The retention of elements in mineral pseudomorphs in lateritic saprolite from granite — a weathering budget

Abstract

Saprolite formed by lateritic isovolumetric chemical weathering of granite contains pseudomorphs of primary mineral grains which can be separated and analysed. For most elements the chemical composition of rock and saprolite matches the sum of the contributions by the separated primary mineral grains in the rock and their pseudomorphs in saprolite. This agreement allows an element 'budget' to be calculated, which shows that the behaviour of some elements during alteration of granite is determined by the mineralogy of the primary mineral grains and their various alteration products. Most of the Si, Ba, Sr, Pb and Cu present in feldspars in granite was lost on weathering to halloysite, whereas Ga, Al and V were retained. Most of the Zn, Mn and Co that was present in biotite in granite was also lost during alteration, whereas the pseudomorphs retained Fe, Cr and Ni. Zn, Fe, Mn, Cr and V present in magnetite were wholely retained within the hematite pseudomorphs, whereas Ni was lost. Thus Zn and Mn exhibit a mobile behaviour when initially present in biotite and a residual behaviour when present in magnetite. Ni shows an opposite behaviour; it is mobile in magnetite but residual in biotite. These differences are a consequence of the formation of different alteration products from the primary minerals.