Quantification of weathering processes in an acid brown soil developed from tuff (Beaujolais, France): Part I. Formation of weathered rind

Abstract

The petrographical and mineralogical composition of tuff in the studied area was determined by optical microscopy, X-ray analysis, bulk chemical analysis and mean chemical composition of each mineral obtained from the electron microprobe. The tuff stones sampled on the surface and in the pits of soil are usually surrounded by a bleached rind. From the results of chemical weathering mass balance (isovolumetric calculations), the chemical and mineralogical transformation are quantified during the formation of this weathered rind. This tuff from the Upper Visean was composed of about 45±5% phenocrysts (quartz, andesine, albite and biotite) and about 55±5% devitrified groundmass (spherolites: quartz, K-feldspar, albite, andesine and biotite). This series was a broad monotonous series although some facies diversity occurred, i.e., colour and density of plagioclase phenocrysts, evidence of fluidality. A local heterogeneity in the tuff mineralogy and a variation in its chemical composition (K 2O, CaO, Na 2O and loss on ignition) have been induced by devitrification of the groundmass and hydrothermal alteration. The contact metamorphism seemed to have caused the groundmass devitrification and recrystallization of biotite phenocrysts as small biotite crystals (<0.1 mm). Hydrothermal alteration has led to biotite chloritization, transformation of plagioclase (albite and andesine) into white mica and K-feldspar, and a few veins (quartz, chlorite, epidote). In the `Montagne des Aiguillettes', tuff weathering appeared as a thin bleached rind around all stones. The weathering gradient inside rinds corresponded to a progressive dissolution of tuff minerals: quartz<K-feldspar<white mica<biotite<albite<apatite<andesine. The weathering processes were characterized by a strong dissolution of plagioclases and apatite (increase of voids by about 12%) and to a lesser extent by the transformation of micas (biotite) into hydroxy-interlayered vermiculite (HIV) and mica-HIV mixed-layer minerals and by kaolinite (KA) precipitation. The results of the petrographical study were consistent with the calculated mass balance. The weight losses were about 18% and corresponded to a strong exportation of CaO, Na 2O, Sr, P 2O 5, Al 2O 3 and SiO 2 (74; 62; 51; 42; 17 and 25%, respectively) and an increase in the water content. Eighty-two percent of andesine, 41% of apatite, 40% of albite, and biotite (<3%) were dissolved. The increase in the total water content corresponded to the formation of hydrated weathered minerals (2.6% of HIV and 4.3% of KA).