyms of the term ‘MDO-polytype’ which has the advantage of being exactly defined. The method used to obtain the MDO-polytypes is also applicable to more complicated cases.

Chemical changes during the natural alteration of micas were studied by electron microprobe and classical chemical analyses of fresh and altered portions of mica flakes from 10 Canadian mineral deposits. Results of 50 new analyses are discussed in five examples, starting from simple changes in the interlayer followed by exsolution of titania and ending with complex replacements of anions and cations in all layers of the mica structure. Alteration of micas starts along 001 cleavage planes and fractures and gradually extends into the entire flake leaving some or no remnants of the host mica. The removal of ions from the mica structure and from the flake takes place by gradual depletion, by exsolution of oxides, and/or by alternating removal and redeposition of a newly-formed oxide, illustrated in the following example of the removal of Ti;

1. (1) removal of Ti from the mica structure and exsolution of rutile in the parent mica

2. (2) destruction of rutile and recrystallization as anatase on the surface of the mica; and

3. (3) destruction of anatase, removal of Ti from the surface of the host and crystallization of anatase away from the parent mica.

Residual minerals replacing the original micas are secondary and recrystallized micas, chlorite, vermiculite, serpentine, talc and depleted, optically amorphous flakes. Newly-formed minerals which may incorporate the released ions into their structures are rutile, anatase, sericite, hydronepheline, chlorite-serpentine aggregates, goethite and jarosite. The following partly-altered micas are indicative of sulphide mineralization;

1. (a) bright-green, partly altered phlogopite from ultrabasic rocks and from Co-Cu-Fe-Ni sulphide assemblages. The green phlogopite formed by replacement of K, Al and Ti by Fe, Mg and OH

2. (b) depleted mica coated with jarosite and Ni-goethite in the oxidation zone of a nickel deposit.

   The jarosite formed from released S and Fe from decomposed Fe-Ni sulphides and K released from the host mica; and

3. (c) chlorite-sericite-biotite alteration zones adjacent to Pb-Zn-Cu-Fe deposits.

The clay fraction of an Attic soil has been fired at temperatures up to 1100°C and the transformations induced by firing were studied by Mössbauer spectroscopy. The unfired clay contained iron in the form of small particles of α-Fe2O3 and β-FeOOH and as a substitutional ion in the clay mineral structure. A transformation of β-FeOOH to α-Fe2O3 is observed in the region of 200–400°C followed by dehydroxylation of the clay mineral between 500 and 800°C. A disintegration of the clay mineral structure is inferred above 800°C, followed by recrystallization in an iron-rich phase, most probably in the form of very fine particles. The room temperature spectra of the fired samples indicate that an increase in particle size of the oxides occurs upon firing.

Garnierite from the Tocantins Complex at Niquelandia, Brazil, is a 15Å, dioctahedral clay mineral, nickeliferous nontronite. The principal octahedral cations are Fe3+, Al and Ni. The ferric state of the iron has been verified by ESCA. Ni occupies both the octahedral site and an exchange site. The garnierite formed (and is still forming) by the weathering of nickeliferous pyroxenite. Although the garnierite is a secondary product of weathering, it undergoes further change as weathering progresses: Ni and silica decrease, Fe3+ and Al increase, and the color changes from bright yellow green to red brown. Eventual breakdown of the garnierite leaves mainly hydrated oxides of iron and aluminum.