A quantitative determination of the relative marcasite/pyrite contents
in virgin coals is possible by means of Fe-57 Mossbauer spectroscopy.
Complications arise, however, when iron-containing silicates, carbona
tes, or other salts are present. The application of a mild chemical tr
eatment involving hydrofluoric acid has been employed to remove these
Fe-containing phases while leaving the iron-disulfide phases unaffecte
d. Several South African coal samples with non-iron disulfide, Fe-cont
aining phases ranging from 18 to 30 weight percent have been subjected
to a hydrofluoric acid leaching at room temperature. The loss of mine
ral matter with HF leaching correlates well with the mineral matter re
sidue following low temperature ashing. The Fe-57 Mossbauer spectra of
the resulting coal samples, collected at 297K, indicate that only FeS
2 phases are present. The Mossbauer parameters for these samples (0.61
9 less than or equal to E-Q less than or equal to 0.622 mm s(-1); 0.30
6 less than or equal to delta less than or equal to 0.309 mm s(-1)) in
dicate the absence of appreciable quantities of marcasite in the coals
; These Mossbauer parameters differ slightly, but systematically, from
those of pyrite for which a quadrupole splitting of 0.6110 +/- 0.0020
mm s(-1) has been established. On the basis of previous studies, thes
e increased E-Q values suggest the presence of As substitution in the
pyrite phases. Fe-57 Mossbauer spectra of virgin coals exhibit phase a
ssemblages comparable to those observed by x-ray diffraction (XRD), e.
g. pyrite and (Ca,Fe)CO3, even though the presence of pyrite is less d
efinitive in the XRD data.