Abstract

DE ABRISQUETA, Ander et al. Some weathering processes in the Las Mercedes schist, Miranda State, Venezuela. Rev. Fac. Ing. UCV [online]. 2011, vol.26, n.3, pp.55-62. ISSN 0798-4065.

The Las Mercedes Schist is the more extensive geological unit of the Coastal Cordillera and is characterized by graphite schist/phyllite usually cut by numerous carbonate veins. During the weathering process the phyllite changes from black to white and the white veins change to reddish and orange colors. Mineralogical and geochemical studies were carried out to understand the processes acting in these color changes. Under SEM, the unaltered graphite phyllite is highly conductive and texturally homogeneous, while the altered whitish side contains numerous pores and steps and losses electrical conductivity. It is interpreted that during the advance of the weathering front, the rock undergoes a partial or total loss of graphite, which affects the color and conductivity. Both the reddish and white fractions of the carbonate veins were identified as pure calcite, but the concentrations of Fe and Mn are significantly higher in the reddish fraction. The analysis by infrared spectroscopy ruled out the influence of organic compounds in the reddish fraction so a sample was added to HCL (10%) and the resultant insoluble residue was identified by X-ray diffraction as goethite. Consequently it is interpreted that the advance of the colored front is due to the deposit of microparticles of goethite, as aqueous solutions permeate the vein through intergranular surfaces and structural defects in the calcite. Goethite comes from the oxidation of pyrite and other primary minerals of the bedrock. Field observations clearly show that in both cases the color variations are due to weathering processes.

Keywords : Calcite; Phyllite; Weathering; The Las Mercedes Schist; Elemental analysis; SEM.