Fractures induced by shock in quartz and feldspar

Abstract

SUMMARY. This study is devoted to fractures produced by natural and artificial shock processes in non-porous rocks consisting essentially of quartz and feldspar. Petrological and textural investigations were performed using optical and scanning electron microscopic techniques. A microfracturation index is adapted from Short (i966 , I968a, b) in order to compare the fracture densities in different materials shocked in different ways. In all cases, the density of fractures in quartz and feldspar increases with increasing pressure to about 200 kb. At higher pressure this trend is reversed. Fracturing is more intense in feldspar than in quartz. Plane shock waves produced in laboratory scale experiments induce more fracturing than natural shock waves. However, such an increase is no larger than the scatter among the data and the experimental technique used in the laboratory can be considered realistic in terms of fracturing. Finally the correlation between pressure and fracture density is too poor to be of use for quantitative pressure calibrations of naturally shocked materials.There is no direct correlation between the density of fractures and the number of planar elements observed. There is a negative correlation between fracturing and formation of diaplectic glass, Diaplectic glasses are remarkably weakly fractured compared with shocked minerals. The abrupt change in the slope of the curve giving the dependency of the density of fractures with pressure corresponds to the pressure at which diaplectic glass is formed. From petrographic considerations it is deduced that fracturing occurs at the end of the shock sequence, on pressure release, while diaplectic glasses are forming or already formed. Hypotheses of mechanical and thermal fracturing are examined; both are plausible, but a thermal origin may be preferred. The mechanism of formation of diaplectic glass is discussed with respect to results and deductions obtained by the study of fracturing. Diaplectic glass could represent a decompressed highdensity glass resulting from a single state transformation of a mineral, at high pressure, FRACTURES in minerals are a common feature of natural (impact craters ) and artificial shock events (chemical-nuclear explosions, laboratory scale impacts, etc.). However, as the same effects are often generated by endogenic processes and by static experiments, fractures due to impact are at the threshold of non-ambiguous shock features. Because of this, they are relatively poorly studied compared with other shock effects. We still do not Copyright the Mineralogical Society know exactly why, how, and when fractures occur in the minerals of shocked rocks. The purpose of this paper is to try to answer these questions, restricting the study to the behaviour of quartz and feldspar in non-porous rocks. Arguments will be supported by the examination of the correlations between fractures and the shock geometry, fractures and other shock effects, and also fractures and pressure. Previous studies have demonstrated a general increase in th...

Relationships among magnetic fabrics, microfractures and shock pressures at an impact crater: A case study from Lockne crater, Sweden

“…The rocks farther than about 6.5 km from the crater centre, with low shock pressures (b0.3 GPa) show random distribution of microfractures. These observations are in agreement with Hörz (1969) and Lambert (1979), who demonstrate correlation between fracture density and shock pressure at ≥ 0.2 GPa, and suggest that below the pressure limit, the fractures cannot be characterized.…”

“…On the other hand, the microfractures, in rocks between 3.5 km (the inner crater rim) and 6.5 km from the centre of the crater show trend characteristic of impact generated microfractures. The results, when interpreted in light of experimental studies (Ahrens, 2002;Gattacceca et al, 2007;Hörz, 1969;Lambert, 1979;Nishioka and Funaki, 2008;Nishioka et al, 2007), suggest shock pressure in the range of 0.5 to 0.2 GPa in rocks up to about 6.5 km from the centre of the crater, and in the order of b0.2 GPa in the rocks farther away.…”

“…The comparisons unambiguously demonstrate that the radial fractures form during the compressive phase due to accumulation of tensile stresses at mechanical flaws (Ahrens and Rubin, 1993;Polanskey and Ahrens, 1990;Sammis and Ashby, 1986) and the concentric fractures form due to sudden pressure release during the decompressive phase (Ahrens and Rubin, 1993;Rubin and Ahrens, 1991;Selberg, 1952;Shibuya and Nakahara, 1968). Hörz (1969) and Lambert (1979) suggest that, in general, a systematic relationship between the impact generated fractures and the shock pressures is valid only in the range of 0.2-20 GPa. The impact origin of the fractures may, therefore, be confirmed by investigating their orientations with respect to the impact crater and if the microfractures are indeed generated by shock wave we may quantify the shock pressures to be in the range of 0.2-20 GPa.…”

Shock pressure estimates in target basalts of a pristine crater: A case study in the Lonar crater, India

“…Review of published literature reveals that brittle deformation in response to shock waves depends upon the various rock properties such as grain size, porosity, preexisting fractures, and density (e.g., Ahrens and O'Keefe, 1977;Lambert, 1979;Ahrens and Rubin, 1993;Huffman et al, 1993;Kumar and Kring 2008). Moreover, in the pressure range of 0.2-20 GPa, a direct correlation between shock pressure and fracture density is observed in rocks with similar mineralogical and physical properties (e.g., Hörz, 1969;Lambert, 1979).…”

A Raman spectroscopic study of shocked single crystalline quartz

“…Samples shocked to peak pressures below approximately 36 GPa exhibit "planar features" visible in optical microscopy, formed by single or multiple lamellae or sub-planar elements, generally oriented along families of crystal planes. For crystalline samples (as opposed to powders) shocked to peak pressures below approximately 10 GPa, all of these planar features could be attributed to fractures or cleavages, appearing as the quartz was shocked within the elastic region of the Hugoniot (H6rz 1968;Ahrens and Rosenberg 1968;von Engelhardt and Bertsch 1969;St6ffier 1972St6ffier , 1974Lambert 1979). Some of these features, such as the basal lamellae parallel to (0001), could be produced in static deformation experiments at temperatures of 500-900 ~ C (Christie et al 1964;McLaren et al 1967;Carter 1971).…”