Thermodynamic and structural behavior of analcime-leucite analogue systems.

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 H2O 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

A new era in hydrofluoric acid solution calorimetry: Reduction of required sample size below ten milligrams.

Significant advances have been made in hydrofluoric acid solution calorimetry at Lafayette College in the past 15 years. To determine the degree to which these developments enable the reduction of sample size, calorimetric experiments were performed on hexagonal germanium oxide as a function of sample weight. The resulting calorimetric data indicate that the highest degrees of reproducibility (60.1%) are maintained down to sample sizes of 50 mg, and that precisions of 61%, acceptable for many applications, are observed to sample sizes of 10 mg. Because silicate systems produce weight-based heats of solution that are about twice that of germanium oxide, the required sample size for these will be even less. The new minimum required sample size of 5 to 25 mg (depending on application) is about two orders of magnitude less than that used 20 or 30 years ago. This makes possible many new kinds of projects for HF solution calorimetric investigation, including those on high-pressure materials