A simple atomistic Monte Carlo simulation suggests that there were up to four stages in the evolution of an etch pit in the (001)-surface of an idealised regular lattice. During the first stage, the etch pit was an inverted pyramid; its horizontal and vertical dimensions increase at a constant rate; the apparent horizontal (vh) and vertical (vd) growth rates were faster than during all subsequent stages but nevertheless less than the step retreat rate (vs) on account of surface etching (vv). The pyramid apex was truncated in the second stage; it was thereafter bounded by an expanding bottom plane and shrinking lateral walls; this was accompanied by a gradual decrease of vh; vd drops to a negative value indicating a slow decrease of the etch-pit depth; the bottom plane acquires a concave-up curvature; the outward curvature of the walls, initiated during the first stage, increased. During the third stage the etch pit consists of a single concave-up bottom plane; vh and vd decrease at declining rates; consecutive etch-pit profiles were scalable in the horizontal direction. The hypothetical fourth stage was inferred but not documented by the simulations; it sets in when vh was reduced to zero; unless this corresponds to an as yet unidentified steady-state condition, the etch pit from here on forth shrinks until it eventually disappears altogether. The sole cause for this succession was the process of stochastic rounding of confined steps and faces. The triangular footprint of recoil-track, fission-track, ion-track and dislocation etch pits in trioctahedral mica and its compliance with the monoclinic symmetries implies that the relevant periodic bond chains were O–Mg/Fe–O chains in the octahedral layer. The size distribution of etched recoil tracks was due to (1) depth variations resulting from the size distribution of the latent tracks, (2) the random truncation of the surface tracks, (3) the variable rate of etch-pit enlargement and (4) the fact that new tracks were exposed at the surface due to surface etching. The greater size of dislocation, fission-track and ion-track etch pits was due to their greater extent below the surface. The increase of the number of etched tracks with etching time due to bulk etching was non-linear because the bulk etch rate vv was not constant. The evolution of etch-pit shape with continued etching could also cause loss of tracks due to observation effects related to loss of contrast.

Revelation of Nuclear Tracks and Dislocations - a Monte Carlo Simulation of Mineral Etching. K.Stübner, R.Jonckheere, L.Ratschbacher: Geochimica et Cosmochimica Acta, 2008, 72[13], 3184-99