THE RELATIONSHIPS BETWEEN CRYSTAL-CHEMICAL AND HYPERFINE PARAMETERS IN MEMBERS OF THE ASTROPHYLLITE-GROUP: A COMBINED 57FE Mö SSBAUER SPECTROSCOPY AND SINGLE-CRYSTAL X-RAY DIFFRACTION STUDY

Abstract

A suite of 22 members of the heterophyllosilicate astrophyllite-group from SiO 2 -over-and undersaturated igneous rocks were studied by single-crystal X-ray diffraction, wave-length dispersive electron microscopy and room temperature 57 Fe Mossbauer 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 57 Fe Mossbauer 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] Fe 2+ and [6] Fe 3+ doublets, (2) the high-energy lines from [6] Fe 2+ doublets, and (3) the high-energy lines from [6] Fe 3+ doublets, respectively. Average centre shift values (〈CS〉) for [6] Fe 2+ range from 1.124 to 1.154 mm/s and average quadrupole splitting (〈QS〉) values from 2.202 to 2.416 mm/s. No evidence for [4] Fe 3+ was found in the samples studied. Fe 3+ /Fe tot ratios range from 0.01 to 0.21, corresponding to 0.05 to 0.56 apfu Fe 3+ . A general trend in quadrupole splitting distributions (QSDs) is observed from narrow distributions with large 〈QS〉 and QS peak values in Fedominant samples, to broad distributions with small 〈QS〉 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 D ϕ 6 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 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 (Mossbauer, 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.