Crystal chemistry and surface features of a 2M1 paragonitic muscovite

• Main Authors: ELMI, Chiara, BRIGATTI, Maria Franca, PASQUALI, Luca, MONTECCHI, Monica, NANNARONE, Stefano, MALFERRARI, Daniele
• Other Authors: Elmi, Chiara, Brigatti, Maria Franca, Pasquali, Luca, Montecchi, Monica, Nannarone, Stefano, Malferrari, Daniele
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2011
• Subjects: muscovite; surface; crystal chemistry; Antarc*; Antarctica
• Online Access: http://hdl.handle.net/11380/739071

The paragonitic muscovite considered [XII](K1.722 Na0.251 Ba0.010) [VI](Fe2+0.140 Al3+3.769 Cr3+0.003 Mg0.074 Ti0.012 Mn0.002) [IV](Al1.897 Si6.103) O20 (F0.109 OH3.891) came from Antarctica, it 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) (°).By using a multi-analytical approach this study describes the crystal chemical features of muscovite determined both in the bulk and on the uppermost surface layers. The bulk structure was refined by single crystal X-ray diffraction and the final refinement yielded the following agreement factor R = 0.0299. The two tetrahedral sites, T1 and T2, present similar distances ( = 1.643 Å and = 1.647 Å) and distortion parameter values, thus evidencing a disordered distribution of [IV]Si and [IV]Al in tetrahedral sites. Tetrahedral ring is significantly distorted (α = 11.2°) and the tetrahedral basal oxygen atoms plane is remarkably wavy (Δz = 0.224Å). tetrahedral ring distortion affects not only the topology of the cleavage plane, but also the coordination of the interlayer cation, which reduces from twelve to eight. Crystal chemical modifications at mineral surface were investigated via X-ray Photoelectron Spectroscopy (XPS), by comparing data related to surface to the ones characterizing the bulk. A peculiarity of muscovite from Antarctica is a limited Na for K interlayer substitution, which was found in the bulk as well as on the mineral surface. It is well known that in micas the coordination of the interlayer cation can vary from twelve to six, depending on the distortion of the hexagonal tetrahedral ring, as measured by  angle, as observed and predicted by Weiss et al. [1].The interlayer cation K (binding energy = 293.36 eV) is confirmed to be eight-fold coordinated [2]. Na (binding energy = 1071.02 eV) substitutes K both in the bulk and at the surface. Na coordination, at mineral surface, reduces from eight to six [3]. The six-fold coordination presented by Na is consistent with ...