L'alteration lateritique debutante des pyroxenites de Jacuba, Niquelandia, Bresil

Abstract

RESUME: L'alteration lateritique d~butante des orthopyroxenes et clinopyrox6nes de Niquelandia, Bresil, montre plusieurs stades. Le premier stade facultatif est la formation de produits 'amorphes" dont la composition chimique proche de celle des min~raux parentaux, montre d6j~i un enrichissement en nickel. Le deuxi6me stade est marqu6 par l'apparition de phyllosilicates dont la nature et la composition chimique varient scion le degr~ de porosit6 de la roche parentale. Cette porosit~ contr61e 6galement la distribution du nickel dans les diffbrents phyllosilicates, (saponite, talc, pimelite). Les pyrox6nes ne contenant pas ou tr6s peu de nickel, il est clair que le nickel qui s'accumule darts les produits de l'alt~ration est totalement import6 avec les eaux de percolation. L'origine de ce nickel est fi rechercher plus haut dans les profils ou plus en amont dans la sequence, l/l oti les dunites sont pr~sentes. A B S TR A C T:The initial weathering products under lateritic conditions of orthopyroxenes and clinopyroxenes in Jacuba, Niquelandia, Brasil, are amorphous and have chemical compositions very near those of the parent minerals but with enrichment of Ni. Later products are phyllosilicates such as talc, saponite and pimelite, the nature and composition of which vary with the development of porosity and fissuring of the parent rock. It appears that Ni distribution between these phyllosilicates is also controlled by fissure development. As the parent pyroxenes are Ni-depleted, it is clear that Ni accumulation in the structures of the phyllosilicate weathering products is a result of solution transfer from other parts of the profile.

Weathering of Hornblende to Ferruginous Products by a Dissolution-Reprecipitation Mechanism: Petrography and Stoichiometry

“…"Sawtooth" or "denticulated" terminations characteristic of pyribole remnants at intermediate stages of weathering (Figures 4-6) have been observed by numerous workers in other study areas (e.g., Cleaves, 1974;Delvigne, 1983;Colin et al, 1985;Wilson, 1986). Berner et aL (1980) showed that etch pits (see Figure 3) reflect selective attack at defects and dislocations; the sharp-pointed denticulations (see Figures 4-6) probably resulted from side-byside coalescence of lenticular etch pits (Berner et al, 1980;Berner and Schott, 1982).…”

“…(4) The absence of 2:1 clay minerals in saprolites of Coweeta is apparently due to intense leaching in the study area, which prevented solutions in the weathering profiles from becoming saturated with respect to 2:1 clay minerals. Previous workers (Wilson and Farmer, 1970;Cleaves, 1974;Colin et al, 1985) reported the formation of 2:1 clays only from pyribole weathering in poorly drained weathering environments and/or microenvironments, whereas 1:1 clays and/or oxyhydroxides have been reported from well-leached macro-and microenvironments (Cleaves, 1974;. Natural solutions in previously studied watersheds in the study area ( Figure 1) are in equilibrium with kaolinite, but not with 2:1 clays (Velbel, 1985b).…”

Domain Segregation in Ni-Fe-Mg-Smectites

“…Of the samples analyzed by Colin et al (1985), only those extracted from poorly fractured pyroxenite rocks and which presented a smectitic clay as a weathering product were examined in the present study. Pyroxene crystals were hand picked under a binocular microscope, and the smectitic clay sample was extracted from the pyroxenes by ultrasonic cleaning and then dried and prepared for XRD, TEM, and spectroscopic analyses.…”

“…In a previous paper, Colin et al (1985) studied the Samples from 1 to 18 refer to point microprobe analyses of undisturbed smectitic clay in cracks. Sample 19 is the smectitic clay extracted from weathered pyroxenes by ultrasonic methods and used for X-ray powder and electron diffraction and spectoscopic studies.…”

“…The Jacuba quarry (48~ and 14~ is in the western part of the ultramafic body, which here is strongly weathered into a 30-m-thick, argillaceous and ferruginous, lateritic mantle. Colin et al (1985) reported that the chemical weathering of pyroxenes leads either to Fe-rich, noncrystalline products and then to goethite, or to a phyllosilicate clay. They showed that the phyllosilicate clay consists of either smectite in relatively unfractured parent rock or a mixture of pimelite and smectite in strongly fracCopyright 9 1987, The Clay Minerals Society tured rock.…”