The interplanar d-spacings calculated for volcanic α-cristobalite (d101 = 4.039-4.064 Å) by XRD (Table 5.3) are generally larger than those reported for synthetic α-samples (e.g., d101 = 4.04 Å) (Chao and Lu, 2002a). Most likely, these result from structural cation substitutions observed by EMPA. Chao and Lu (2002a) have shown a displacement in the primary α-cristobalite peak proportional to the level of (Al2O3 + Na2O)-co-doping during cristobalite synthesis. The lattice spacing in their experiments exhibits an increasing trend from lower doping levels to higher doping levels, from d101 = 4.040 Å at 2.91 mol. % Na2O-Al2O3 to d101
= 4.058 Å at 4.65 mol. %. Doping levels used by Chao and Lu (2002b)were on the order of those seen for volcanic cristobalite (up to 2.4 mol. % Al2O3), and the observed monotonic trend suggests any amount of cation substitution can lead to structural discontinuities and a corresponding shift in lattice spacing (towards a lower degree 2θ in X-ray diffraction patterns).
The sharpness and intensity of an X-ray diffraction peak can generally be regarded as an indication of the degree of crystallinity and perfection of atomic arrangement within the structural framework of a mineral. Peak widths for volcanic cristobalite isolates are wider than the two cristobalite standards used, indicating a larger degree of disorder for the
149
Structure and Composition 5
volcanic crystals and variability in short-range interplanar spacings, which also likely results from incorporation of Al and Na into the crystal lattice. While a homogenous distribution of substituted cations would shift the 2θ diffraction position entirely, EMPA on single cristobalite crystals verifies a heterogeneous distribution of substitutions throughout a crystal (Figure 5.4). The resultant, non-uniform d-spacings lead to a broader XRD reflection, suggesting a poorly ordered lattice and further supporting the interpretation of structurally incorporated cations.
5.4.2.1 Characteristics of the α-β cristobalite transition
The nature of the α-β transition can provide information on the cristobalite structure. The transition-temperature range for MBA5/6/99 observed by XRD (170-210 °C) and DSC (onset at 175 °C, peak at 202 °C) are in close agreement, and likely only differ due to the different instruments and heating techniques. These temperature ranges are lower than generally reported for synthesized cristobalite samples cristobalite (e.g. 227-267 °C, Mosesman and Pitzer, 1941), which supports the hypothesis of a lower degree of crystallinity.
The breadth of the transition reflects the heterogeneity of cristobalite in the sample. Both DSC and thermal XRD data typically show a gradual transition interval with upper transition-boundary temperatures of approximately 270 °C (Stevens et al., 1997). The transition breadth observed for Soufrière Hills ash generally corresponds to that of synthetic samples, e.g., ~20-40 °C (Leadbetter and Wright, 1976; Mosesman and Pitzer, 1941);
however, the transition interval observed for Santiaguito dome rock was much wider, ~80
°C. The increased transition breadth in the dome rock samples could result from a difference in grain size; cristobalite is preferentially enriched in the fine fraction (Horwell et al., 2003b) in ash samples which may not be replicated by grinding dome rock samples by hand for XRD. The difference could also represent a more heterogeneous distribution of lattice perfection for cristobalite from Santiaguito compared to Soufrière Hills as Santiaguito contained a greater abundance of cation substitutions. No systematic difference in d-spacings by XRD were noted to support this interpretation; however, Thompson and Wennemer (1979) report a sharp peak in heat capacity at 262 °C for a highly crystalline synthetic sample (prepared at 1500 °C for 20 hours), indicating that a narrow transition range may reflect a homogeneous cristobalite crystal population.
The cristobalite inversion involves an enthalpy of transition, which is indicated by the endo- and exothermic reactions on the thermal DSC curve (Figure 5.8): the peak at 200 °C on heating and 183 °C on cooling marks the phase transition. The difference between the transition temperatures upon heating and cooling is due to transition hysteresis, and is well
Structure and Composition 5
documented for cristobalite (Leadbetter and Smith, 1976; Schmahl, 1993). The susceptibility of cristobalite to hysteresis also eliminates concern over the difference in heat flow for the three recorded transitions. Further, there is no correlation between the α-β and β-α temperature ranges (Leadbetter and Smith, 1976). Hysteresis is associated with strain and surface free energy in the transforming crystals, which are dependent on the direction of thermal change. Stevens et al. (1997) showed marked hysteresis on heating and cooling for cristobalite, and this increased with firing temperature. The increase in inversion temperature and the extent of hysteresis with firing temperature are evidence of the production of better ordered cristobalite crystals at high temperatures (Stevens et al., 1997). Even the most crystalline samples obtained by Leadbetter and Smith (1976) still contained considerable strain due to disorder. As such, hysteresis is always expected to be present for cristobalite;
however, the extent should be lower for less-ordered volcanic samples compared to better-ordered synthetic samples.
Based on the position and width of the (101)α reflection before and after the transition, cristobalite reverts back to the original isomorph on cooling with little change in the structure. A similar occurrence was seen by Butler and Dyson (1997), whereby disordered cristobalite reverted to the same defect α-cristobalite structure following a transition to the β-phase. The most likely reason is that the foreign ions and vacancies that stabilise the disordered form cannot be spatially varied during the transition. This means that, in a volcanic dome, any continuous cooling and reheating should result in minimal change to the impurities observed here and by Horwell et al. (accepted; Appendix 4).
The athermal phenomenon, whereby the transition halts when held isothermally, has been observed for cristobalite (Leadbetter and Wright, 1976; Schmahl, 1993) as well as with quartz held isothermally for 20 hours, where no additional shift of the prominent (101)α reflection occurred under isothermal conditions (Wahl et al., 1961). With an additional temperature increase, the d-spacing continues to increase in both cristobalite and quartz.
Further, the finite width of the transition range is not simply a kinetic effect (Leadbetter and Wright, 1976). A transformation that does not proceed isothermally is analogous to martensitic transformations (i.e., diffusionless, displacive transformation in the solid state) in close-pack metals and alloys (Schmahl, 1993), which can result from the need of a crystal structure to accommodate the minimum energy state for a given temperature (Otsuka and Wayman, 1998). Since no redistribution of atoms occurs, time-dependent diffusional processes are absent, and the transformation is temperature, rather than time, dependent. This observation suggests self-pinning at the domain walls between the α- and β-phases, resulting in the thermal hysteresis and non-quenchable structural phase transition (Schmahl, 1993).
151
Structure and Composition 5