Summary: Clay-like, hydrous nickel silicates from Niquelandia, Goias, Brazil, and from Jacupiranga, São Paulo, Brazil, are 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 (Mg0·15Ni0·06Ca0·015Na0·005)(Fe0·92IIIFe0·025IIAl0·55Mg0·37Ni0·22)(Si3·88Al0·12)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 nontronites, 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 Ia 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 R1·75(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 defined for the Jacupiranga chlorites to be given this name.