Ab Initio Mossbauer Parameter Calculations (MSc & PhD)

A suite of 22 members of the heterophyllosilicate astrophyllite-group from SiO2-over- and undersaturated igneous rocks were studied by single-crystal X-ray diffraction, wave-length dispersive electron microscopy and room temperature 57Fe Mössbauer spectroscopy. This is the first study of its kind for this mineral group and presents the relationships between average and distribution-related crystallographic and hyperfine parameters. The room temperature, thin-limit, thickness-corrected 57Fe Mössbauer spectra of astrophyllite-group minerals are characterized by two strong absorption peaks centered at ≈ -0.1 and 2.3 mm/s and a third, weaker shoulder at ≈ 0.9 mm/s, corresponding to (1) the sum of the low-energy lines from [6]Fe2+ and [6]Fe3+ doublets, (2) the high-energy lines from [6]Fe2+ doublets, and (3) the high-energy lines from [6]Fe3+ doublets, respectively. Average centre shift values ( ) for [6]Fe2+ range from 1.124 to 1.154 mm/s and average quadrupole splitting ( ) values from 2.202 to 2.416 mm/s. No evidence for [4]Fe 3+ was found in the samples studied. Fe3+ /Fetot ratios range from 0.01 to 0.21, corresponding to 0.05 to 0.56 apfu Fe3+. A general trend in quadrupole splitting distributions (QSDs) is observed from narrow distributions with large and QS peak values in Fe-dominant samples, to broad distributions with small and QS peak values in Mn-dominant samples. The low edge of the distribution moves towards higher values as the width of the distribution decreases, while the high edge remains relatively constant. This behavior has been noted in experimental QSDs of many layered silicates, and has been shown by ab inito electronic structure calculations to be the result of a combination of a maximum in the QS versus octahedral flattening (Ψ) curve and a maximum in the electric field gradient due to chemical disorder in the first nearest-neighbour octahedra. In both astrophyllite-group minerals and trioctahedral micas, the dominant octahedral structural parameter affecting the QSD is Ψ. The presence of the Dφ6 bridging octahedron in the astrophyllite structure imposes an additional distortion effect on the sheet of octahedra, resulting in two distinct QSD populations based on the Zr content of the sample. Zr-rich samples (Zr > 0.40 atoms per formula unit, apfu) have overall larger QS compared to Zr-deficient samples (Zr < 0.40 apfu). This shift in QSDs of Zr-rich samples to higher average values corresponds to a higher maximum local distortion environment. The existence of such a variation in local distortion environments is further supported by the existence of a discontinuous relationship in chemical and structural parameters (Mössbauer, single-crystal X-ray diffraction and EMPA) between astrophyllite-group minerals which crystallized in hydrothermal (Zr-deficient) versus magmatic or post-magmatic (Zr-deficient/Nb-rich) environments.

We present a chemical and mineralogical explanation, derived from powder X-ray diffraction and Mössbauer spectroscopy measurements of synthetic samples, of the P:Fe = 1:2 limiting ratio of P incorporation (as PO4) that was previously observed in natural aquatic oxic iron precipitates. The 57Fe Mössbauer hyperfine parameters are interpreted with the help of state-of-the-art ab initio electronic structure calculations. We find that there is a strong tendency for solid solution P–Fe mixing in the P-bearing hydrous ferric oxide (P-HFO) aqueous coprecipitate system, interpreted as occurring between the P-free (ferrihydrite) end-member and an inferred P:Fe = 1:2 end-member beyond which P is not incorporated in the structure of the P-HFO solid. Up to and somewhat beyond the limiting end-member P:Fe ratio, all available P is scavenged by the coprecipitation reaction, suggesting strong P–Fe complexation in the precipitation-precursor dissolved species. The P-HFO solids are more stable (i.e., have stronger chemical bonds) than the P-free ferrihydrite end-member. We show that in coprecipitation the P specifically incorporates within the nanoparticle structure rather than complexing to the nanoparticle surface. Our results are relevant to the question of the mechanisms of coupling between the Fe and P cycles in natural aqueous environments and highlight a strong affinity between Fe and P in aqueous environments.

We report ab initio electronic structure calculations that directly relate given local chemical and distortion environments to corresponding hyperfine electric field gradients (EFGs) in ⁵⁷Fe Mössbauer spectroscopy, thereby giving needed interpretive power to the technique in characterizing VIFe²⁺ environments in minerals. Changes of the EFG with various distortions were investigated on model clusters, including the bare octahedra FeO¹⁰⁻6 and Fe(OH)6⁴⁻, and various seven-octahedra sections of an octahedral sheet, through self-consistent charge Xα ab initio calculations. Distortions examined for all clusters were flattening, counter-rotation, and bond scaling, as well as changes in neighbor bond lengths and the identity and ordering of neighbor cations for the seven-octahedra clusters. The evolution of the EFG with distortion was derived at T = 0 K and T = 300 K as a function of the distortion parameters. We find that the percent change in the EFG over the range of distortion parameters found in 1M trioctahedral micas is greatest with flattening for the clusters compared, suggesting that flattening is the most important structural distortion in determining the EFG. The EFGs for the seven-octahedra cluster as a function of flattening were compared for thirteen configurations of Mg²⁺ and Al³⁺ cations in the first nearest-neighbor octahedra. The percent change in the EFG for flattening and cation substitution was found to be of similar magnitude. In comparing EFG vs. flattening curves with measured quadrupole splitting distributions (QSDs), the magnitudes of EFGs in the theoretical curves agree well with experiment. The sharp high quadrupole splitting edge is explained by the presence of a maximum in the EFG vs. flattening curve. These model calculations are a necessary first step in establishing a firmer link between local structural distortions in minerals and measured QSDs.