XVI Reunión Científica de la Sociedad Española de Arcillas - Libro ...

Comunicaciones SEA 2001 – Baeza (Jaén)
SOCIEDAD
ESPAÑOLA DE ARCILLAS
MÖSSBAUER SPECTROSCOPIC INVESTIGATION OF VERMICULITES
FROM ANDALUSIA (SPAIN)
Lerf A b , Wagner FE b , Poyato J c , Perez Rodríguez JL c , Justo A c
a
Meissner-Institut der Bayer. Garching.
b
Physics Department. Garching.
c Instituto de Ciencia de Materiales de Sevilla, CSIC.
We report on a Mössbauer spectroscopic investigation (ME) of three
vermiculites from Andalusia with increasing iron content (Santa Olalla,
Almonaster, Ojén) and the phlogopite from Santa Olalla. A typical ME spectrum is
shown in figure 1. The spectra of all samples can be interpreted with the
superposition of three doublets. One of them is assigned to Fe 3+ , the other two to
Fe 2+ (all in octahedral positions). According to the chemical formula we could not
identify Fe 3+ in tetrahedral sites. Interestingly enough the isomer shift (IS,
measured versus (Fe)Rh source) and the quadrupole splitting (QS) of the iron
sites in all three samples vary only in a very small range of values depending on
the type of the sample (powder, single crystal) or the method of data analysis
(gaussian or lorentzian profiles): IS of Fe 3+ 0.24-0.29 mm/s, IS of Fe 2+ 1.00-1,10
mm/s; QS of Fe 3+ 1.02-1.10 mm/s, QS of Fe 2+ 2.51-2.61 mm/s; the only exception
is the quadrupole splitting of the phlogopite from Santa Olalla which amounts to
2.35 mm/s. With the exception of the phlogopite there is more Fe 3+ than Fe 2+ . The
Fe 3+ /Fe 2+ ratio determined by ME deviates more or less strongly from the ratio
determined by chemical analysis. In all spectra one observes also a broad
unresolved component that is assumed to result mainly from slow paramagnetic
relaxation of Fe 3+ .
The values of IS and QS found for the samples investigated are in excellent
agreement with the parameters found previously for di- and trioctahedral micas [1]
and vermiculites [2]. It should be noted that the quadrupole splitting of the Fe 3+
site is exceptionally high for trioctahedral 2:1 clay minerals. The quadrupole
splittings for the two Fe 2+ sites correspond very well with those given by Rancourt