Thermodynamic and structural behavior of analcime–leucite analogue systems

Abstract

Two synthetic solid-solution series, analcime to Rb-leucite and analcime to Cs-leucite (pollucite), have been investigated to understand more fully the thermodynamic and structural behavior of analcime-leucite and similar mineral systems. Unit-cell dimensions and volumes in these series expand with the substitution of analcime component in either Rb-leucite or pollucite, as H 2 O molecules structurally replace the smaller entities Rb +1 or Cs +1 , respectively. Unit-cell volumes vary linearly as functions of composition, but with changing slopes over several segments of compositional space, akin to thermal expansion in K-, Rb-, and Cs-end-member materials studied by previous workers. When symmetry changes displacively from tetragonal to isometric, as in the Rb-bearing series, the slope of volume expansion changes. Once structures have reached full expansion, volume slopes flatten and are little affected by additional analcime component. Enthalpies of solution measured at 50 ∞C in 20.1 wt% hydrofluoric acid show single-slope linear relationships over the entire compositional ranges of both series. Thus, despite positive volumes of mixing, there are no enthalpies of mixing in either series, nor is there energetic evidence of displacive tetragonal/isometric inversion or the various stages of structural expansion. Overall, the data suggest that the analcime-leucite system also can be modeled as close to thermodynamically ideal. The limited solid solution between natural analcime and leucite must be attributed to energetically favored heterogeneous equilibria involving minerals such as feldspars and other feldspathoids, and not to immiscibility between the end-members.

Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

“…In support of this assumption, Teertstra et al (1994) argued that decreased symmetry in analcime resulted from "a slight, systematic rotation of the tetrahedra during growth, which does not involve ordering or even change during cooling from a higher-temperature, more symmetrical phase." Further support comes from Hovis et al (2002), who suggested that at room temperature, analcime is near the transition temperature for a displacive transformation from a cubic to a tetragonal structure.…”

“…We therefore contend that igneous analcime need not be non-cubic, in disagreement with Gottardi & Galli (1985), and that by analogy with leucite, analcime from higher-temperature parageneses should be cubic. Any non-cubic nature in igneous analcime would result from small amounts of solid solution that increase the temperature of a displacive cubic-tetragonal transition (Hovis et al 2002).…”

“Soil for Soil Remediation” Strategy Driven on Converting Natural Soils into Fe2O3-CAN-Type Zeolite Composites for Dual Ionic Heavy Metal-Contaminated Soil Remediation: Universality, Synergistic Effects, and Mechanism

“…The calculated ion-exchange energies are listed in Table , where ΔE for Cd 2+ and AsO 2 – are −5.56 and −0.30 eV, respectively. These negative values suggest that the ion-exchange behaviors between Cd 2+ /AsO 2 – and Na + /OH – within CAN interior channels are energetically favorable and happened spontaneously. − …”

Investigation of Thermal and Structural Properties of Natural and Ion-Exchanged Analcime

“…Analcime (ANA) structures formed in this way include channels parallel to <110> that are connected to 8-member tetragonal rings. Unlike the big extraframework positions and channel openings which are commonly found in many zeoliteslike clinoptilolitechannels in analcime are pretty small, with openings ~ 1.6 Å × 4.2 Å [9,10]. These small channels affect both the cation exchange behavior and dehydration/hydration behavior of analcime.…”