Cretaceous and lower Tertiary glauconite in the South Island of New Zealand

Petrographic examination was carried out on glauconites of Cretaceous or Lower Tertiary age from 142 localities in the South Island of New Zealand, Campbell Island and Chatham Island. Samples from which concentrated glauconite extracts could be obtained were further analysed by x-ray fluorescence, x-ray diffraction, and Infrared techniques. Scanning electron microscope and Mӧssbauer studies were also performed on selected samples. Existing morphological classification schemes for glauconite are inadequate for describing many of the glauconites encountered. Two new morphological classes are therefore defined: 'fragmentary glauconite', grains which are fragments of once larger grains which have been broken along planes of weakness (eg. lobes broken off lobate grains); 'corroded glauconite', grains which have a porous spongy surface form. A new internal textural class is also defined as: 'semioriented glauconite', which has a largely random microcrystalline internal texture but contains one or more oriented microcrystalline patches. The current crystallographic classification scheme for glauconites is criticised, and the disorder coefficient (DC) is defined as a new parameter which more closely reflects crystal disorder. Using the value of DC, the % expandables, and the presence or absence of any minerals other than glauconite; the crystallographic classes of glauconite are redefined in quantitative terms. Some iron in glauconite, analysed in detail by Mӧssbauer methods, is found to have a variable oxidation state at room temperature, in that electrons hop between Fe²⁺ and an adjacent Fe³⁺, thus reversing their oxidation states. Greater than 98% of all iron in non-oxidised glauconite is found to be in octahedral coordination; none was detected in tetrahedral or non-structural positions. The data derived are integrated with previously published work to produce a theory of glauconite genesis, which is broadly based on the 'layer lattice' model of Burst (1958a, 1958b). The modified 'layer lattice' model presented here, resolves several limitations inherent in the original proposition. With respect to New Zealand stratigraphy two aspects of glauconite chemistry appear to be important. Firstly, divergent glauconite compositions suggest that there was a significant difference in the geochemistry of the genetic environment for Cretaceous and Lower Tertiary glauconites formed South of the Rangitata River, compared with glauconites formed to the North of this boundary. Secondly, the concentrations of Ti, V, Mn, Zr and Yin glauconite appear to decrease with increasing distance of the genetic environment from metamorphic or igneous basement rocks