Análisis e interpretación de resultados del Estudiante NA

If net internal energy increase had no influence on the peak shock pressure needed to form shock features, samples PTFE.2, A1.2 and St.2 would all contain the same shock features.

etc.) show this is not the case. A comparison o f the shock features observed in the three samples

is summarised in table 6.2

Table 6.2, Comparison of shock effects observed in samples PTFE.2, A1.2 and St.2 Shock features

PTFE.2

Sample

A1.2 St.2

Grains containing no shock effects none none many

‘Discontinuous’ PDFs none none some

Single sets of PDFs none none some

Multiple sets of PDFs all all occasional

Minimum no. of sets of PDFs observed within a single grain 2 2 0 Maximum no. of sets o f PDFs observed within a single grain 3 3 2 Number of different PDF orientations observed within the

sample

6 limited data (2)

3

PDFs parallel to jlO ls j yes limited data

(no)

yes

PDFs parallel to j l O l l j yes limited data

(no)

no

‘Ladder structure’ PDFs yes yes no

Crystallites of stishovite infilling PDFs yes no no

Amorphous silica at grain boundaries yes yes no

Patches o f amorphous silica within grains not observed but likely

yes no

Under the optical microscope all o f the grains in samples PTFE.2 and A1.2 contained multiple PDFs that were clearly visible in crossed polarised light and plane polarised light. The grain boundaries in both samples were surrounded by amorphous silica. In contrast, sample St.2 contained a large number o f grains free from any observable shock features and was observed to contain only two grains containing multiple sets o f PDFs and a few a scattered grains containing single sets o f PDFs. No amorphous silica was seen at the grain boundaries for this sample. Thus, from optical observations o f the spatial density o f the PDFs and the presence o f observable amorphous silica at grain boundaries samples PTFE.2 and A1.2 would be classified as having been shock metamorphosed to a greater extent than sample St.2.

These observations are backed up by the XRD analysis o f samples PTFE.2 and St.2 (sample A1.2 was not analysed by XRD). The results show a prominent peak representing amorphous silica for sample PTFE.2 indicating it contains a significant quantity o f amorphous silica. The analysis for St.2 contains no amorphous silica peak. From the classification used by Kieffer (1971) where the extent o f shock metamorphism was determined by the percentage o f the rock that had been transformed to amorphous silica, sample PTFE.2 would be classified as being shocked to a greater extent than sample St.2.

When comparing the TEM results o f the samples, differences in the shock features that formed can be seen between all three samples. Sample St.2 exhibits features that indicate that it suffered a lesser extent o f shock metamorphism than samples PTFE.2 or A1.2. Consistent with

the optical microscopy results a large number o f the grains examined in St.2 were free from shock features including PDFs and the maximum number o f sets o f PDFs observed in one grain was two. This in contrast with samples PTFE.2 and A1.2 in which the spatial density o f the PDFs was much greater; no grains were free from PDFs and the num ber o f sets o f PDFs simultaneously occurring within a single grain in either sample varied from two to three. The crystallographic orientations o f the PDFs in samples PTFE.2 and St.2 echo the results from the spatial density. Despite having a very slightly lower peak pressure, sample PTFE.2 was observed to contain grains with both ‘marker’ PDF orientations jlOls} and {1OI2 }, while sample St.2 was only observed to contain PDFs parallel to the ‘lower pressure’ orientation {1013}. Very limited observations and PDF orientation measurements were made for sample A1.2 in the TEM, therefore there is no comparable data for this sample.

While the grains in sample St.2 were mainly free from shock features and the areas where the highest pressure and temperatures occurred are represented by the presence o f PDFs and strain contrast in the untransformed crystalline quartz, in sample A1.2 the majority o f grains contained PDFs and showed strain contrast and the areas that experienced the greatest pressure and temperature conditions were represented by patches o f amorphous silica. As discussed earlier no patches o f amorphous silica were observed in sample PTFE.2 in the TEM but it seems likely they occur.

PDFs infilled with crystallites o f the high-pressure polymorph o f silica stishovite occur in sample PTFE.2 but are not present in samples A1.2 and St.2.

The cumulative observations o f the shock features present in these three samples indicate that sample PTFE.2 is the most strongly shock metamorphosed, A1.2 shows intermediate amounts o f shock metamorphism and sample St.2 shows the least amount o f shock metamorphism. The extent o f shock metamorphism shown by the samples appears to be in direct relation to their net internal energy increase, with the sample exhibiting the greatest net internal energy (-1625 J/g) PTFE.2 being the most shock metamorphosed, sample A1.2 having experienced an intermediate net internal energy increase (-1040 J/g) exhibits an intermediate degree o f shock metamorphism, and sample St.2 with the lowest net internal energy increase (-650 J/g), exhibiting the lowest degree o f shock metamorphism.

Though the shock conditions needed for the formation o f ‘ladder structure’ PDFs has not been characterised, their occurrence in samples PTFE.2 and A1.2 and absence from sample St.2 suggests increasing the net internal energy increase experienced by the sample will lower their pressure o f formation. Despite its relatively high peak pressure sample St.l (-1 9 GPa) did not contain ‘ladders structure’ PDFs suggesting its relatively low net internal energy increase may have been the limiting factor in their formation.