HR-AFM: STUDY OF (001) MICA SURFACES.

Anna Maria Lanfranco

Institute of Mineralogy and Petrography, CH-1700 Fribourg, annamaria.lanfranco@unifr.ch

Keywords:micas, AFM, plane group

The Atomic Force Microscopy (AFM) was used in this study to image at near atomic resolution the (001) cleavage plane of several micas.

This instrument results at-today the simplest one to reach very high resolution (sub-A order, according to Topometrix) in vacuum and in air without special sample preparation.

The samples selected were Alpine micas, different in origin and chemical composition. They were chemically characterised by XRF (major and trace elements with Z>11); Multiphase Carbon Determinator (water content) and PIXE method (halogens measurements); the mineralogical composition of the samples was determined by means of XRD powder method. The XRD spectra were also used to refined the cell parameters applying the Rietveld refinement.

HR-AFM images were collected in air using a Nanoscope III Multimode AFM, setting the contact scan mode; the Si₃N₄ cantilever (0.58 N/m as spring force) scanned areas of about 20nm side with as soft as possible force. The tip-surface contact force was always checked to verify the presence of water layer on the surfaces.

Mica basal planes were easily recognised when molecular resolution of 5A were reached. Good images were considered those in which the two tetrahedra are distinguished, corresponding to a lateral resolution of about 2.5A.

Several pictures captured on the same sample were processed by mathematical methods as flatten, planefit and zoom when the noise had to be reduced. The 2D-Fast Fourier Transform showed the periodicity of the images and allowed to identify their crystallographic plane group.

The measured surface parameters were then statistically weighted and compared with those calculated for a bulk (001) plane.