MOSSBAUER-SPECTROSCOPY OF MINERALS .1. INADEQUACY OF LORENTZIAN-LINE DOUBLETS IN FITTING SPECTRA ARISING FROM QUADRUPOLE SPLITTING DISTRIBUTIONS

We compare two different approaches to fitting the Fe-57 Mossbauer spectra of paramagnetic state minerals: the widespread practice of using Lorentzian line doublets with adjustable GAMMA widths and a new method (Rancourt and Ping, 1991) based on quadrupole splitting distributions (QSDs). We argue that there is no physical justification for the former and that the latter is a theoretically correct approach. With the same number of free parameters, the QSD method performs better. The Lorentzian doublet method 1) significantly overestimates the background, 2) puts overly large wings or tails on the main absorption peaks, and 3) gives unphysically large values of GAMMA. Whereas Lorentzian doublets are often arbitrarily (and incorrectly) assigned to model sites in the average unit cell, QSDs represent the most information that can be distilled from the spectra and are caused by the various local and long-range structural and chemical environments acting concomitantly.