Textural Transition and Genetic Relationship between Precursor Stevensite and Sepiolite in Lacustrine Sediments (Jbel Rhassoul, Morocco)

Abstract

Abstract--In the Tertiary lacustrine sediments of the Jbel Rhassoul (Morocco), stevensite and sepiolite, confined to the dolomitic facies, are commonly mixed at lower parts of the so-called "Formation Interm6diaire". Transmission electron microscopy (TEM) observations reveal a relation between these 2 magnesian clay minerals. One can observe the different transition steps, from the initial folded layers of stevensite to the fibers emerging from the layers, and finally to the complete replacement of stevensite by sepiolite. That transition can also be observed by scanning electron microscopy (SEM), where the fibers seem to grow at the expense of stevensite. Thermodynamic calculations have been applied to provide geochemical conditions for the formation of sepiolite after stevensite. Early deposition of the "Formation Interm6diaire" occurred under climatic conditions varying between dry and wet. During dry periods, the relative silica enrichment and the pH decrease in the lake water should destabilize stevensite, leading to the formation of sepiolite. Subsequently, a perennial wet climate would lead to the formation of pure stevensite without any trace of sepiolite.

Study of traditional Tunisian medina clays used in therapeutic and cosmetic mud-packs

“…2a), made up of very thin plates with irregular outline and curled edges (Fig. 2b), typical of Moroccan stevensite (Chahi et al, 1997). Gabes-2 showed a more irregular appearance (Fig.…”

Preservation in microbial mats: mineralization by a talc-like phase of a fish embedded in a microbial sarcophagus

“…Saturation indexes (SI) of the solutions with several Mg-silicate phases, defined as the decadic logarithms of the ratio of the ion activity product over the solubility product constant (Ks), were calculated. Solubilities (expressed as Log Ks), estimated at 25 • C, of kerolite, sepiolite stevensite and talc were 25.79, 15.76 (Stoessell, 1988), 25.45 (Chahi et al, 1997), and 23.02 (Jones, 1986), respectively. Solutions in the microbial mat and the control tanks were significantly oversaturated with these different Mg-silicate phases, with SI-values of 5.54 for kerolite, 3.52 for sepiolite, 5.71 for stevensite, and 8.31 for talc in the microbial mat tank (see Supplementary Tables S1-S4).…”

Adsorption of metal ions onto Moroccan stevensite: kinetic and isotherm studies

“…We present in this work a study of the adsorption of some bivalent metal ions (Cd(II), Cu(II), Mn(II), Pb(II), and Zn(II)) by a abundant Moroccan stevensite called locally rhassoul. Recent studies were devoted to the geochemical and mineralogical characterization of the Moroccan stevensite [2,3]. At present, this mineral is essentially marketed as a detergent or traditional shampoo.…”