Comprehensive physicochemical study of dioctahedral palygorskite-rich clay from Marrakech High Atlas (Morocco)

摘要

This study is devoted to the physicochemical and mineralogical characterizations of palygorskite from Marrakech High Atlas, Morocco. The raw clay and its Na+-saturated &2 μm fraction were characterized using chemical, structural, and thermal analytical techniques. Measurements of specific surface area and porous volume are reported. The clay fraction was found to be made up of 95 % of palygorskite and 5 % of sepiolite. An original feature of this palygorskite is its deficiency in zeolitic H_2O. The half-cell structural formula of its dehydrated form was determined on the basis of 21 oxygens to be (Si_(_(7.92))Al_(0.08))(Mg_(2.15)Al_(1.4)Fe_(0.4)Ti_(0.05)□_1)(Ca_(0. 03)Na_(0. 08)K_(0.04))O_(21), while the hydrated form could be formulated as (Si_(7.97)Al_(0.03))(Mg_(2.17)Al_(1.46)Fe_(0.40)Ti_(0.05))(Ca_(0.03)Na_(0.07)K_(0,03))O_(20.18)(OH)_(1.94)(OH_2)_(3.88)·2.43 H_2O. These formulas show that the (Al~(3+)+Fe~(3+))/Mg~(2+) ratio is around 0.84, revealing a pronounced dioctahedral character. Further, inside its octahedral sheet, it was determined that the inner M1 sites are occupied by vacancies, whereas the M2 sites are shared between 90 % of trivalent cations (78 % for Al~(3+) and 22 % for Fe~(3+)), 7.5 % of Mg~(2+), and 2.5 % of Ti~(~(4+)), all of them linked to 1.94 of structural hydroxyls. The two remaining Mg~(2+) by half-cell occupy edge M3 sites and are coordinated to 3.88 molecules of OH2. Channels of this palygorskite are deficient in zeolitic H_2O since they contain only 2.43 H_2O molecules. A correlation was found between these results and the observation of very intense and well-resolved FTIR bands arising from dioctahedral domains (mainly Al_2OH, Fe_2OH, and AlFeOH) along with very small responses from a trioctahedral domain (Mg_3OH). Accordingly, a schematic representation of the composition of the octahedral sheet was proposed. The cation exchange capacity, specific surface area, and total pore volume were also assessed to be ca. 21.2 meq/100 g, 116 m~2/g, and 0.458 cm~3/g, respectively.