The degree o f shock metamorphism has been determined between samples PTFE.2, A1.2 and St.2 and between samples St.2 and Poly.8. By combining the summaries o f observed shock features in these samples, with a summary o f the shock features in samples S t.l, PTFE.3
and PTFE.x (table 6.4) the relative extent o f shock metamorphism between all seven samples is
assessed.
Table 6.4, Comparison o f shock features used in assessing shock extent observed in recovered samples n/d = no data, 1/d = limited data and results in parenthesises indicate that these were the observed results but that they are uncertain without further investigation.
Shock features Sample
Poly.8 PTFE.x PTFE.3 PTFE.2 A1.2 St.2 St.l
~8 GPa - 1 2 GPa - 1 2 GPa - 1 6 GPa - 1 7 GPa - 1 7 GPa - 1 9 GPa -1 3 7 0 J/g -1 2 4 5 J/g -1 2 5 0 J/g -1 6 2 5 J/gi -1 0 4 0 J/g - 6 5 0 J/g - 7 2 0 J/g
Grains containing no shock effects none n/d none none none many few
‘Discontinuous’ PDFs some n/d yes none none some none
Single sets of PDFs some yes yes none none some some
Multiple sets o f PDFs some (no) yes all all occasional many
Minimum no. of sets o f PDFs observed 1 n/d 1 2 2 0 0
within a single grain
Maximum no. o f sets of PDFs observed 4 n/d 3 3 3 2 3
within a single grain
Number of different PDF orientations 4 n/d 3 6 1/d (2) 3 7 observed within the sample
PDFs parallel to no n/d no yes 1/d (no) yes no
PDFs parallel to |l0 1 2 | yes n/d no yes 1/d (no) no yes
‘Ladder structure’ PDFs no n/d no yes yes no no
Stishovite infilling PDFs no n/d no yes no no no
Amorphous silica at grain boundaries yes yes yes yes yes no no Patches of amorphous silica within no n/d yes 1/d yes no no
grains (yes?)
Samples Poly.8 and PTFE.3 appear to exhibit a similar extent o f shock metamorphism as the shock features they contain and the spatial density o f their PDFs are similar. All o f the grains in samples Poly.8 and PTFE.3 contained PDFs that occurred as both discontinuous and continuous features and in both single and multiple sets. Their range o f simultaneously occurring sets o f PDFs are very similar; Poly.8 they range from one to four, in PTFE.3 they range from one to three. Both samples were observed to contain patches o f amorphous silica within their grains and their grain boundaries were surrounded by amorphous silica.
The only difference in shock features between Poly.8 and PTFE.3 is the presence o f ‘marker’ PDFs parallel to the |l0 1 2 | crystallographic plane in sample Poly.8 suggesting a much greater extent o f shock metamorphism occurred in Poly.8 than in PTFE.3, which is at odds with other observations. There could be two possible explanations to explain this anomaly. First, it
may be that PDFs parallel to |l0 1 2 | do occur in sample PTFE.3 but were not observed in this study. Or, second, that the formation o f the PDFs parallel to {1OI2 } is more strongly dependent on the net internal energy increase experienced by the sample than the other shock features discussed above. Hence it formed in sample Poly.8 because o f its higher net internal energy increase (-1370 J/g), but not in sample PTFE.3 (-1250 J/g), despite the lower peak pressure o f Poly.8 ( -8 GPa) compared to PTFE.3 (-1 2 GPa). To determine the correct explanation further studies would need to be carried out. For the purpose o f classifying the relative extent o f shock metamorphism o f these two samples for this study only the available data will be used, and it will be interpreted that Poly.8 is shock metamorphosed to a slightly greater extent than PTFE.3 as it was observed to contain PDFs parallel to {1OI2 }.
Under the optical microscope samples PTFE.2 (-1 6 GPa and -1625 J/g), A1.2 (-1 7 GPa and -1 0 4 0 J/g) and St.l (-1 9 J/g and -7 2 0 J/g) appear similar apart from the lack o f amorphous silica at the grain boundaries in sample S t.l. The TEM results would suggest that PTFE.2 and A1.2 have undergone a greater amount o f shock metamorphism than St.l because the PDFs in PTFE.2 and A1.2 are all continuous and occur as multiple sets, whereas sample St.l contains grains free from PDFs, ‘discontinuous’ PDFs and PDFs occurring as single sets. Amorphous silica was observed to surround the grain boundaries in samples PTFE.2 and A1.2, and A1.2 was observed and PTFE.2 was assumed to contain patches o f amorphous silica within the grains. Neither amorphous silica at the grain boundaries or regions o f amorphous silica were observed in sample St. 1 by optical microscopy, TEM or XRD.
The presence o f amorphous silica in samples PTFE.2 and A1.2 and its absence in sample St.l (excluding its occurrence infilling PDFs) would suggest that PTFE.2 and A1.2 are more strongly shocked than S t.l. The observed presence o f PDFs parallel to the ‘m arker’ orientations appears anomalous compared to the extent o f shock metamorphism suggested by the other shock features in samples PTFE.2 and S t.l. St.l contains PDFs parallel to the {1OI2 } crystallographic orientations, while sample PTFE.2 contains PDFs parallel to both jlOls} and I1OI2 } crystallographic orientations. This would suggest PTFE.2 is less shocked than St.l if the relative extent o f shock metamorphism experienced by these two samples was judged solely on this criterion.
As was suggested previously this result may be an artefact o f the limited number o f measurements o f crystallographic orientations o f the PDFs and while the observed presence o f a specific orientation is a positive result, the lack o f an observation o f PDFs parallel to a specific orientation is not a negative result. In other words it may be possible that St.l also contains
PDFs parallel to {iOb }, but they were not present in the grains examined under the TEM.
by net internal energy increase than the spatial density o f PDFs or the transformation o f quartz to the ‘higher pressure’ polymorphs amorphous silica and stishovite.
From the discussion in section 5.4.2 it is clear the formation o f PDFs parallel to the
‘m arker’ orientations {lOls} and {lOll} are affected by net internal energy increase. It is beyond the scope o f this study to determine the extent o f the effect pressure and net internal energy increase play on their formation because o f the limited number o f measurements o f PDF orientation. A better understanding o f the shock conditions at which PDFs parallel to these orientations form is needed because their first appearance are used as ‘m arkers’ in examining extent o f shock metamorphism.
From the comparisons o f spatial density o f PDFs, the amount o f amorphous silica observed in the samples and the presence o f stishovite, o f the three samples, sample PTFE.2 is interpreted as having undergone the greatest extent o f shock metamorphism followed by sample A1.2, and St.l is interpreted as exhibiting the lowest degree o f shock metamorphism.
The extent o f shock metamorphism in sample Poly.8 (~ 8 GPa and -1 3 7 0 J/g) appears to be similar to that in St.l. In both samples, PDFs occur as single and multiple sets, and PDFs parallel to the ‘m arker’ crystallographic orientation {1OI2 } have been observed. Under the optical microscope the PDFs in sample St.l appear much more prominent than those in sample Poly.8. Sample St.l was observed to contain some grains that were completely free from PDFs while others contained closely spaced continuous PDFs. All o f the grains in sample Poly.8 were observed to contain PDFs, although some o f these sets o f PDFs were ‘discontinuous’.
Sample St.l did not contain amorphous silica at its grain boundaries or occurring within its grains. Sample Poly.8 had small amounts o f silica surrounding its grain boundaries and occurring within its grains suggesting a slightly higher degree o f shock metamorphism.
Sample PTFE.x was observed under the optical microscope but not examined in the TEM or by XRD so the observations for this sample are less detailed than the other samples in
table 6.4, because its shock conditions were very similar to those o f sample PTFE.3 it is assumed they are shock metamorphosed to a similar extent.
6.3.4 Summary of the relative extent of the shock metamorphism exhibited by the