Tracks in Mica, 50 Years Later: Review of Evidence for Recording the Tracks of Charged Particles and Mobile Lattice Excitations in Muscovite Mica

Abstract

Large crystals of the layered mineral muscovite mica often contain fossil tracks of charged positrons emitted from radioactive potassium atoms that make up 3 atomic % of mica. The tracks are made visible naturally by decoration with the black mineral magnetite coming from an impurity of iron that is precipitated after the crystals have formed deep underground. Positively charged high energy muon tracks created by cosmic rays also are recorded. The layered structure of mica allows thin transparent sheets to be peeled off to reveal a bewildering array of black lines, of which only 1 % are the tracks of charged particles. Lying mostly in random directions the charged particle tracks were identified in four years. The remaining 99 % of lines lying exactly parallel to chains of potassium atoms defied explanation for another 25 years until evidence was found for them being caused by recoil of potassium nuclei following emission of positrons. It was proposed the recoils created mobile highly-localised, self-focussing, non-linear lattice excitation of the lattice, called quodons, involving only a few atoms with energies up to tens of eV. After 10 more years the existence of quodons was shown in a laboratory experiment, confirming their stability against thermal motions of atoms. 10 years later, it was shown that atomic cascades, created by energetic nuclear scattering of swift particles, generate atomic-size kink-pulses that can gain energy from the metastable mica lattice. These cascades give rise to fan-shaped patterns containing multiple parallel tracks called striae. The possibility that ’ultra-discrete kinks’ might explain the striae is examined. These and similar energetic lattice excitations should assist in annealing radiation-induced defects in crystals. The lines in muscovite mica remain the only way to observe the flight and behaviour of these excitations and illustrate the remarkable properties of quasi-2-dimensional atomic structures.