Characterization and origin of two Fe3+ EPR spectra in kaolinite

Abstract--A method is proposed to measure the absolute concentration of paramagnetic Fe 3+ ions in kaolinite from various geochemical environments using powder X-band electron paramagnetic resonance (EPR) data. An Fe3+-doped corundum sample is used as a concentration standard. The Fe 3+ signal is calibrated by calculating the powder EPR spectra of Fe 3+ ions in corundum and low-defect kaolinite. The paramagnetic Fe 3+ concentration in other samples is obtained by an extrapolation procedure. This study provides a direct assessment of the iron distribution between isolated structural Fe 3+ ions and other iron species, such as Fe 3+ concentrated phases and Fe 2+ ions. The concentration of isolated structural Fe 3+ ranges between 200-3000 ppm and represents less than half of the total iron within kaolinite crystals.

Section: Resultsmentioning

confidence: 99%

Section: Clays and Clay Mineralsmentioning

confidence: 99%

Quantitative Measurement of Paramagnetic Fe³⁺ in Kaolinite

Balan

2000

“…The Q-band spectrum of low-defect kaolinite shows additional resolution of the Fe~u) spectrum for iron. The spectrum is resolved into discrete bands for the two unique octahedral positions in the triclinic unit cell (Gaite et al, 1993). The first-derivative EPR spectra for the clay fractions of six samples are shown in Figure 12.…”

Section: Electron Paramagnetic Resonance Spectroscopy (Epr)mentioning

confidence: 99%

The Relationships between Kaolinite Crystal Properties and the Origin of Materials for a Brazilian Kaolin Deposit

Varajão

Gilkes

Hart

2001

The clay particles in a kaolin deposit from Brazil were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), analytical transmission electron microscopy (ATEM), and electron paramagnetic resonance (EPR) to examine the relationships between morphological and chemical properties of the crystals and to relate these properties to formation conditions. The XRD patterns show the dominant presence of kaolinite with minor amounts of gibbsite, illite, quartz, goethite, hematite, and anatase. ATEM observations show two discontinuities in the deposit as indicated by changes in morphology and size of the kaolinite crystals. At the base of the deposit, hexagonal platy and lath-shaped particles (mean area of 001 face = 0.26 p,m 2) maintain the original fabric of the parent rock which characterizes an in situ evolution. In the middle of the deposit a bimodal population of large (mean area of 001 face > 0.05 ixm:) and small (mean area of 001 face < 0.05 p.m 2) sub-hexagonal platy kaolinite crystals occurs. This zone defines the boundary between the saprolitic kaolinite and the pedogenic kaolinite. Near the top of the profile, laths and irregular plates of kaolinite, together with sub-hexagonal particles, define two different depositional sources in the history of formation of the deposit. Crystal thickness as derived from the width of basal reflections and the Hinckley index are compatible with the morphological results, but show only one discontinuity. At the base of the deposit, kaolinite has a lowdefect density whereas in the middle and at the top of the profile, kaolinite has a high-defect density. Likewise, EPR spectroscopy shows typical spectra of low-defect kaolinite for the bottom of the deposit and typical spectra of high-defect kaolinite for the other portions of the deposit. Despite the morphological changes observed through the profile, the elemental composition of individual kaolinite crystals did not show systematic variations. These results are consistent with the deposit consisting of a transported pedogenic kaolinite over saprolite consisting of in situ kaolinized phyllite.

“…These samples are listed in Table 1 together with their sources, mineralogy, and indices relating to structural order. Two samples (DCV and GB3) were recently the subject of detailed EPR investigation (e.g., Gaite et al, 1993Allard and Muller, 1998).…”

Section: Samplesmentioning

confidence: 99%

“…As the fourth-order parameters (n = 4) are weaker than the second-order parameters (Gaite et al, 1993), they can be neglected in Equation (2). The integration over the fine-structure parameters may be thus approximated with a summation over discrete values of B ~ and B~.…”

Section: Epr Spectroscopymentioning

confidence: 99%

Structural Fe³⁺ in Natural Kaolinites: New Insights from Electron Paramagnetic Resonance Spectra Fitting at X and Q-Band Frequencies

Balan

Allard

Boizot

et al. 1999

Abstract--Structural Fe 3+ in kaolinites and dickites covering a broad range of disorder was investigated using electron paramagnetic resonance (EPR) spectroscopy at both the X and Q-band frequencies. A procedure based on a numerical diagonalization of the spin Hamiltonian was used to accurately determine the second and fourth-order fine-structure parameters. A least-squares fitting method was also developed to model the EPR spectra of Fe 3+ ions in disordered local environments, including multimodal site-tosite distributions. Satisfactory fits between calculated and observed X and Q-band spectra were obtained regardless of the stacking order of the samples.In well-ordered kaolinite, Fe 3+ ions are equally substituted in sites of axial symmetry (Fe(ii)sites, namely Fe(m a and Fe(mb) which were determined to be the two non-equivalent All and A12 sites of the kaolinite structure. In dickite, Fe 3+ ions were also found to be equally substituted for AP + in the two non-equivalent A1 sites of the dickite structure. In poorly ordered kaolinites, the distribution of the fine-structure parameters indicates that Fe 3+ ions are distributed between Fe(,> sites and other sites with the symmetry of the dickite sites.Hence, when stacking disorder prevails over local perturbations of the structure, the near isotropic resonance owing to Fe 3+ ions in rhombically distorted sites (Fe(i) sites) is a diagnostic feature for the occurrence of C-layers in the kaolinite structure, where C refers to a specific distribution of vacant octahedral sites in successive layers.