• journal_title：The Canadian Mineralogist
• Contributor：Chiara Elmi ; Maria F. Brigatti ; Stephen Guggenheim ; Luca Pasquali ; Monica Montecchi ; Daniele Malferrari ; Stefano Nannarone
• Publisher：Mineralogical Association of Canada
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.51.1.5
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：51
• issue：1
• firstpage：5
• section：Article

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](Fe²⁺_0.14 Al³⁺_3.77 Mg_0.07 Ti_0.01) ^[IV](Al_1.91 Si_60.9) O₂₀ (F_0.11 OH_3.89). The sample is monoclinic, 2M₁ polytype, with symmetry C2/c and unit cell parameters a = 5.1969(1), b = 9.0138(3), c = 20.0835(7) Å, and β = 95.763(2)^°. The two tetrahedral sites, T1 and T2, present similar distances (<T1–O> = 1.643Å and <T2–O> = 1.647 Å) 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 (Δz = 0.224 Å).

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.