In Situ Genesis of Alumino-Ferruginous Nodules in a Soil Profile Developed on Garnet Rich Micaschist in the High Reliefs of South Cameroon Rainforest Zone (Central Africa)

Abstract

The aim of the study is to highlight in detail using microscopic observations, coupled with mineralogical and geochemical analyses, the genesis of alumino-ferruginous nodules formed during weathering of a garnet rich micaschist in the high reliefs of the south Cameroon plateau. Morphologically, the studied profile is characterized by four horizons from bottom to top: (i) an isalteritic horizon in which there are numerous crystals of garnet with intragranular fracture coatings, which isolate either the original garnet grain fragments or alveolar voids with crystals of gibbsite; (ii) an alloteritic horizon with slightly indurated nodules surrounded by a discontinuous perinodular micaceous cortex which marks the original flow structures of the original bedrock, and where intragranular fracture coatings still isolate alveolus with gibbsite crystals; (iii) a nodular horizon characterized in addition to the properties noted below either by an orange-brown birefringent micromass or a darker and undifferentiated micromass; and (iv) a set of clayey and loose horizons with few nodules, which always show gibbsite crystallarias as in other horizons. Mineralogically, traces of kaolinite are recorded only in the weathering garnet in the isalteritic horizon. Other nodules are made of three secondary minerals gibbsite, hematite and goethite. Geochemically, there is a high expression of aluminium in nodules whose composition remains dominated by silica. Globally, those nodules are formed in situ from original garnet grains as well illustrated by the presence of discontinuous perinodular micaceous cortex and the compartmentalized structure. They are named alumino- ferruginous nodules due to their enrichment in gibbsite, hematite and goethite. This is a typical example of in situ formation of nodules from primary minerals which characterize the south Cameroon plateau high reliefs. This process may be extended to the high reliefs of the intertropical rainforest zone not capped in the past by iron duricrust, thus without any iron duricrust relicts.