Microscopical aspects of garnet weathering in a humid tropical environment

A sequence of different weathering stages of garnet under humid tropical conditions (Yaounde area, Cameroon) is described micromorphologically and mineralogically. The fissures, already observed in ‘fresh’ garnet in the rock, increase in number and size in the saprolite, and tend to get filled with well crystallized goethite. In this way a boxwork structure is formed. The isolated garnet fragments undergo a congruent dissolution, and the created space may be filled by gibbsite and/or globular Fe-oxihydrates. In the A and B horizons a further dissolution of garnet and gibbsite takes place and all available space is filled with microcrystalline Fe-oxihydrates. In this way a nodule, pseudomorph after garnet and with still recognizable boxwork-structure, is formed. Near the soil surface these nodules may be transformed to haematite and/or undergo physical decay. Fresh garnets entering directly into the soil environment mainly undergo a physical decay and transformation to amorphous or microcrystalline Fe-oxihydrates, without formation of a pseudomorph.