J. M. Martín Pozas

Acid activation of a palygorskite with HCl : development of physico-chemical, textural and surface properties

Abstract A palygorskite from Bercimuel (Segovia, Spain) was treated with 1.0, 3.0, 5.0 and 7.0 N solutions of HCI for 1 hour under reflux. The solids obtained were characterized by XRD, FT-IR spectroscopy, thermal analyses, TEM and nitrogen adsorption-desorption isotherms at 77 K. The acid treatment runs with the removal of the octahedral cations—Mg(II), Al(III) —and the formation of amorphous silica from the tetrahedral sheet. An important increase in the specific surface area was observed during treatment, reaching a maximum of 286 m2/g in the sample treated with 5.0 N HCI. The silica obtained after the treatments maintains the fibrous morphology of natural palygorskite. No creation of microporosity is observed during the treatment.

Textural and structural modifications of saponite from Cerro del Aguila by acid treatment

Abstract The physicochemical properties of clays can be modified by acid treatment with inorganic acids. This treatment is usually referred to as ‘acid activation’ , because it increases the specific surface area and the number of active sites of the solids. In the present study, the acid activation of saponite from Cerro del Aguila (Madrid, Spain) with HCl solutions was measured. Illite, quartz and small amounts of feldspar were found as impurities in the raw saponite. Acid treatments were carried out with different concentrations of HCl solutions. The samples obtained were characterized by mineralogical and chemical analyses, XRD, FT-IR spectroscopy, N2 adsorption-desorption isotherms and TEM. The acid attack, under the conditions employed, produced a progressive destruction of the structure of saponite by partial dissolution of the octahedral Mg(II) cations. Amorphous silica coming from the tetrahedral sheet of saponite was generated. The specific surface area of the most intensely treated sample (2.5% for 24 h) was doubled with respect to that of natural saponite. This increase in the surface area is due to the increase in both the external and internal surface areas.

Geochemistry and EPR of cassiterites from the Iberian Hercynian Massif

Abstract This paper is based on microprobe analysis (Ta, Nb, Fe, Mn, Ti, W, Sn) and Fe3+ EPR spectra of cassiterite samples from 14 tin deposits from the Iberian Hercynian Massif. The intelpretation of microprobe data, atomic ratios (Fe+Mn)/(Nb+Ta), and Fe3+ EPR spectra reveal that the coupled substitution: 3Sn4+ = (Fe,Mn)2+ + 2(Nb,Ta)5+ is dominant in cassiterites from pegmatites, and high-temperature quartz veins and in the dark zones of cassiterites. The mechanisms: 2Sn4+ = Fe3+ + (Nb,Ta)5+ and Sn4+ + O2- = Fe3+ + OH- are dominant in those from medium-low temperature quartz veins and in the light zones of cassiterites.