A Golden-Colored, Ferri-Nickel Chloritic Mineral from Morro Do Niquel, Minas Gerais, Brazil

G. W. Brindley and Jefferson Vieira de Souza
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.

Abstract: A golden-colored, flaky mineral from Morro do Niquel, Minas Gerais, Brazil, gives an X-ray diffraction pattern of a IIb chlorite polytype, with basal spacing 14·21 ± 0·02 Å and b = 9·23 Å. Thermogravimetric analysis shows a progressive weight (water) loss up to 500°C, followed by a rapid weight loss corresponding to dehydroxylation of the interlayer material and a slower weight loss due to dehydroxylation of the 2:1 layer. The structural formula derived from the chemical analysis on the basis of O10 (OH)8, or total cation valence of +28, shows 5·3 total octahedral cations, i.e. probably 2·3 in the interlayers where normally 3 cations are found. This deficiency together with the appreciable loss of water below 500°C suggests a partially vermiculitized interlayer. A new method for deriving the interlayer composition gives R1·67(OH)4·08(H2O)0·59, and a ratio (OH + H2O)/R = 2·80, which approaches that of a dioctahedral interlayer and is consistent with a predominance of R3+ ions. The mineral may resemble the golden, vermiculitized biotite described by Walker and others.

Clays and Clay Minerals; March 1975 v. 23; no. 1; p. 11-15; DOI: 10.1346/CCMN.1975.0230102
© 1975, The Clay Minerals Society
Clay Minerals Society (www.clays.org)