CRYSTAL CHEMISTRY AND SURFACE CONFIGURATIONS OF TWO IRON-BEARING TRIOCTAHEDRAL MICA-1M POLYTYPES

The aim of this paper is to explore the crystal chemical features of both the bulk and the uppermost surface layers of two trioctahedral Fe-rich mica-1M (space group C2/m) polytypes: a tetra-ferriphlogopite from an alkali-carbonatic complex near Tapira (Belo Horizonte, Minas Gerals, Brazil) and a Fe2+-bearing phlogopite also containing Fe3+ in tetrahedral sites from the Kovdor carbonatite-bearing alkaline-ultrabasic complex (Kola Peninsula, Russia). The chemical formulae are [XII](K0.99) [VI](Fe2+0.08 Fe3+0.15 Mg2.76 Ti0.01) [IV](Fe3+0.82 Si3.18) O10.37 (F0.24 OH1.39) and [XII](K0.94 Na0.06) [VI](Fe2+0.17 Fe3+0.05 Mg2.75 Mn0.01 Ti0.05) [IV](Fe3+0.16 Al0.84 Si3.00) O10.21 (F0.35 OH1.44) for tetra-ferriphlogopite and Fe-bearing phlogopite, respectively. The two minerals show a pronounced difference in tetrahedral composition, where Fe for Si substitution is at 20.5% in tetra-ferriphlogopite and at 4% in Fe-bearing phlogopite. Fe3+ substitution produces an increase of the tetrahedral sheet thickness and of mean tetrahedral edge lengths in tetra-ferriphlogopite with respect to Fe-bearing phlogopite. The tetrahedral rotation angle (α) changes remarkably from tetra-ferriphlogopite (α = 10.5°) to the Fe-bearing phlogopite (α = 8.5°), thus indicating a significantly greater tetrahedral ring distortion in the tetra-ferriphlogopite with respect to the Fe-bearing phlogopite. The interconnection between α and tetrahedral flattening angle () may indirectly suggest the variation of  due to Fe for Si tetrahedral substitution. Crystal chemical modifications at the mineral surface were investigated via X-ray Photoelectron Spectroscopy by comparing data related to the surface to data originating from the bulk. The local population at the anionic position can be obtained by considering that the binding energy of photoelectron peaks is dependent on the chemical state of atoms and their local environment. If oxygen is substituted by fluorine, the binding energy of Mg was observed to shift to higher values. By considering the Mg1s binding energies of tetra-ferriphlogopite and Fe-bearing phlogopite, Mg is preferentially coordinated to F and not to OH groups. This is consistent with higher BE values observed for Fe-rich phlogopite characterized by higher F content with respect to tetra-ferriphlogopite.