Diaplectic glass

Diaplectic glass is densified, amorphous silica produced by a solid to solid

transformation from crystalline silica (DeCarli and Jamieson, 1959; Gratz et al. 1992) and is

unique to shock compression. It is distinctly different to other silica glass such as lechatelierite and synthetic silica glass which are formed via a liquid phase, and because its formation does not include a liquid phase it never contains flow structures or vesicles and it can contain inclusions o f polyciystalline aggregates o f the high pressure silica polymorphs, coesite and stishovite (Stoffler, 1971). Diaplectic glass is characterised by its preservation o f the pre-shock morphology and texture o f the precursor crystal, including the grain boundaries, orientated inclusions and twin boundaries. It also displays a higher degree o f long-range order than glasses formed as a quenched melt (Stoffler, 1972: Schneider, 1978; Goltrant et al., 1992; Mashimo et al., 1980; Langenhorst, 1993) and thermal annealing experiments (Rehfeldt- Oskierski, 1986; Rehfeldt- Oskierski and Stoffler, 1986) have shown the structure o f the diaplectic glass retains a memory o f the crystalline state.

Diaplectic glass is denser and has a higher refractive index than glass formed from a melt o f the same chemical composition, with a density range o f 2.276- 2.206 g/cm^ and a refractive index (R.I.) between 1.461 and 1.468, compared to synthetic silica glass that has a density o f 2.202 g/cm^ and an R.I. o f 1.459, or lechatelierite which has densities in the range o f

2 .200- 2.205 g/cm^ and R.I. between 1.458- 1.460 (Engelhardt et al., 1967; Chao, 1967, 1968

Engelhardt and Bertsch, 1969; Stoffler and Hornemann, 1972; Xiande and Chao, 1985

Rehfeldt- Oskierski, 1986; Rehfeldt- Oskierski and Stoffler, 1986; Grothues et al., 1989

Huffman et al., 1989; Langenhorst et al., 1992; Langenhorst and Deutsch, 1994).

It has been found at numerous impact structures (e.g. papers in French and Short, 1968; Stoffler, 1972, 1984), in nuclear explosion craters (Short, 1966, 1968, 1970) and synthesised in many laboratory shock experiments (DeCarli and Jamieson, 1959; Horz, 1968; Défoumeaux, 1968; Kleeman and Ahrens, 1973; Mashimo et al., 1980; Rehfeldt- Oskierski, 1986; Rehfeldt-

Oskierski and Stoffler, 1986; Grothues et al., 1989; Gratz et al. 1988, 1992; Langenhorst et al.,

1.8.4 Lechatelierite

Lechatelierite is a silica melt glass formed by shock and is alm ost indistinguishable from synthetically produced melt glass o f the same compostion or Lechatelierite formed by other processes (Stoffler and Langenhorst, 1994 and papers therein). It is the product o f the highest degree o f shock (bar vaporisation) (Grieve et a l, 1995) and forms highly vesiculated glass with flow structures from unconsolidated sedim ents or sedimentary target rocks (papers in French and Short, 1968; Kieffer et a l, 1971, 1976; Hôrz et al., 1989; Stoffler and Langenhorst, 1994). Where the target rocks are non porous and crystalline, lechatelierite only occurs as inclusions and schlieren in whole rock melts (Engelhardt, 1972, fig u re 1.27).

Figure 1.27, Lechatelierite (light-coloured elongate areas in upper left of image) in a whole rock melt glass from the suevite at the Otting Quarry in the Ries Crater in Germany. Schlieren, circular vesicles and angular mineral fragments are also present in the glass matrix. Reproduced from French (1998), image originally published in Engelhardt and Stoffler (1968).

Lechatelierite occurs as ‘bom b-shaped’ inclusions in ejecta blankets from natural impacts and nuclear explosion craters (e.g. Short, 1970). It also occurs as subm icroscopic veins or tilling fractures in untransformed crystalline material as a result o f localised pressure and tem perature peaks (Gratz et a i, 1988, 1988, 1992; M artinez et a l, 1993) in naturally and experim entally shocked material, and has been produced by numerous laboratory- scale, shock experim ents (DeCarli and Jam ieson, 1959; Stoffler, 1972, 1974, 1984; Stoffler et a l , 1975; M ashimo et al., 1980; R ehfeldt-O skierski, 1986).

1.9 Aims o f this study

In natural impacts, the minerals are shocked via a direct single shock loading path. Most laboratory calibration experim ents have used reflected shock loading paths. Com parison o f the single shock results from Florz (1968) with reflected path shock studies indicate that loading path may be important. The objective o f the present work was to further explore the effect o f loading path variations on shock m etam orphism o f quartz.

The majority o f the shock loading experim ents on silica, including those used to calibrate shock effects with pressure, have used single crystal quartz that has been loaded using the reverberation technique, i.e. loaded with a reflected loading path. This means that the net internal energy increase in the sam ple is unrelated to the final peak pressure. The net internal energy increase in a sample loaded with a reflected loading path will be much lower than the energy that would be deposited with a direct loading path. The series o f experim ents conducted

for this study is intended to determine whether the transformation o f quartz during shock processes is affected by the value o f the net internal energy increase, and whether varying the net internal energy increase has an effect on the pressures at which shock effects will occur.

If net internal energy increase does play a role in shock transformations, it would imply that the peak pressures that shock transformations occur at will vary with the loading path used. It would also mean that the shock effects produced by direct loading paths in laboratoiy experiments would be most comparable with shock effects produced in nature.

If it proves that the formation o f shock effects is independent o f net internal energy increase it would imply that the shock metamorphic effects in quartz would be solely dependent on peak pressure and that a silica grain would exhibit the same shock effects at a specific pressure regardless o f the loading path or the lithoiogy or porosity o f the surrounding rock.

In the following chapters the theory and shock physics behind laboratory shock

experiments are introduced (Chapter 2) and the experimental methods used in this study are

detailed (Chapter 3). The calculated shock conditions o f the samples o f porous quartz shock

loaded for this study are presented (Chapter 4) and the observations o f the recovered samples by

optical microscopy, x-ray diffraction and transmission electron microscopy, summarised

(Chapter 5). To test the hypothesis that loading path may affected the peak pressures at which shock features form, the pressures at which observed shock features were formed were compared between samples with different loading paths within this study and the implications