Crystal chemistry, Mossbauer spectroscopy and paragenesis of astrophyllite group minerals from over- and undersaturated alkaline rocks.

摘要

The crystal-chemistry and paragenesis of astrophyllite group minerals (AGM) from over- and undersaturated alkaline rocks has been documented by a combination of electron microprobe analyses, single-crystal and powder X-ray diffraction, Mössbauer spectroscopy, Fourier-transform infrared spectroscopy, nuclear reaction analysis, thermal gravitational analysis, thermal decomposition, wet-chemical methods and optical measurements. A standardized general formula for AGM has been developed, based on 31 anions, and is of the form A₂BC₇D ₂T₈O₂₆(OH)₄X _0-1, where [10]–[13]A = K, Rb, Cs, H₃O+, H₂O, or □; [10] B = Na or Ca; [6]C = Mn, Fe2+ Fe3+ Na, Mg, or Zn; D = [6]Ti, Nb, or Zr; [4]T = Si or Al and X = &phis; = F, OH, O, or □. Results from Mössbauer spectroscopy, bond valence sums and thermodynamic calculations indicate F to order at X [&phis;(16)] whereas the monovalent anion sites in the O-sheet are host solely to OH− . On this basis, eight species and two AGM polytypes are recognized, the most recent, niobokupletskite, identified from Mont Saint-Hilaire during the course of this study. The AGM heterophyllosilicate structure consists of two composite sheets stacked along [001] in a 2:1 ratio resulting in a layered HOH structure. Single-crystal X-ray refinements indicate the existence of two kupletskite polytypes, kupletskite-1A and kupletskite-Ma2b2c, the result of polytypic stacking of identical HOH layers. Mössbauer spectroscopy indicates all Fe to be present in the O-sheet, and Fe3+/Fe _tot ratios in AGM to range from 0.01 to 0.21, corresponding to 0.05 to 0.56 apfu Fe3+, confirming that Fe 2+ is the dominant valence state for iron in the AGM structure. AGM from oversaturated alkaline intrusions are characterized by a restricted range of Mn/Mn+Fe_tot (0.03 to 0.69) and enrichments in K, Rb, Ti (with positive values of Nb-Zr) and Si. AGM from undersaturated intrusions display a wide range in Mn/Mn+Fe_tot, (0.09 to 1.00) and have been further subdivided into a kupletskite subgroup (Mn/Mn+Fe_tot ≥ 0.50) and an astrophyllite subgroup (Mn/Mn+Fe_tot 0.50). Kupletskite subgroup samples are enriched in Na, Mn, Zn, Fe3+, Nb and Zr (with positive values of Nb-Zr), whereas astrophyllite subgroup samples are enriched in K, Ca, Fe2+, Ti, Zr (with negative values of Nb-Zr) and F. Crystal-chemical parameters in AGM (such as Mn/Mn+Fe_tot and Nb/Nb+Zr) can be used as petrogenetic indicators. At Mont Saint-Hilaire (Quebec), two distinct paragenetic trends are recognized in the AGM population: (1) an oxidizing magmatic to post-magmatic oxidizing trend resulting in enrichment of the AGM in Na, Mn, Fe3+ and Nb, and (2) a reducing trend resulting in enrichment in Ca, Fe2+, Zr and F by hydrothermal or metasomatic processes. The second evolutionary trend is thought to be the result of interaction of an exsolved magmatic fluid with either meteoric waters or sediments. Both trends lend evidence for late-stage mobilization and concentration of high field-strength elements (such as Nb and Zr) in highly alkaline systems.