Análisis e interpretación de resultados del Estudiante NM

All the PDFs observed in this study were composed o f amorphous silica except for

some o f those in sample PTFE.2 that were filled with crystallites (figures 5.47a,b and c, 5.49a,

b) that were very beam sensitive and transformed rapidly to amorphous silica under the electron

beam (figures 5.47c and 5.49c). It is assumed that these crystallites are stishovite, as stishovite

is commonly found ‘decorating’ PDFs. The possibility that they were coesite was discounted because it is rarely observed in samples shocked on the scale o f laboratory experiments and is

usually associated with diaplectic glass (Kieffer et al., 1976) rather than occurring as

decorations in PDFs.

The presence o f stishovite is in agreement with the results o f DeCarli and Milton (1966) who observed the presence o f trace amounts o f stishovite in a number o f experimentally

shocked quartz and quartz rock samples summarised in table 6.1.

Table 6.1, Summary of samples and shock conditions that produced trace amounts of stishovite under experimental conditions in experiments by DeCarli and Milton (1966). Ranges for pressures and temperatures are limits o f possible values, in parentheses are best estimates (DeCarli and Milton, 1966). Table modified after DeCarli and Milton, 1966.

Sample material Density (g/cm^) Shock Conditions

Pressure (GPa) Temperature (°C) Shock Rarefaction

Sandstone 2.0 15-30(24) 600-1100 (900) 550-800 (650)

Novaculite 2.6 24-30 (28) 250-400 (300) 150-300 (200)

Sandstone 2.0 12-17(15) 450-600 (500) 400-550 (450)

Single crystal quartz 2.65 14-17(16) 100-250(100) 100-250 (100)

These samples were shock loaded in stainless steel sample containers via reflected

loading paths. All o f the samples listed in table 6.1 had shock durations o f less than 10 psec

(DeCarli and Milton, 1966), which is in the same order o f magnitude as the shock durations experienced by the samples in this study. The very fine size o f the stishovite crystallites in PTFE.2 is explained by the very short shock duration experienced by sample PTFE.2 (DeCarli

et a l, 1966) to which their growth is limited.

Other TEM studies on single crystal quartz experimentally loaded via reflected loading paths (e.g. Ashworth and Schneider, 1985: Langenhorst, 1994) only report the presence o f amorphous silica in the PDFs.

Despite reporting the presence o f no high pressure polymorphs in the PDFs that they observed in quartz recovered from the ejecta blanket o f the Sedan nuclear test crater Cordier and Gratz (1995) published a TEM image o f PDFs with very small, strongly refracting areas that

have the appearance o f crystallites (figure 6.2). The argument that these strongly refracting

presen ce o f ‘d a m a g ed ’ layers ob served in P D F s exa m in ed at h igher m agn ification s (section

Figure 6.2, PDFs in quartz recovered from the Sedan nuclear test crater by Cordier and Gratz (1995). Though not commented on by Cordier and Gratz, the PDFs appear to be composed o f very small, strongly refracting areas that have the appearance o f crystallites. TEM image reproduced from Cordier and Gratz (1995).

6.2.8‘D am aged’ layers of silica in PDFs

The crystallites of stishovite filling the PDFs of sam ple PTFE.2 were very beam sensitive, and transformed rapidly to amorphous silica (figure 5.47). The crystallites situated in the centre of the PDFs transformed most rapidly with the result that images taken after the transformation had comm enced show crystallites present at one or both of the edges of the PDFs. This has a very similar appearance to the layers of ‘dam aged’ material in PDFs described by Cordier and Gratz (1995) (figure 1.22). In common with the remaining crystallites seen in this study Cordier and Gratz (1995) describe their ‘dam aged’ layer as occurring “ ...either on a single side of the lamellae or on both.” This perhaps suggests that they may have been observing crystallites some of which had just transformed to am orphous silica. The electron diffraction pattern they took of the ‘dam aged layer’ confirm s that it is mostly composed of crystalline material, but unfortunately then add “ ...n o further information could be gained because of the strong strain contrast of these [‘dam aged’] layers.” This unfortunately gives no evidence as to w hether these layers are “ ‘dam aged’ m aterial” as suggested by Cordier and Gratz (1995) or perhaps related to the partially transform ed crystallite filled with a similar appearance seen in this study.

6.2.9 A m orphous silica at grain boundaries

Observations under the optical m icroscope of samples PTFE.2, PTFE.3 PTFE.x, A1.2 and Poly.8 reveal the presence of amorphous silica at the grain boundaries. Kieffer (1970 and 1971) and Kieffer et al. (1976) noted the presence of high-pressure phases o f silica decorating the edges of the original quartz grains in the initially porous Coconino sandstone. They noted that these areas appeared yellow/ gold in plane polarised light (figure 6.3) and were composed of submicron to m icron-sized crystallites of coesite, areas of glass and quartz. Kieffer et al.

b eca u se the silica p hases present are “ ...in te r m ix e d w ithin m icron s and appear in a verm icu lar h a b it ...”

Figure 6.3, Optical micrograph (plane polarised light) of symplectic material (s) occurring at the grain boundaries in shocked Coconino sandstone. Optical micrograph reproduced from Kieffer et al., 1976.

The ‘am orphous’ silica at the grain boundaries o f the sam ples recovered in this study is not colourless with very low relief as is the expected appearance o f a silica glass in transm itted light. Instead the material appears a little darker or perhaps o f higher relief than the quartz grain it is surrounding; it appears greyish and translucent (figures 5.10, 5.11, 5.14 and 5.18) (also DeCarli and Bowden, 2001). However as would be expected o f am orphous silica it appears isotropic in cross-polarised light. This slightly unusual appearance o f the am orphous silica may suggest that like the material at the grain boundaries in the shocked Coconino sandstone (K ieffer et al., 1976), it may not be solely amorphous silica but could contain a mixture o f unshocked quartz, am orphous silica and perhaps a high-pressure polymorph o f silica.

Kieffer et a i, (1976) classified the sample in which sym plectic material at the grain boundaries first appeared as a “Class 2 rock” and described it as weakly shocked. In “ class 3 rocks” that had experienced a greater degree o f shock metamorphism nearly every grain was bordered by the symplectic or opaque material (a m ixture o f coesite, stishovite and glass) (Kieffer, 1971). The thickness o f the symplectic material at the grain boundaries increased with increasing shock extent (Kieffer, 1971) indicating that greater quantities o f am orphous silica or symplectic material at the grain boundaries indicate a greater extent o f shock m etamorphism .

6.2.10 Regions of am orphous silica

Patches o f amorphous silica were observed within grains in samples AI.2, PTFE.3 and Poly.8 were observed to be associated with areas o f high strain (figures 5.45 and 5.55) and were interpreted to be due to heterogeneous tem perature conditions over small areas. Kieffer et a l,

(1976) noted evidence for a sim ilar heterogeneity o f the extent o f shock conditions in their studies o f the originally porous Coconino sandstone, for exam ple the concentration o f phase transform ations and fracturing at grain boundaries and the coexistence o f quartz, coesite and stishovite for which no equilibrium P- T - Vconditions exist where all three phases are stable.

b ecau se the silica p hases present are “ ...in te r m ix e d w ithin m icrons and appear in a verm icular h a b it ...”

Figure 6.3, Optical micrograph (plane polarised light) o f symplectic material (s) occurring at the grain boundaries in shocked Coconino sandstone. Optical micrograph reproduced from Kieffer et a i, 1976.

The ‘am orphous’ silica at the grain boundaries of the samples recovered in this study is not colourless with very low relief as is the expected appearance of a silica glass in transmitted light. Instead the material appears a little darker or perhaps of higher relief than the quartz grain it is surrounding; it appears greyish and translucent (figures 5.JO, 5.J1, 5.14 and 5.18) (also DeCarli and Bowden, 2001). However as would be expected of amorphous silica it appears isotropic in cross-polarised light. This slightly unusual appearance of the am orphous silica may suggest that like the material at the grain boundaries in the shocked Coconino sandstone (Kieffer et a l, 1976), it may not be solely am orphous silica but could contain a mixture of unshocked quartz, amorphous silica and perhaps a high-pressure polym orph of silica.

Kieffer et a l, (1976) classified the sample in which symplectic material at the grain boundaries first appeared as a “Class 2 rock” and described it as weakly shocked. In “class 3 rocks” that had experienced a greater degree of shock m etam orphism nearly every grain was bordered by the symplectic or opaque material (a mixture of coesite, stishovite and glass) (Kieffer, 1971). The thickness of the symplectic material at the grain boundaries increased with increasing shock extent (Kieffer, 1971) indicating that greater quantities of am orphous silica or symplectic material at the grain boundaries indicate a greater extent of shock metamorphism.

6.2.10 Regions of am orphous silica

Patches of amorphous silica were observed within grains in samples A1.2, PTFE.3 and Poly.8 were observed to be associated with areas of high strain (figures 5.45 and 5.55) and were interpreted to be due to heterogeneous temperature conditions over small areas. Kieffer et a l,

(1976) noted evidence for a similar heterogeneity of the extent of shock conditions in their studies of the originally porous Coconino sandstone, for example the concentration of phase transform ations and fracturing at grain boundaries and the coexistence of quartz, coesite and stishovite for which no equilibrium P- T - V conditions exist where all three phases are stable.

Ashworth and Schneider (1985) noted “irregular patches o f featureless glass” started to appear in their single crystal quartz that was experim entally shocked to 30 GPa via a reflected loading path (figure 6.4). As is also seen in samples A1.2, PTFE.3 and Poly.8, the patches of am orphous silica appeared to be closely associated with multiple sets of PDFs and untransformed quartz that exhibits a severe strain contrast (Ashworth and Schneider, 1985).

Figure 6.4, Irregular patches of amorphous silica (p) in single crystal o f quartz shocked to 30 GPa with a reflected loading path (Ashworth and Schneider, 1985). As with samples Poly.8, PTFE.3 and A1.2 from this study the amorphous silica was observed to form in association with PDFs and strained untransformed quartz. TEM image reproduced from Ashworth and Schneider, 1985.

Kieffer (1971) used the percentage of am orphous silica within shocked samples of Coconino sandstone as one of the criteria to classify the extent of shock m etamorphism the sample had undergone, with the samples containing greater quantities of am orphous silica having undergone a greater extent of shock metamorphism.

TEM observations of sample PTFE.2 did not reveal the presence of ‘patches’ of amorphous silica despite the sample having experienced a greater shock pressure and a greater net internal energy increase than samples A1.2 and PTFE.3 in which it was observed. This seeming anomaly has been interpreted as having been a com bination of the heterogeneous distribution of the shock features, including the patches of am orphous silica through the sample and an artefact of the limited area of the sample foil that was exam ined in the TEM . This explanation is backed by the XRD results (figure 5.28) that dem onstrate that sample PTFE.2 contains much greater quantities of am orphous silica than sam ple PTEE.3. Also the large quantity of amorphous silica present at the grain boundaries suggests the presence of amorphous silica within the grains is very likely.

6.2.11 ‘B ent’ crystal lattice

Samples S t.l, St.2 and PTFE.2 contained grains in which the crystal lattice of the untransform ed crystalline quartz had been strained so that the electron diffraction pattern gradually and continuously altered as the sam ple was moved across the electron beam indicating the lattice was slightly ‘bent’. This characteristic was not specifically looked for in the samples so may be present in the other samples but was unnoticed in the TEM study. For example, under the optical microscope sample PTFE.3 exhibited an undulatory extinction in some of its grains

(figures 5.15a and 5.15b).

The same effect was observed by Ashworth and Schneider (1985) in their experimental

samples and described as “ ...n o t sharp changes, but gradual flexures o f the lattice...” . Kieffer et

al. (1976) also noted a patchy undulose extinction in their studied specimen o f weakly shocked

Coconino sandstone.

The crystal lattices o f the quartz within the samples in this study also exhibited much shorter-range strain effects such as mosaicism and strain contrast.