Nickel-containing montmorillonites and chlorites from Brazil, with remarks on schuchardtite

Abstract

Summary: Clay-like, hydrous nickel silicates from Niquelandia, Goias, Brazil, and from Jacupiranga, São Paulo, Brazil, arc examined by X-ray diffraction, chemical analysis, and thermogravimetric methods. The nickel silicates from Niquelandia are ferric iron montmorillonites, with nickel in the octahedral positions and, to a lesser extent, in interlayer positions. The chemical compositions of samples dried at 110°C average (Mg_0.15Ni_0.08Ca_0.015Na_0.005) (Fe_0.92^IIIFe_0.025^IIAl_0.55Mg_0.37Ni_0.22) (Si_3.88Al_0.12)O₁₀(OH)₂(H₂O)_0.93. The layer charge originates largely in the octahedral sheet. The cation-exchange capacities average 107 me/100 g clay (110°C). These minerals differ from non-tronites, in which the layer charge originates mainly in the tetrahedral sheets.The nickel silicates from Jacupiranga resemble chlorites, with a non-expanding basal spacing of 14·2-14·3 Å. The better crystalline forms give X-ray data in good agreement with a la polytype structure, which is consistent with their formation by low-temperature processes. Chemical formulae derived on the basis of O₁₀(OH)₈ give an average of 5·47 cations in octahedral positions. Thermogravimetric measurements show up to 4% H₂O lost between 110 and 500 °C. These results suggest a partial vermiculitization of the interlayer sheets. A new method of calculating a structural formula is developed that gives an interlayer composition of the form R_1.75(OH)_3.75(H₂O)_1.14, with (OH + H₂O)/R = 2·80, which approaches the ratio for a dioctahedral sheet structure.A comparison of these defect chlorites with available data on schuchardtites shows many similarities; however, schuchardtite is not yet sufficiently well denned for the Jacupiranga chlorites to be given this name.