XRD, micro-XANES, EMPA, and SIMS investigation on phlogopite single crystals from Mt. Vulture (Italy)

摘要

Selected phlogopite flakes from Mt. Vulture in southern Italy were studied using a combination of single-crystal techniques: electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS), single-crystal X-ray diffraction (SCXRD), and micro-X-ray absorption near-edge spectroscopy (XANES). The latter technique was employed to analyze the structure of the Fe-K absorption edge over the region from 7080-8100 eV and to determine Fe~(3+)/SFe at a micrometer scale, albeit with large error bars due to known effects of orientation on pre-edge energy. The annite component, Fe/(Mg+Fe), of the samples studied ranged from 0.16 to 0.31, the Ti content from 0.11 to 0.27 atoms per formula unit (apfu) and the Ba content from 0.03 to 0.09 apfu. SIMS analysis showed H_2O (wt%) = 1.81-3.30, F (wt%) = 0.44-1.29, and Li2O (wt%) = 0.001-0.027. The intra single-crystal chemical variability for major/minor elements (Mg, Fe, Al, Ba, Ti, and K) was found particularly significant for samples VUT191-11 and PG5-1, less significant for the other samples of the set. SIMS data relative to crystals VUT187-24, VUT191-10, VUT191-11, and VUT187-28 showed a noteworthy variation in the concentrations of some light elements (H, Li, and F) with coefficient of variation CV (as 1s%) up to ～18% for H _2O. The analyzed micas belong to the 1M polytype. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 3.08 ≤ R ≤ 3.63, 3.32 ≤ Rw ≤ 3.98%. Micro-XANES results yielded Fe~(3+)/ΣFe from 51-93%. Previous M?ssbauer data from powdered samples suggested Fe~(3+)/ΣFe values ranging from 49-87%. However, the Fe~(3+) content determined by both techniques is sometimes remarkably different, in part because of the large errors (±10-15%) presently associated with the micro-XANES technique and in part because the Fe~(3+) content of a single crystal may significantly depart from the average value obtained from routine M?ssbauer analysis. The combination of EMPA, SIMS, and micro-XANES resulted in the characterization of the samples at a comparable spatial scale. By means of in-situ data and the results of crystallographic investigations, the occurrence of different relative amounts of M~(3+)-oxy [~(VI)M~(2+) + (OH)- ? ~(VI)M~(3+) + O~(2-) + ~(1/2)H_2↑], Ti-oxy substitutions [~(VI)M ~(2+) + 2(OH)- ? ~(VI)Ti~(4+) + 2O~(2-) + H_2↑], and Ti-vacancy (□) substitution (2~(VI)M ~(2+) ? ~(VI)Ti~(4+) + ~(VI)□) was ascertained for the studied samples.