1. Ámbito problémico
3.4 Sobre el concepto Physis
The grains in sample PTFE.3 were observed to contain between one to three sets o f PDFs with crystallographic orientations including those parallel to the basal plane and the
rhombohedral planes {1011} and {10 1 4 } (table 5.14). The planar microstructures were
parallel to the {0001} plane and were observed to be filled with amorphous silica (figures
5.50a and 5.57) and therefore interpreted as being PDFs rather than Brazil twins. Those that
occurred as single sets were observed to be discontinuous and terminate arbitrarily (figures 5.54
and 5.53) to leave areas o f the grain free from shock features. Relatively wide dominant
amorphous planar features parallel to {OOOl}, interpreted as PDFs (figure 5.52), were present
in two o f the grains (%een in figures 5.50 a, h and 5.51, 5.52, 5.3). In both examples, fine PDFs
-0.01 p,m in width, parallel to the {1011} crystallographic plane (figures 5.50), or its
enantiomorph {oiîl} (figure 5.51) were observed to terminate against the PDF parallel to
{OOOl}. In the grain in figure 5.52 a third orientation o f PDFs, parallel to { 1 0 1 4 } , were
present and were observed to terminate against the PDFs parallel to both { OOOl} and {1011},
indicating the order o f formation was { OOOl}, {1011} then { 1 0 1 4 } {see fig u re 5.52).
Table 5.14, Summary of the orientations of the PDFs observed in sample PTFE.3.
figure Number of sets of Crystallographic orientations
PDFs Set 1 Set 2 Set 3
5.50 a & b 2 {OOOl} {1011}
5.51, 5.52 & 5.53 3 {0001} {lO T l} { 1 0 l4 }
5.54 1 {0001}
5.55 & 5.56 2 ? ?
5.57 a & b 1 {OOOl}
Generally, the grains in sample PTFE.3 did not appear to have a continuous population o f PDFs across their extent, but instead contained regions that are free from shock features, and
show little or no strain contrast. The one exception is the grain in figures 5.55 and 5.56 which
contains two sets o f PDFs and shows a strong strain contrast. The strength o f the strain o f the crystal lattice is also illustrated by the strong degree o f asterism shown by the electron
diffraction pattern. A region o f amorphous silica is present in this grain (figure 5.55). Vesicular
material possibly lechatelierite, is present in the grain in figures 5.50a & b.
The combination o f areas o f grains being free from shock features, the confined areas o f shock metamorphic features represented by the relatively thick PDFs parallel { OOOl} with the fine PDFs terminating against them, and the confined areas o f grains showing strong strain contrast is an indication o f the heterogeneity o f the heat distribution in this sample.
Figure 5.50a, Bright field TEM image looking down the ^lOO^zone axis. This grain is largely free from PDFs. A single wide amorphous planar microstructure parallel to {OOOl} and up to -0.16 pm wide cuts across the grain. For about a 0.05 pm stretch along its length fine planar microstructures parallel to jlOll j intersect and terminate against it
(figure 5.50b). Appearing to cross at ninety degrees across the feature parallel to {OOOl} are three features tilted relative to the zone axis. They are irregular, rather than planar and have 'bubbly' or vesiculated textures; one of these features can be seen in more detail at higher magnification in figure 5.50b.
micron
Figure 5.50b, Bright field TEM image looking down the (lOO) zone axis of the area in image in figure 5.50a
where PDFs parallel to jlOll j terminate against the planar microstructure parallel to {OOOl}. The PDFs parallel to jlOllj are -0.04 pm in width. The electron diffraction pattern of the quartz either side of the planar feature parallel to {OOOl} is the same, so this feature is not at a grain boundary. Although there seem to be no other planar microstructures parallel to the same orientation, it seems that this feature may be a PDF. It may also be possible that this feature could be a very thin melt vein.
The feature with the vesiculated texture is tilted with respect to the zone axis, thus it is not possible to glean much information about the structure or identify the vesicular material. However, in porous quartz that has had the porosity crushed out it is unlikely that the vesicles predate the dynamic loading, i.e. they are themselves a product of shock. If this is the case, a possible explanation is that the vesicles are present in lechatelierite that has formed as a vein or along a fracture.
Figure 5.51, Bright field TEM image looking down zone axis (lOO). Figures 5.52 and 5.53 are slightly lower magnification images of the areas slightly below and above, respectively, the area imaged in this figure. A planar feature parallel to {OOOl} dominates this image, it varies in width along its length from -0.04 pm to -0.22 pm (figure 5.53). The PDFs that terminate against this feature are parallel to the crystallographic orientations |o i ï l | and |o i l 4 |, and are all -0.01 pm in width. Those parallel to |o il4 j are shorter than and terminate against those parallel to jo 111 I indicating that the order of formation of the planar microstructures started with the feature parallel to {OOOl}, was followed by those PDFs orientated parallel to
| o i l l | ,
then finally the PDFs parallel to| o i l 4 | .
The untransformed quartz situated between the PDFs parallel to the rhombohedral planes exhibits a great degree of strain contrast.5 0 m i c r o n s
1 micron
Figure 5.52, Bright field TEM image looking down the (lOO) zone axis. At the top left of the image the feature parallel to {OOOl} ‘forks’ to produce the area the PDFs parallel to {oillj and {0114} (figure 5.51). Thin PDFs, -0.01 pm wide and parallel to jo illj, branch off or terminate against both edges of the feature parallel to {OOOl} along its length. In the widest portion of the feature parallel to {OOOl} are some round fragments of crystalline silica (centre of image). They have the appearance of quartz that did not transform during the shock process. It is possible that these relics of
untransformed quartz could be remnants of the crystalline quartz between PDFs, like those parallel to jo illj and |o 114 j in figure 5.51, that did not fully coalesced to produce the {OOOl} amorphous feature. These observations suggest that this feature is a PDF rather than a melt vein.
Figure 5.53, Bright field TEM image looking down the (lOO) zone axis of the area above that in figure 5.51.
Discontinuous planar microstructures parallel to the {OOOl} crystallographic orientation, which are irregularly spaced, appearing to occur in clusters. Their lengths range from ~l pm upward. The structure of these planar microstructures was not determined.
200nm
Figure 5.54, Bright field TEM image down the (lOO) zone axis. This grain contains a single set of PDFs parallel to the {OOOl} crystallographic plane. These PDFs are not continuous across the whole grain but terminate abruptly and arbitrarily. The 'speckles' in the untransformed quartz (bottom left) are the result of beam damage. A fracture cuts the grain; it has a curved, wedged shape and is therefore not edge on to the plane of the image.
Figure 5.55, Bright field TEM image, the zone axis down which the image was taken cannot be determined due to the overexposure and the asterism of the electron diffraction pattern. This grain has clearly experienced a heterogeneous heat distribution, as the area of the grain in the centre of this image appears to have undergone more extreme shock metamorphism than the areas towards the top and bottom of the image, which appear to be free from any shock features. In the centre of the image, by contrast, are two sets of planar microstructures (figure 5.56). The untransformed quartz between these features shows extreme strain contrast as reflected by the severe asterism in the electron diffraction pattern. Two irregularly shaped patches of amorphous silica (left of centre and above right of centre) exist within this region, which exhibits more extreme strain. Apart from the inclusion of a small ‘block’ of what appears to be untransformed quartz in the patch of amorphous silica (positioned left of centre of the image) the amorphous silica is featureless.
i JH I Figure 5.56, The zone axis that
this bright field TEM image is
I looking down is undefined due to the over exposure of the electron diffraction pattern and its severe asterism. This image is of the same grain seen in figure 5.55, of the area slightly to the right of that seen in figure 5.5. The strain in the crystal lattice can also be seen in the crystalline silica by the strong strain contrast it exhibits. Two sets of planar microstructures are present with unknown orientations. One set orientated nearly North- South in this image have the appearance of regularly spaced PDFs. The second set of planar microstructures are represented by two parallel, sub-planar features approximately 0.4 pm apart and may also be PDFs.
i
■
Figure 5.57a, Bright field TEM image (-1.5 fim in width) taken looking down the jlOloj crystallographic plane. A single set of amorphous PDFs parallel to {0001} is cut by a later-formed fracture (see figure 5.57b).
Figure 5.57b, Higher magnification bright field TEM image of PDFs to right of the fracture in figure 5.57a.
This image looking down the (100) zone axis if the crystal and is therefore tilted relative to the image in
figure 5.57a. At this magnification it can be seen that the planar microstructures parallel to {OOOl} are filled with amorphous silica, thus they are PDFs rather than Brazil twins despite their crystallographic orientation.
5.3.7 TE M observations of Poly.8
Between one to four sets o f PDFs were observed to occur sim ultaneously within the grains in sample Poly.8 (table 5.15). All o f the PDFs observed in sample Poly.8 were filled with am orphous silica. The single set o f PDFs seen in the grain in fig u r e 5.58 were orientated parallel to the basal plane, { OOOl} and are -0.01 pm in width. W here m ultiple sets o f PDFs occurred in a single grain (figures 5.59, 5.60a and b and 5.61) they had crystallographic orientations parallel to the rhombohedral planes { 0 1 1 2 } , { 1 0 1 1 } and its enantiom orph { 0 1 1 1 } and the basal plane {OOOl}.
Table 5.15, Summary of the orientations of the PDFs observed in sample Poly.8.
figure Number of sets of
PDFs
Crystallographic orientations
Set 1 Set 2 Set 3 Set 4
j . j # 1 { 0 0 0 1 }
j.J P 2 9 ?
5.60 a & b 2 { 0 0 0 1 } { l o T i }
1 6 / 4 { 0 0 0 1 } { O i T i } { l O T l } { 0 1 T 2 }
The PDFs in the grains containing more than one set, are discontinuous across the extent o f the grain and either appear to term inate arbitrarily (figure 5.59) or term inate against PDFs in a set o f a different orientation (figures 5.60 and 5.61). Relatively wide PDFs, -0 .2 pm width, parallel to the basal plane, { OOOl } act as a domain boundary for another set o f much finer PDFs, -0.01 pm width, parallel to { 1 0 1 1 } in fig u re 5.60b. In the grain containing four sets o f PDFs, PDFs parallel to certain orientations acted as dom ain boundaries for sets o f PDFs with other specific crystallographic orientations, indicating the order o f formation was; {101 1} followed by {01 11} and {01 1 2 } then lastly { OOOl } (figure 5.61).
Examples o f two types o f am orphous silica outside the boundaries o f the PDFs were observed in this sample. In fig u re 5.58 an example o f a fracture containing vesicular amorphous material is seen, this material has been interpreted as being lechatelierite. In fig u re 5.61, a patch o f featureless am orphous silica is present in association with PDFs and untransformed quartz that exhibits strain contrast.
The degree o f strain displayed by the untransform ed crystalline quartz in this sample varies from showing no shock strain and retaining pre-shock features such as dislocations
(figures 5.58 and 5.59), to the electron diffraction pattern exhibiting a m inor am ount o f asterism
(figure 5.61) and to confined areas that exhibit severe strain contrast (figure 5.60b).
Figure 5.58, Bright field TEM image (-2.2 pm in width) looking down the
(lOO) zone axis of the crystal. A single set of planar microstructures is parallel to {OOOl}, at higher magnifications the amorphous silica filling the features is resolvable indicating they are PDFs rather than Brazil twins. These PDFs are cut by an irregular fracture containing vesicular amorphous silica similar to that seen in sample PTFE.3 (figures 5.50 a and b). Though the quartz foil thickens
rapidly, the PDFs can be seen to be continuous across the fractures indicating that the fracturing occurred after PDF formation. The strain of the crystal lattice is shown by asterism of the diffraction pattern.
before again tracing the
Figure 5.59, Bright field TEM image of two orientations of PDFs. The diffraction pattern for this image was lost in the processing therefore the
crystallographic orientations of these PDFs is unknown. A fracture runs along a PDF boundary before cutting discordantly across it and three other PDFs parallel to a different orientation, boundary of a PDF. The fracture causes a slight offset of the PDFs it cuts.
Figure 5.60a, Bright field TEM image taken down the (l OO) zone axis. The quartz foil thickens rapidly leaving only a limited area that is electron transparent. In these areas, two orientations of PDFs
(figure 5.60b) are {OOOl} and evident;
200nm
Figure 5.60b, Higher
magnification bright field TEM image of PDFs orientated parallel to jlOllj and {0001} in the electron transparent area to the left in
figure 5.60a. The PDFs parallel to {OOOl} are wider than the set of PDFs parallel to jlOllj, with a maximum width o f -0.18 pm. The PDFs parallel to {OOOl} must have formed first as they act as domain boundaries for those PDFs parallel to jlOllj. The PDFs parallel to jlOllj are thin with widths o f -0.01 pm and their spatial varies either side of the PDFs parallel to {OOOl}. Towards the centre of the image, two PDFs parallel to {OOOl} enclose a 0.2 pm wide region of relatively closely spaced PDFs parallel to jlOllj. The untransformed crystalline quartz between the PDFs exhibits strong strain contrast. Coexisting with the strained quartz and PDFs in this area is a patch of amorphous silica (upper right of image). This amorphous silica is featureless apart from the inclusion of two fragments of untransformed quartz. This area has a very similar appearance to the region observed in (figure 5.51) of sample PTFE.3. On the left- hand side of the electron transparent region, three PDFs with jlOllj orientations have been cut and offset. The feature responsible is not edge on to this zone axis, but it is presumably a fracture with a slight shear element.
200nm
Figure 5.61, Bright field TEM image taken down the
(100) zone axis. Four orientations of PDF co-exist in this grain. By examining which orientations of PDFs act as domain boundaries for PDFs of other orientations, the order of their formation was determined. The PDFs parallel to jlOllj formed first (widths up to -0.04 pm), second to form were the orientations jo il2 j and
!011lj in unknown order as
there is not an example of their intersection. Examples o f PDFs parallel to {OOOl} terminate against all three other orientations indicating that this particular orientation were the last to form. They are also the thinnest o f the sets of PDFs with widths o f -0.01 pm. In this sample there is a correlation between the order o f formation of the PDFs and their thickness, with those having formed first being wider. This suggests that the final widths of PDFs may depend on shock duration. All orientations of PDF are discontinuous and do not cross the grain uninterrupted. The asterism o f the diffraction pattern indicates that the crystal lattice is strained.