Action of laser radiation on rocks

Abstract

This paper deals with certain physical effects of laser radiation on strong rocks.The work was performed on specimens of minerals from the silicate and carbonate groul~, hornblende, biotite, quartz, microcline, oligoclase, and calcite. The mineral surface was ground, which enabled us to determine the change in structure and elucidate the character of the effect of laser radiation by preparing polished sections from the irradiated sectors. The experiments were performed in a ruby-neodymium laser; the pulse duration was 10 -~ or 10 "s see, the pulse energy was 8J (free generation) or 0.5 J (modulation), and the laser radiation was focused on the specimen. The fractures produced in the specimens were studied in reflected light under the microscope, and on polished sections in a polarizing microscope. The strength of the material was determined from the microhardness of the minerals.The results of laser action on minerals must be largely determined by the thermal effect and the shock wave [1, 2]. The first or second effects will evidently be more marked in different minerals. From this viewpoint, the minerals used can be divided into two groups. The first group (thermal effect) includes biotite and hornblende from the group of banded and layered silicates, in which we observed fusion with spatmring of the fused mass from the crater. Around and within the crater, we observed sintered, previously fused mineral. Figure 1 displays these characteristics by the example of biotite. When the axis of the laser beam was perpendicular to the biotite plates, the crater was conical. At the edges of the crater, the biotite was fused and converted to a sintered blistered mass. In a polished section prepared from a slice parallel to the crater mouth (see Fig. lb), in crossed Nicols we observed in the crater center an accumulation of accessory minerals, giving the impression of having been filtered out from the evaporated biotite. These were not neogenic products because such minerals are found in the initial rock. Figure lc shows a cross section of the crater obtained by the action of a laser beam parallel to the biotite plates (perpendicular to the third pinacoid). Here we can frequently see an altered zone, the thickness of which reaches 0.1 mm atthe crater edges, and 0.14 mm at the bottom. The alterations at the edges of the zone are reflected in the sintering of biotite into a blistered mass; in the inner part of the zone, the biotitm became almost colorless. The transition to unaltered biotite is not always sharp; the boundary between both parts of the zone is uneven and pockety. This unevenness may be due to differences in the microstructures of adjoining sectors. To determine the microhardness (the change in biotite strength due to laser irradiation), we used the natural polish of biotite flakes on the cleavage planes because we failed to obtain satisfactory polishing owing to delamination of the biotite, tt will be seen from Fig. 2 that around the crater, at a distance l 3 = 3r (r is the crater radius), there is a zone of in...