Stacking Order in a 14.30-Å Mg-Vermiculite

Abstract

Abstract--The stacking order ofa 14.30-/~ Mg-vermiculite from Santa-Olalla, Spain, has been determined from Weissenberg photographs. The results prove that the Mg-vermiculite structure is not a 2-layer polytype structure, but a semi-ordered one. Because the structure is semi-ordered, its resolution needed a dual approach: (1) a direct approach using an electron density projection along the y axis in conjunction with a one-dimensional electron-density projection onto the z axis; and (2) an indirect approach in which the observed intensities along the (0,k) and (1 ,k) reciprocal rods were compared to the calculated intensities given by model defect structures. The semi-ordered structure of the Mg-vermiculite results from +b/3 shifts in the stacking of the silicate layers. The shifts are randomly either along +b or -b.

Vermiculite: Structural Properties and Examples of the Use

“…The CEC varies between the 120 and 200 cmol+/kg (air dried vermiculites) or 140 and 240 cmol(+)/kg (dehydrated vermiculites) [14,15]. Water molecules associated with internal surfaces evokes hydration of interlayer cations.…”

“…The hydration properties are controlled by the interlayer cations Mg 2+ and minor amounts of Ca 2+ , Na + , and K + . The cation radius and charge influence the degree of hydration state in the interlayer and the stacking layer sequences [9,15,19,22,28]. The hydration state of vermiculite was defined by the number of water layers in the interlayer space.…”

“…For semi-ordered stacks the reciprocal space cannot be described by a set of hkl indexes but rather by modulated reciprocal hk with the variable intensity along them [15]. The regular alterations of 1.150 and 0.902 nm domains can create the regular interstratification with d = 2.060 nm.…”

“…(Si2.86 Al1.14)(Mg2.83Al0.15Fe 3+ 0.02 )O10(OH)2 Mg0.41 · 3.72 H2O (Llano), [9] (Si2.72 Al1.28)(Mg2.36Al0.16Fe 0.58)O10(OH)2 Mg0.32 · 4.32 H2O (Kenya), [19] (Si2.72 Al1.28)(Mg2.59Fe 2+ 0.03 Al0.06Fe 3+ 0.24 Ti0.08)O10(OH)2 Mg0.39 Ca0.02 ·4.7 H2O (Santa Olalla, Spain), [15] (Si2.64Al1.36)(Mg2.48 Fe 2+ 0.04 Al0.14 Fe 3+ 0.32Ti0.01Mn0.01)O10(OH)2 Mg0.44 (Santa Olalla, Spain), [35] (Si2.69Al1.31)(Mg2.48Fe 3+ 0.324Fe 2+ 0.036Al0.14 Ti0.01 Mn0.01)O10(OH)2Mg0.39, (Santa Olalla, Spain), [57] …”

Effects of Iron Oxidation State on the Texture and Structural Order of Na-Nontronite Gels

“…Annabi-Bergaya et al (1980) and Ben Rhaiem et al (1987) observed a similar phenomenon in low-charge Na-smectite. Studies in which either vermiculite or Ca-smectite was used (e.g., Ben R h~e m et al, 1987;de la Calle et al, 1976de la Calle et al, , 1988 would have failed to show an appreciable effect of either methanol or ethanol, because the layers would already have been largely collapsed due to other forces (e.g., presence of divalent cations), leaving the possibility for only a minor influence from the low-dielectric solvents.…”