Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28s.315a&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 315
Mathematics
Logic
History, Thermal, Thermometry, Troilite
Scientific paper
Troilite (FeS) exhibits two solid-state phase transformations, which, according to conventional thermodynamic literature [1,2], occur at 411 +- 3 K (alpha/beta) and 598 +- 3 K (beta/gamma). In principal, the thermal history of a particular troilite sample might be expected to impart structural or strain characteristics that could be measured upon experimental inducement of the phase transformations. To investigate that possibility, we applied differential scanning calorimetry (DSC) to determine the temperatures and enthalpy changes for phase transitions of various troilite samples subjected to controlled heating [3]. Post-heating residues were prepared as polished grain mounts and petrologically characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS). Troilite grains separated from the Mundrabilla (octahedrite), PAT91501 (L7 chondrite), and EET83213 (L3 chondrite) meteorites were compared with terrestrial troilite from an ultramafic complex in Del Norte Co., California. Both the alpha/beta and beta/gamma transitions were easily measured by DSC applied to samples on the order of 5 milligrams. Extrapolated-onset temperatures (the DSC approximation of equilibrium transition temperatures) for the alpha/beta transition display a systematic progression as follows: Mundrabilla, PAT91501, EET83213, Del Norte (Fig. 1). At least for Mundrabilla, PAT91501, and Del Norte, there also exists an apparent correlation of transition enthalpy with onset temperature (Fig. 1). (Enthalpy measurements for EET83213 troilite were systematically low, by dilution effects from admixed Ni-Fe metal, and require correction before such comparison.) Although Mundrabilla troilite contains on the order of 1 wt% Cr, and some grains of PAT91501 troilite contain on the order of 1 wt% Ni, it is doubtful that compositional differences can fully account for the measured differences in thermodynamic properties. Instead, it is likely that first-order differences are the consequence of different thermal histories. Multiple samples of Del Norte troilite were used to determine the influence of laboratory-scale thermal histories on DSC signatures by heating and cooling each sample under different programmed conditions. In reconnaissance experiments, maximum temperature achieved during heat treatment appears to be more influential than does either the time maintained at temperature or the heating/cooling rate. The experimentally measured alpha/beta onset temperature shows a systematic decline with maximum temperature achieved during prior heating, suggesting that high onset temperatures are indicative of low maximum temperatures in the natural histories of the troilite samples. That trend is at least qualitatively consistent with the petrologic rankings of the meteorites in which troilite from the relatively unmetamorphosed L3 chondrite shows a higher onset temperature than does troilite from either the highly metamorphosed L7 chondrite or the octahedrite. Additional work should define the limits of a quantitative calibration that might ultimately permit derivation of meteorite thermal histories by calorimetric thermometry of troilite. Samples were kindly provided by E. R. D. Scott (Mundrabilla), C. B. Moore (Del Norte), and the Meteorite Working Group (PAT91501; EET83213 powder from E. Jarosewich). References: [1] Chase M. W. Jr. et al. (1985) JANAF Thermochemical Tables, 3rd ed., 1194. [2] Robie R. A. et al. (1979) Geol. Surv. Bull. 1452, 125. [3] Allton J. H. and Gooding J. L. (1993) LPS XXIV, 21-22. Fig. 1, which appears here in the hard copy, shows the thermodynamics of troilite alpha/beta phase transformations measured by DSC during first-heat cycles.
Allton Judith H.
Gooding James L.
Wentworth Sue J.
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