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

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, Na 2 O and MgO, slight losses of SiO 2 while Al 2 O 3 and K 2 O 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.

Tropical chemical weathering of a garnet rich micaschist in the rainforest zone of Cameroon

“…Nesbitt and Young (1982, 1984, 1989 and Nesbitt et al (1996) used ternary A-CN-K and A-CNK-FM diagrams to deduce weathering trends. These authors demonstrated that the bulk compositions of diverse rocks composition follow simple trends that are largely unaffected by the climatic conditions under which weathering took place (Aristizábal et al, 2005;Wouatong et al, 2013). In addition to providing a method of visualizing the degree of weathering, the A-CN-K system permits the evaluation of the weathering trends from fresh rock compositions through their weathering products (Fedo et al, 1995;Aristizábal et al, 2005;Wouatong et al, 2013).…”

“…Chemical weathering indices based on quantitative measurements based on whole-rock chemical analyses are commonly used to characterize the intensity and mechanisms of weathering (Nesbitt and Young, 1982;Irfan, 1996Irfan, , 1999Ng et al, 2001;Gupta and Rao, 2001;Voicu and Bardoux, 2002;Aristizábal et al, 2005;Wouatong et al, 2013). Recent works have shown that weathering indices based on the ratio of a group of mobile elements to one or more immobile elements are useful parameters for characterizing changes induced by weathering (Duzgoren-Aydin et al, 2002;Aristizábal et al, 2005;Wouatong et al, 2013).…”

“…Chemical weathering in tropical regions favours extensive lateritization which is the cementation of sesquioxide-rich soil horizons (Helgren and Butzer, 1977;Aristizábal et al, 2005) and formation of deeply weathered profiles (Irfan, 1996;Voicu and Bardoux, 2002). The main chemical factors that usually control the weathering of rocks include solution pH, parent material composition and crystallinity, microenvironment, oxidation-reduction potentials, temperature and ionic strength (Wouatong et al, 2013).…”

“…These processes lead to the formation of laterites, which are the main constituents of weathering mantles in the tropical zone (McFarlane, 1976). They are complex materials derived from the weathering of various rock types (Tardy, 1993;Thomas, 1994;Beauvais, 2009;Ndjigui et al, 2008;Tsozué et al, 2011;Wouatong et al, 2013). Their profiles result from intense chemical weathering in intertropical zones and have commonly developed over several millions of years (Tardy, 1993;Tardy and Roquin, 1998).…”

“…Recent works have shown that weathering indices based on the ratio of a group of mobile elements to one or more immobile elements are useful parameters for characterizing changes induced by weathering (Duzgoren-Aydin et al, 2002;Aristizábal et al, 2005;Wouatong et al, 2013). The elements more mobile during weathering are generally used to quantify the effects of chemical weathering on heterogeneous weathering residua (Price and Velbel, 2003;Aristizábal et al, 2005;Wouatong et al, 2013). Nesbitt and Young (1982, 1984, 1989 and Nesbitt et al (1996) used ternary A-CN-K and A-CNK-FM diagrams to deduce weathering trends.…”