STUDY OF EXPERIMENTALLY DYNAMICALLY COMPRESSED DOLOMITE USING TEM AND X-RAY POWDER DIFFRACTION

R. Skála¹, N. Miyajima², F. Langenhorst³ and F. Hörz⁴

¹Geologický ústav AVČR, v.v.i., Rozvojová 269, 16500 Praha

²Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

³Institut für Geowissenschaften, Bereich Mineralogie, Friedrich-Schiller-Universität Jena, Fürstengraben 1, D-07743 Jena, Germany

⁴SN2, NASA Johnson Space Center, Houston, TX 77058, USA

The response of carbonate-dominated sediments to transient strong dynamic compression and subsequent rapid unloading is essential for understanding atmospheric CO₂ pollution and environmental consequences of large-scale asteroid impacts on the Earth. The experimental data on the deformation behavior of dolomite under strong dynamic (i.e., shock) compression are scarce and the threshold shock pressures and temperatures for partial to complete decomposition are completely missing.

Consequently, we have carried out shock-recovery experiments at the Johnson Space Center, NASA, Houston, USA, using a 20-mm-caliber powder propellant gun. The starting material was a dense (~0.04% porosity) dolomite rock composed of equi-granular grains, typically 25 μm across. Pressures attained by multiple shock reverberation technique covered the range from 4 to 61 GPa (Table 1). To characterize the shock defects over the entire range of conditions we prepared five shock-loaded samples (20, 25, 29, 42 and 61 GPa) and the undeformed starting material for TEM observation.

Deformation features recorded in shock-loaded dolomite samples include perfect and partial dislocations, stacking faults, and microtwins. Dislocations are omnipresent in all samples; they are already present in the unshocked starting material but their density significantly increases with shock pressure. Highest dislocations densities on the order of 10¹⁴ m^–2 are observed in samples shocked to medium to high shock pressures (30 – 61 GPa). At lower pressures (< 30 GPa), c-type dislocations dominate; at higher pressures (> 30 GPa), f- and r-type dislocations become more important though c-type dislocations are still present (Fig. 1). Distinct narrow twin lamellae on f-planes occur exclusively in the sample shocked to 42 GPa. Partial dislocations were observed in the twin walls, indicating their mechanical nature. Stacking faults occur in all samples, however, they are more frequent in materials shocked to higher pressures. The sample shocked to 61 GPa shows weak diffuse streaks or superstructure reflections in selected area electron diffraction patterns, which might be the result of cation disordering at high post-shock temperatures (Fig. 2).

In powder diffraction patterns, the most notable feature is a systematic broadening of the peaks with increasing peak shock pressure due to an increase of micro-strain and decrease in size of coherently diffracting domains. The unit-cell dimensions systematically increased by ~0.5% at 30 GPa; at still higher pressures, cell size remains invariant. The diffraction data, in agreement with those obtained by TEM, do not indicate decomposition of dolomite under even the highest dynamic loads.

In summary, the defect microstructures document strong deformation of dolomite under dynamic compression with high strain-rates but features indicating significant outgassing or melting were not observed in dolomite shock-loaded to the pressures as high as 61 GPa. These results indicate that decomposition may only be possible if the porosity of starting materials were high or if shock pressures were much higher.

Table 1. Summary of shock recovery experiments indicating the pressures attained on the cover plate/flyer plate (CP/FP) interface and in the sample (in GPa).

Shot #	vi (km/s)	Material		Pressure (GPa)
		Flyer	Cover	CP/FP	sample
2064	1.131	Lexan	Al2024	4.2	4.2
2065	1.694	Lexan	SS304	7.4	7.4
2066	1.345	Al2024	SS304	17.0	17.0
2067	1.371	Al2024	SS304	17.4	17.4
2068	1.34	SS304	SS304	28.8	28.8
2069	1.478	Al2024	SS304	19.0	19.0
2070	1.54	Al2024	SS304	20.0	20.0
2071	1.121	SS304	SS304	23.5	23.4
2072	1.192	SS304	SS304	25.2	25.1
2073	1.345	Al2024	SS304	18.8	18.8
2094	1.26	W	FS77	55.0	43.1
2096	1.413	SS304	FS77	42.7	42.3
2097	1.461	W	FS77	65.4	51.6
2099	1.533	W	FS77	69.0	61.2

Fig. 1. Weak-beam dark-field image of dislocations of f-, c- and r-slip systems.

Fig. 2. High resolution TEM image of the sample shocked to 61 GPa, indicating some stacking faults along the (10-14) plane. The inset is the selected area electron diffraction pattern illustrating weak diffuse streaks.