TY - JOUR
T1 - Rare earth element mineralogy and geochemistry in a laterite profile from Madagascar
AU - Berger, Alfons
AU - Janots, Emilie
AU - Gnos, Edwin
AU - Frei, Robert
AU - Bernier, Felix
PY - 2014/2
Y1 - 2014/2
N2 - In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE+Y concentrations are typical of granitoids (299-363ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9wt% SO3 and 7wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite-jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE+Y reach 2194ppm with high Ce concentrations (1638ppm; 9*Cebedrock) compared to other REE (3-4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile.Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing.
AB - In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE+Y concentrations are typical of granitoids (299-363ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9wt% SO3 and 7wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite-jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE+Y reach 2194ppm with high Ce concentrations (1638ppm; 9*Cebedrock) compared to other REE (3-4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile.Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing.
U2 - 10.1016/j.apgeochem.2013.12.013
DO - 10.1016/j.apgeochem.2013.12.013
M3 - Journal article
SN - 0883-2927
VL - 41
SP - 218
EP - 228
JO - Applied Geochemistry
JF - Applied Geochemistry
ER -