Effects o f loading path on the formation of shock features; a comparison with previous experimental studies

The pressure and net internal energy increase (P- E conditions) formed in quartz or a

quartz rock under experimental shock conditions are dependent on two intrinsic variables; the shock impedance o f the quartz (a function o f its density) and the shock loading path. In this

section a variety o f P- E conditions for porous quartz (1.43 cc/g) with loading paths that would

be produced by using the sample container materials used in this study are compared to

estimates o f the P- E conditions that could occur in single crystal quartz (2.65 cc/g) shock

loaded in the commonly used stainless steel containers via reflected loading paths and with single crystal quartz shock loaded in a matched impedance container via a direct loading path.

The shock conditions at which shock features were observed to occur in this study will then be compared to their published occurrence in single crystal quartz experim entally loaded via reflected and direct loading paths.

Simplified calculations have been made o f net internal energy increases for a theoretical peak pressure range o f 8 GPa to 20 GPa for porous quartz ( 1.43 cc/g) with a variety o f reflected and direct loading paths and for single crystal quartz with direct loading paths, and for a pressure range o f 8 GPa to 30 GPa for single crystal quartz loaded via a reflected loading path. The results were plotted on a pressure vs. net internal energy increase {P- E) graph {figure 6.5)

(where the lines represent the locus o f approxim ate net internal energy increases against pressure, and not a pathway) to dem onstrate the differences in the range o f net internal energy increases that occur in quartz by varying the shock impedance and the loading path.

The calculations o f net internal energy were sim plified by assum ing the release adiabats for all o f the samples approxim ate the Hugoniot curve o f non-porous quartz. This approxim ation does not take into account any phase changes that the quartz may undergo. Phase transform ations that occur with increasing extent o f shock metam orphism , w hether recoverable or not, would result in the sample experiencing a greater net internal energy increase because compression o f the material to a higher density phase would ‘shift’ the path o f the release adiabat to the left i n a f - F plot (see fig u re 2.12) producing a greater area between the loading path and the release adiabat, i.e. a greater net internal energy increase. Therefore the approximate net internal energy increases at the higher pressures represent a minimum and in reality would be greater than presented in this simplified example.

Figure 6.5, Plot of loci of approximate net internal energy increases a sample would experience over a peak pressure range of 8 GPa - 20 GPa for porous and non-porous quartz loaded with a variety of direct and reflected loading paths and for non-porous quartz shocked in a pressure range from 8 GPa to 30 GPa via a reflected loading path.

represents approximate shock conditions for porous quartz (1.43 cc/g) shock loaded in a polyethylene container via a direct loading path. --- represents shock conditions in porous quartz (1.43 cc/g) shock loaded in a Teflon container via a reflected loading path. --- represents shock conditions in porous quartz (1.43 cc/g) shock loaded in an aluminium container via a reflected loading path represents shock conditions in porous quartz (1.43 cc/g) shock loaded in a stainless steel container via a reflected loading path represents shock conditions in single crystal quartz shock loaded in a matched impedance container via a direct loading p ath . represent shock conditions produced in single crystal quartz shock loaded within a stainless steel container via a

3 4 0 0 3 2 0 0 3 0 0 0 2 8 0 0 2 6 0 0 2 4 0 0

i

reflected loading path. The calculations that produced these values were simplified (see text), thus net internal energy increase values should not be treated as absolute but rather as an indication of the approximate range.

It is not possible to directly compare the shock effect vs. pressure calibrations from previous experiments because calculations o f the net internal energy increases have not been published. To give an indication o f the effect o f net internal energy increase on the pressures at which shock features occur, published observations o f the occurrence o f single and multiple sets o f PDFs, amorphous silica and stishovite and PDFs occurring parallel to the (l 013} and {1OÎ2} crystallographic planes calibrated against pressure have been superimposed on the plot o f

pressure vs. approximate net internal energy increase (figure 6.5) in fig u res 6.6 to 6.11. These

figures therefore compare the approximate, not the actual, shock conditions.

It is clear from the plots in figures 6.6, 6.7 and 6.8 that the formation pressures o f single

sets o f PDFs, multiple set o f PDFs and the occurrence o f PDFs parallel to the {1OÏ2} crystallographic planes, are lowered by increasing the net internal energy increase o f the sample. This may be determined as the first occurrence o f these features have been recorded in more than one study for more that one set o f experimental conditions. A comparison o f the results o f previous studies support the results o f this study as shock features were observed to form at lower pressures in the single crystal quartz shocked via a direct loading path, than in a single crystal shocked via a reflected loading path. The first occurrence o f single sets o f PDFs have been recorded in previous studies for shock experiments on single crystal quartz that have been loaded via both direct and reflected loading paths (Horz, 1968 and Reimold, 1988). The first appearance o f single sets o f PDFs first occurred in single crystal quartz shock loaded via the reflected loading path at ~17 GPa (Reimold, 1988), while in the single crystal quartz shock loaded via a direct loading path their first appearance was recorded at ~10 GPa (Horz, 1968). Single sets o f PDFs were observed to occur at lower pressures in the porous quartz with a direct loading path than in the single crystal quartz with the direct loading path; ~8 GPa compared to -1 0 GPa.

The pressure at which multiple sets o f PDFs form is also lowered by increasing the net

internal energy increase (figure 6.7). This is demonstrated by the recorded first appearance at

-2 5 GPa o f multiple sets o f PDFs in single crystal quartz shocked via a reflected loading path (Reimold, 1988). While the first formation o f multiple sets o f PDFs is recorded at a lower pressure o f -1 2 GPa in the single crystal quartz loaded via a direct loading path (Horz, 1968). In the porous quartz sample loaded via a direct loading path they were observed to occur at an even lower pressure o f - 8 GPa.

The first occurrence o f PDFs parallel to the |l0 1 2 | plane was recorded at -2 5 GPa in single crystal quartz shocked via a reflected loading path (Langenhorst and Deutsch, 1994)