Sodian muscovite-2M1: Crystal chemistry and surface features

Abstract

This study explores crystal chemical features of a sodian muscovite from Antarctica, both in the bulk of the mineral and at the uppermost surface layers. These features may support a deeper comprehension of processes affecting mineral surface and mineral physical properties. The muscovite under analysis is characterized by limited celadonite and paragonite substitutions and shows the following chemical formula: [XII] (K 1.73 Na 0.27 ) [VI] (Fe 2+ 0.14 Al 3+ 3.77 Mg 0.07 Ti 0.01 ) [IV] (Al 1.91 Si 60.9 ) O 20 (F 0.11 OH 3.89 ). The sample is monoclinic, 2M 1 polytype, with symmetry C 2/ c and unit cell parameters a = 5.1969(1), b = 9.0138(3), c = 20.0835(7) A, and β = 95.763(2) ° . The two tetrahedral sites, T1 and T2, present similar distances ( = 1.643A and = 1.647 A) and distortion parameter values, evidencing a disordered distribution of Si and Al at tetrahedral sites. The tetrahedral ring is significantly distorted (α = 11.2 ° ) and the tetrahedral basal oxygen atom plane is remarkably wavy (Δ z = 0.224 A). Crystal chemical modifications at mineral surface were investigated by X-ray photoelectron spectroscopy. The interlayer cation K is eight-fold coordinated both in the bulk and at the surface, and its amount decreases at the surface. Sodium substitutes K and its coordination at the mineral surface is six. This evidence, together with a significant Na increase at the surface, is consistent with Na clustering close to the cleavage plane.