The Origin of Etch Pits Recorded on Residual Grain Surfaces from Kaolinized Granitic Rocks West Region Cameroon

A S L Wouatong,B P K Yerima,R Yongue Fouateu,A Mvondo Ze,G E Ekodeck

doi:10.5539/esr.v2n2p93

Abstract

The weathering of some granitic rocks in the West Region of Cameroon (Central Africa) was examined by X-ray diffraction (XRD) and X-ray fluorescence Spectrometer (XRF) in conjunction with Scanning Electron Microscopy (SEM). Kaolinite and illite were the most predominant weathering products whereas gibbsite, goethite and maghemite were also found in small amounts; only residual quantities of quartz and potassium feldspar were observed. The geochemical study shows the mobility of major elements during the progressive kaolinization of granitic rocks. The main trend is the loss of elements, with high losses of CaO, Na2O and MgO, slight losses of SiO2 while Al2O3 and K2O appear to increase with weathering intensity. Chemical index of alteration (CIA) shows a high value (68-95%). The scanning electron microscope analysis reveals the existence of several surface features that reflect the effect of mainly chemical actions on the quartz and K-feldspar grains. Quartz and K-feldspar crystals from kaolinized granitic rocks show low to very high chemical etching attributed to weathering under moderate to severe environmental conditions such as low pH, high temperature and humidity and extreme leaching typical of tropical climatic conditions. Etched pit densities on quartz surfaces increase with weathering, as a result of increasing silica concentrations in pore fluids.

Highlights

IntroductionA tremendous amount of literature exits on the mechanism of chemical weathering under different geomorphic and paleoclimatic conditions, provided by many investigators (Stanley, 1968; Lelong, 1969; Pedro, 1964, 1968; Millot, 1971; Krinsley & Cavallero, 1970; Berner & Holdren, 1979; Nahon et al, 1979, 1982; Tardy, 1993; Kitagawa et al, 1994; Wilson, 1995; Wouatong et al, 1996; Wouatong et al, 2005; Deepthy & Balakrishnan, 2005; Nguetnkam et al, 2008; Ndjigui et al, 2008; Kamgang et al, 2009)
The main trend is the loss of elements, with high losses of CaO, Na2O and MgO, slight losses of SiO2 while Al2O3 and K2O appear to increase with weathering intensity
Quartz and K-feldspar crystals from kaolinized granitic rocks show low to very high chemical etching attributed to weathering under moderate to severe environmental conditions such as low pH, high temperature and humidity and extreme leaching typical of tropical climatic conditions

Summary

Introduction

A tremendous amount of literature exits on the mechanism of chemical weathering under different geomorphic and paleoclimatic conditions, provided by many investigators (Stanley, 1968; Lelong, 1969; Pedro, 1964, 1968; Millot, 1971; Krinsley & Cavallero, 1970; Berner & Holdren, 1979; Nahon et al, 1979, 1982; Tardy, 1993; Kitagawa et al, 1994; Wilson, 1995; Wouatong et al, 1996; Wouatong et al, 2005; Deepthy & Balakrishnan, 2005; Nguetnkam et al, 2008; Ndjigui et al, 2008; Kamgang et al, 2009). Lasaga (1981) and Berner et al (1980) have previously pointed out the importance of active sites concentration in silicate dissolution kinetics. Etch pits have been identified in many naturally dissolved samples (Berner & Schott, 1982; Berner & Holdren, 1979) and used as evidence for interface-controlled dissolution. These poorly characterized “active sites” consist of dislocation edges, micro fractures, point defects, kinks, and grain or twin boundaries (Helgeson et al, 1984)

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