Mössbauer study of a California desert celadonite and its pedogenically-related smectite

Abstract

Celadonite from the northwestern Mojave Desert area of California was examined by detailed Mössbauer spectroscopy at temperatures from 10 K to 400 K. In addition to the predominant Fe³⁺ doublet with isomer shift δ∼0.4 mm s⁻¹ and quadrupole splitting Δ ∼ 0.4 mm s⁻¹, another Fe³⁺ doublet with δ∼ 0.4, Δ∼1.2 mm/s and two Fe²⁺ doublets with δ∼1.1, Δ ∼ 1.7, 2.7 mm s⁻¹ at 300 K were distinguished. The minor Fe³⁺ component is ascribed to dehydroxylated surface sites. Most of the remaining Fe(∼90%) is M2 cis-OH octahedral in an ordered M⁺−M²⁺ array. However, about 10% is M1 trans-OH Fe²⁺. Isomer shift vs. T gives Debye temperatures of 570 K for Fe³⁺ in M2 and 380 K for both Fe²⁺ sites, indicating greater vibrational freedom for Fe²⁺. Quadrupole splitting Δ vs. T for Fe²⁺ gives a valence electronic energy splitting of 760 cm⁻¹ between the ground and first excited state for M2. The M1 sites have a more drastic variation in Δ vs. T which indicates not only a lower first excited state but a rhombic distortion at these sites. A proposed explanation is a neighboring M2 site vacancy. The soil clay formed from this celadonite, which is mostly Fe-rich smectite, was also studied by Mössbauer spectroscopy. About half the Fe²⁺ has been oxidized in the clay, but the isomer shifts and quadrupole splittings are essentially the same as in the original celadonite. A texture orientation in the clay absorber was detected by measuring the absorber at 55° to the source radiation. This texture effect produces asymmetric doublets in the usual 90° measurement.