The first MEMIN shock recovery experiments at low shock pressure (5–12.5 GPa)

with dry, porous sandstone

Astrid KOWITZ1*, Ralf T. SCHMITT1, W. UWE REIMOLD1,2, and Ulrich HORNEMANN3

1Museum fur Naturkunde, Leibniz-Institut fur Evolutions- und Biodiversitatsforschung, Invalidenstraße 43,10115 Berlin, Germany

2Humboldt Universitat zu Berlin, Unter den Linden 6, 10099 Berlin, Germany3Ernst-Mach-Institut, Am Klingelberg 1, 79588 Efringen-Kirchen, Germany

*Corresponding author. E-mail: astrid.kowitz@mfn-berlin.de

(Received 14 March 2012; revision accepted 23 October 2012)

Abstract–As part of the MEMIN research program this project is focused on shockdeformation experimentally generated in dry, porous Seeberger sandstone in the low shockpressure range from 5 to 12.5 GPa. Special attention is paid to the influence of porosity onprogressive shock metamorphism. Shock recovery experiments were carried out with a high-explosive set-up that generates a planar shock wave, and using the shock impedance method.Cylinders of sandstone of average grain size of 0.17 mm and porosity of about 19 vol%, andcontaining some 96 wt% SiO2, were shock deformed. Shock effects induced with increasingshock pressure include: (1) Already at 5 GPa the entire pore space is closed; quartz grainsshow undulatory extinction. On average, 134 fractures per mm are observed. Dark vesicularmelt (glass) of the composition of the montmorillonitic phyllosilicate component of thissandstone occurs at an average amount of 1.6 vol%. (2) At 7.5 GPa, quartz grains showweak but prominent mosaicism and the number of fractures increases to 171 per millimeter.Two additional kinds of melt, both based on phyllosilicate precursor, could be observed: alight colored, vesicular melt and a melt containing large iron particles. The total amount ofmelt (all types) increased in this experiment to 2.4 vol%. Raman spectroscopy confirmed thepresence of shock-deformed quartz grains near the surface. (3) At 10 and 12.5 GPa, quartzgrains also show weak but prominent mosaicism, the number of fractures per mm hasreached a plateau value of approximately 200, and the total amount of the different melttypes has increased to 4.8 vol%. Diaplectic quartz glass could be observed locally near theimpacted surface. In addition, local shock effects, most likely caused by multiple shock wavereflections at sandstone-container interfaces, occur throughout the sample cylinders andinclude locally enhanced formation of PDF, as well as shear zones associated with cataclasticmicrobreccia, diaplectic quartz glass, and SiO2 melt. Overall findings from these firstexperiments have demonstrated that characteristic shock effects diagnostic for theconfirmation of impact structures and suitable for shock pressure calibration are rare. So far,they are restricted to the limited formation of PDF and diaplectic quartz glass at shockpressures of 10 GPa and above.

INTRODUCTION

A significant number of known terrestrial impactstructures occur in quartz-bearing sedimentary targetsthat are characterized by significant porosity and, quitelikely, the presence of pore water. So far, only onesystematic study of shock metamorphism was carried out

of a porous, water-bearing sandstone target at MeteorCrater. This work led to a shock classification scheme for thistype of rock (Kieffer 1971; Kieffer et al. 1976) (Table 1). Thiscalibration scheme is quite different from that for nonporousquartz-bearing rocks. Shock classification of porous, water-bearing sandstones is also, like that for nonporous quartz-bearing rocks, based on five progressive shock stages, but

� The Meteoritical Society, 2012.1

Meteoritics & Planetary Science 1–16 (2013)

doi: 10.1111/maps.12030

uses the complete removal of porosity in shock stage 1;varied amounts of quartz, coesite, and glass in shockstages 2–4; and the occurrence of a vesicular melt withlechatelierite as diagnostic criterion for shock stage 5(Kieffer 1971; Kieffer et al. 1976; and modified bystoffler 1984; Grieve et al. 1996; Stoffler and Grieve2007). Another systematic study with CaCO3-bearingsandstones was carried out at the Haughton impactstructure, Canada, which uses the classification schemeof Kieffer (1971) but added a shock stage 6 for theoccurrence of recrystallized SiO2 glass (Osinski 2007).

These earlier shock pressure classification schemesare not based on shock recovery experiments; instead,pressures were estimated by interpretation of changes inthe Hugoniot curve of Coconino sandstone and itsdeviation from the Hugoniot curve of quartz (Kieffer1971; Kieffer et al. 1976). Previous shock recoveryexperiments on porous and wet quartz-bearing rocks are rareand were not part of a continuous series with increasingshock pressure, and did not provide comprehensivetextural ⁄petrographic analysis. For example, Hiltl et al.(1999, 2000) only investigated the grain size variations inshocked porous and wet sandstone as a function of shockpressure.

As part of the MEMIN (MultidisciplinaryExperimental and Modelling Impact Research Network)research program (Kenkmann et al. 2011, 2013; Poelchauet al. 2013) this study is focused on shock recoveryexperiments to investigate the progressive shockmetamorphism of porous sandstone in the low pressurerange from 5 to 12.5 GPa. This pressure range is importantfor the recognition of impact structures in quartz-bearingporous targets because most of the shocked material withinthese craters belongs to this low shock pressure regime. Inparticular in eroded impact structures, where the crater fillhas been largely or even completely removed, only lowshock-deformed material remains for petrographic

investigations. Another goal of this study is to worktoward an improved calibration and shock classificationsystem for porous, quartz-bearing sandstones, in this lowpressure regime.

METHODOLOGY AND SAMPLE MATERIAL

Analytical Methods

All analytical work was carried out at the Museumfur Naturkunde Berlin. The shock-deformed sandstonespecimens were studied with a Zeiss Axioskop polarizingmicroscope, using high-quality polished thin sectionswith a thickness of 30 lm. The orientation of planardeformation features (PDF) was measured with a fouraxial Universal Stage (Leitz) and indexed by a methoddescribed in Stoffler and Langenhorst (1994).

Scanning electron microscopy (SEM) was performedwith a JEOL JSM-6610LV instrument equipped with aLaB6-cathode and a BRUKER Quantax 800 energy-dispersive X-ray spectrometry (EDX) system. The EDXsystem was used for identification and qualitativeanalysis of minerals. Quantitative mineral analysis andsome imaging were carried out with a JEOL JXA-8500Felectron microprobe equipped with five wavelength-dispersive spectrometers, an EDX system, and a field-emission cathode for enhanced focusing of theelectron beam. Microchemical analysis was based on arange of Smithsonian and Astimex standards.Detection limits (all given in ppm) were determined forSi at approximately 300, Fe approximately 1400, Mgapproximately 300, Al approximately 400, Na approximately300, Ti approximately 700, S approximately 500, Kapproximately 300, and Ca at approximately 300.

Porosity was determined on SEM and microprobebackscattered electron (BSE) images using the imageanalysis software JMicroVision. Additional porosity

Table 1. Shock classification of porous, water-bearing sandstones (based on Kieffer 1971; Kieffer et al. 1976;modified by Stoffler 1984; Stoffler and Grieve 2007).Shockstage

Shock pressurerange* (GPa) Lithology

0 £0.2–0.9 Undeformed sandstone1a 0.2–0.9–�3 Compacted, deformed sandstone with remnant porosity1b �3–�5.5 Compacted, deformed sandstone compressed to zero porosity

2 �5.5–�13 Dense (nonporous) sandstone with 2–5 wt% coesite, 3–10 wt% glass, and 80–95 wt% quartz

3 �13–�30 Dense (nonporous) sandstone with 18–32 wt% coesite, traces of stishovite, up

to 20 wt% glass, and 45–80 wt% quartz4 >�30 Dense (nonporous) sandstone with 10–30 wt% coesite, 20–75 wt% glass, and

15–45 wt% quartz

5 Vesicular (pumiceous) rock with 0–5 wt% coesite, 80–100 wt% glass(lechatelierite), and up to 15 wt% quartz

*Average pressure; locally, peak pressures can be much higher.

2 A. Kowitz et al.

measurements on bulk sandstone specimens ofapproximately 1.5 cm3 size by the Archimedes methodusing purified water as liquid were carried out with aMETTLER-TOLEDO AT 261 DeltaRange balanceequipped with a density kit.

Micro-Raman spectroscopy was performed with anedge-filter-based DILOR LabRam instrument operatingwith an integrated HeNe-Laser (632.8 nm) and employedto identify diaplectic quartz glass and other SiO2 phases(Ferriere et al. 2009; McMillan et al. 1992). Raman spectrawere collected from 100 to 1150 cm)1 wavelength; anenergy of 3 mW on a 2.5 lm spot size was used. Finalspectra represent accumulations of 10 single spectrarecorded with a collection time of 10–20 s each. Anoptical lens of 100x magnification and the LapSpec 5.0software were utilized.

Whole-rock geochemical analysis of the sandstonewas carried out with X-ray fluorescence spectrometry(XRF) on a BRUKER AXS S8 Tiger instrument. Majorelements were measured on glass tablets using an analyticalprogram based on 50 international rock standards. For losson ignition (LOI) determination about 1 g of pulverizedsample material dried for 4 h at 105 �C was used. Thesample material was heated for 4 h at 1000 �C, and the

LOI was calculated using the weight difference beforeand after heating.

Target Material—Seeberger Sandstein

The sample material for the shock recoveryexperiments is a mostly beige but streaky brownish, well-sorted sandstone that is quarried at Seeberg near Gothain Thuringia, Germany, and accordingly known asSeeberger Sandstein (Seidel 2003; and references therein;Stuck et al. 2011). The sandstone unit contains differentbanks, of which bank 5 was used for the currentexperiments. Our samples from bank 5 match those used inthe MEMIN feasibility study (Kenkmann et al. 2011), andare similar to the sandstone cubes quarried in bank 3,which have been the target material for all furtherMEMIN experiments (Kenkmann et al. 2013; Poelchauet al. 2013). Macroscopically, bank 5 consists of fine-grained sandstone displaying a lamination at a spacingbetween 2 and 15 mm, which is mostly defined byweathered, brown-stained (obviously quite iron-rich)layers and laminations (Fig. 1a). Cylinders with adiameter of 15 mm were drilled out of a sandstone block,with cylinder long axes lying in the lamination plane.

Fig. 1. Unshocked Seeberger sandstone, bank 5. a) Macroscopic view of the well-sorted, finely laminated sandstone. b)Microphotograph in plane polarized light. c) BSE image displaying quartz (medium gray) with a selvedge filled with phyllosilicate(dark gray rims), pores (black), and the accessory minerals ilmenite (altered, upper left corner), titanite (upper right corner), andzircon (lower left corner). d) Backscattered electron (BSE) image of a single pore filled with phyllosilicate and subroundedgoethite ⁄ limonite particles (light phases).

Shock effects in dry, porous Seeberger sandstone 3

Microscopically, bank 5 sandstone is an areniticquartz sandstone with a mean grain size of0.17 ± 0.01 mm (Fig. 1b; Kenkmann et al. 2011). Mostquartz grains are subrounded. Besides quartz, accessoryminerals include ilmenite (altered), phyllosilicates,goethite ⁄ limonite, biotite, titanite, rutile, zircon, feldspar,and pyrite (Fig. 1c). The sandstone has an averageporosity of 12.2 and 19.2 vol%, as estimated onbackscattered electron (BSE) images and by measurementsusing the Archimedes method, respectively. This differencein porosity results from the two analytical approaches:image analysis was done on high-magnification images thatonly represent small selected areas of heterogeneoussandstone with a pore space of highly variable pore sizes(approximately 10–100 lm). In addition, there areexperimental uncertainties on the results of the Archimedesmethod (e.g., degree of water and the relatively small bulksamples of approximately 3.5 cm3 may also not berepresentative for the heterogeneous sandstone).Nevertheless, the measured porosities are comparable todata by Stuck et al. (2011) (15.8 vol%). They are in therange given for Coconino sandstone (10–20 vol%; Kiefferet al. 1976), which is important for comparison of the resultsof shock deformation studies. Phyllosilicates occur asaggregates of chaotically arranged flakes in selvedges on oraround quartz grains and as partial or complete fillings ofpores, often in association with goethite ⁄ limonite (Fig. 1d).Phyllosilicate content as determined by image analysis is,on average, 12.6 vol%. Electron microprobe analyses ofaggregates of phyllosilicate flakes indicate that the mainmineral group involved is illite ⁄montmorillonite.

The chemical composition of Seeberger sandstone,bank 5 is presented in Table 2. In keeping with the mostimportant mineral ( ⁄ -groups), quartz and phyllosilicate,the silica and alumina contents, with some minorpotassium, dominate the composition.

Experimental Set-up

These shock recovery experiments were carried outat the Efringen-Kirchen facility of the FraunhoferInstitut fur Kurzzeitdynamik (Ernst-Mach-Institut), withdry Seeberger sandstone under ambient conditions onsample cylinders of 15 mm diameter and 20 mm length.The experimental set-up consists of an explosively drivenflyer plate that impacts a cylindrical ARMCO ironcontainer, in which the sandstone cylinder is shielded(Fig. 2) (for a full description of experimental set-up see,e.g., Langenhorst and Hornemann 2005).

Variations of shock pressure in the sandstonecylinders are achieved through different combinations ofthe thicknesses of the flyer and driver plates, and by theuse of different explosives (see experimental parametersin Table 3). The precision of shock pressure

determination in the ARMCO iron driver plate is in theorder of ±4%. We used the impedance method for ourexperiments to achieve relatively low shock pressures.The shock pressure in the sample is determined bygraphic impedance matching (e.g., Langenhorst andHornemann 2005). In the absence of Hugoniot data forSeeberger sandstone above 5 GPa, we used the Hugoniot

Table 2. Chemical composition of the Seebergersandstone, bank 5.Major elementoxides wt%

Traceelements ppm

SiO2 96.30 V <15TiO2 0.13 Cr 13Al2O3 1.50 Co b.d.

Fe2O3* 0.31 Ni 7MnO b.d. Zn 27MgO 0.09 Rb b.d.

CaO 0.04 Sr 13Na2O b.d. Zr 164K2O 0.11 Ba 50

P2O5 b.d.LOI 0.80Total 99.28

XRF data; detection limits: 1.0 wt% for SiO2; 0.5 wt% for Al2O3;

0.05 wt% for Fe2O3; 0.01 wt% for TiO2, MnO, MgO, CaO, Na2O,

K2O, and P2O5; 15 ppm for Zn; 10 ppm for Zr and Ba; and 5 ppm

for V, Cr, Co, Ni, Rb and Sr. Standard errors: 0.5 wt% for SiO2;

0.1 wt% for Al2O3; 0.05 wt% for Fe2O3, MgO, CaO, Na2O, and

K2O; 0.01 wt% for TiO2, MnO, and P2O5; 30 ppm for Ba; 20 ppm

for Zn; and 5 ppm for V, Cr, Co, Ni, Rb, Sr, and Zr.

*Total Fe as Fe2O3, b.d. = below detection limit, LOI = loss on

ignition.

Fig. 2. Experimental set-up for the shock recovery experiments.a) Complete experimental assembly with high explosives, outersteel block (momentum trap), ARMCO iron flyer plate, spacingring of acrylic glass (diameter 64 mm, height 10 mm, wallthickness 4 mm), and inner ARMCO iron cylinder (diameter48 mm, height 50 mm) containing the sandstone sample. b)Detailed view of flyer and driver plates with thicknesses d and D,respectively, and the sandstone sample.

4 A. Kowitz et al.

data of Coconino sandstone (Ahrens and Gregson 1964;compiled by Stoffler [1982] with additional data fromShipman et al. [1971]) for shock pressure determination.This is thought to lead to an additional error estimatedat about 1–2 GPa in shock pressure with respect to theactual sample material, because the curves do not matchwell above 5 GPa.

This experimental set-up allows complete recovery ofshocked samples, although sandstone may have lostcoherence. ARMCO containers were carefully openedwith a lathe. For subsequent petrographic investigations,polished thin sections were prepared; they were cutperpendicular to the shock front, parallel to the longaxis, and through the middle of the shocked sandstonecylinder.

The pressure-pulse duration for the four experimentswas estimated with the help of time–distance plots forthe shock and rarefraction waves (Fig. 3) using themethod described in Stoffler and Langenhorst (1994).

For these plots, we have used shock (U) and particle (up)velocities calculated from pressure-particle velocitygraphs for ARMCO iron and Coconino sandstone. It isremarkable that the rarefraction wave catches the shockwave somewhere in the middle of the sandstone cylinder;at this depth the initial shock wave is attenuated. Thisapplies to all four shock experiments reported here.Nevertheless, the initial shock wave also propagatesthrough the surrounding ARMCO iron cylinder but witha much higher velocity, and it triggers nonplanar shockwaves at the interface between the ARMCO ironcylinder and the sandstone sample. The maximumpressure-pulse duration (Table 3) was estimated from thetime difference between the arrival time of the shock andthe rarefraction waves at the upper interface betweenARMCO iron and sandstone. The pressure-pulseduration decreases in the sandstone cylinder with depth,until that depth is reached where the rarefraction wavecatches up with the shock wave.

Table 3. Experimental parameters and calculated data for shock recovery experiments.Container ⁄ experiment No. 1–1 1–2 1–3 1–4

Sandstone cylinder Diameter 15 mm, length 20 mmHigh explosive TNT Comp.B Comp.B Comp.BThickness (d) of flyer plate (mm) 4 4 4 3

Thickness (D) of driver plate (mm) 6.5 15 10 8.5Shock pressure at the base of theARMCO iron driver plate (GPa)

18.4 25.0 33.5 42.5

Calculated pressure at the top of

the sandstone cylinder (GPa)*

5.0 7.5 10.0 12.5

Estimated maximum pressure-pulseduration at the top of the sandstone

cylinder (ls)

1.11 0.66 0.88 0.55

*Based on Hugoniot data for Coconino sandstone.

Fig. 3. Time–distance plot for the shock recovery experiment at 5 GPa based on the Hugoniot curve for Coconino sandstone. Theplot displays the distances of shock and rarefraction waves with time in the flyer (light gray) and driver plate (medium gray), and inthe sandstone cylinder (dark gray). Note that the rarefraction wave catches the shock wave within the sandstone cylinder and, thus,cancels the shock wave. vfp = velocity of flyer plate, UFe = shock wave velocity within ARMCO iron, UFe + upFe = rarefractionwave velocity in ARMCO iron, Usst = shock wave velocity within sandstone, Usst + upsst = rarefraction wave velocity insandstone, upsst = particle velocity in sandstone.

Shock effects in dry, porous Seeberger sandstone 5

RESULTS

Significant shock-induced effects in the sandstonecylinder are generated in the experiments (Table 4).These include changes in porosity, optical parameters,the generation of intra- and intergranular fractures,different kinds of melt, and the formation of diaplecticquartz glass. These observations are restricted to theuppermost one millimeter of the shocked sandstone cylinder.Due to interaction of shock and rarefraction waves travelingin the sandstone and in the surrounding ARMCO ironcontainer, the shock pressure determination is problematicin the deeper parts of the sandstone cylinder, so that theseparts were excluded from most of the investigation.However, so-called ‘‘local effects’’ occur also in thedeeper part of the shocked cylinders, and these arediscussed here as well.

Porosity

In the shock experiments all pores are entirely closedor, respectively, filled with a microbreccia, a melt (nowglass), or a mixture of both (Fig. 4). Already at thelowest shock pressure of 5 GPa all pores are closedcompletely and discrimination of individual quartzgrains on BSE images is difficult to impossible becausenow grain boundaries and fractures are indistinguishablefrom one another. Moreover, individual quartz grainsare surrounded by brownish and partially opaque orisotropic regions, which strongly increase in abundance

and size with increasing shock pressure. They are mostlymade up of a highly vesicular melt containing moltenphyllosilicates and ⁄or molten iron oxide minerals.

Optical Extinction of Quartz Grains

In the unshocked sandstone nearly all quartz grainsshow sharp extinction. In contrast, already at a pressureof 5 GPa quartz grains display undulatory extinction,and at pressures of 7.5 GPa and above, the quartz grainsshow weak but prominent mosaicism.

Fracturing Phenomena

Numerous fractures, both intragranular andintergranular, occur throughout the shocked samples. Inthe sample shocked to 5 GPa large intergranularfractures were not observed (Fig. 5a). At pressures of 7.5,10, and 12.5 GPa (Figs. 5b–d) large intergranularfractures originated at the upper edges of the samplecylinders, at contacts to the ARMCO iron container, andtraversed diagonally through the samples converging inthe center of the sandstone cylinder. These fractures arecharacterized by wall displacement, obviously representingshear zones. Several other large intergranular fracturesextend radially toward the centers of the cylinders. Theirnumber increases with increasing shock pressure from 7.5to 12.5 GPa (Figs. 5b–d). The degree of pulverization ofsamples and, thus, loss of material during recovery,increases with increasing shock pressure.

Table 4. Deformation and transformation effects observed in the shocked sandstone samples.Shock pressure*Experiment

Unshocked(1–0)

5 GPa(1–1)

7.5 GPa(1–2)

10 GPa(1–3)

12.5 GPa(1–4)

ObservationsPores �19 vol% crushed crushed crushed crushed

Extinction of quartz grains sharp undulatory weak mosaicism weak mosaicism weak mosaicismFractures (f) �2 f ⁄mm �134 f ⁄mm �171 f ⁄mm �203 f ⁄mm �208 f ⁄mmLarge intergranular fracturesand ⁄or shear zones

) ) + + +

Intragranular fractures extremely rare + + + +Melt (glass) ) �1.6 vol% �2.4 vol% �4.8 vol% �4.8 vol%Dark vesicular melt ) + + + +

Light vesicular melt ) ) + + +Melt + large iron particles ) ) + + +Molten ARMCO iron ) ) ) + +

Diaplectic quartz glass ) ) ) + +Local shock effectsPlanar deformation features

(PDF)

) ) ) + +

Cataclastic microbreccia ) ) + + +Diaplectic quartz glass ) ) (+) + +SiO2 melt ) ) (+) + +

*Calculated pressure at the top of the sandstone cylinder in contact with the driver plate; + = effect is present, (+) = effect is rarely present,

further investigations necessary, ) = effect not observed.

6 A. Kowitz et al.

At the microscale, many irregular intragranular aswell as numerous subplanar intragranular fractures arepresent in quartz. The subplanar fractures appear assingle sets oriented at approximately 50� (Fig. 6a), 0�(Fig. 6b), or 35� (Fig. 6c) to the shock front (± parallelto the surface of the sandstone specimen). They alsoappear as directional sets where the larger one is orientedat approximately 50� (Fig. 6d) and the shorter one atapproximately 22� to the shock front, and at approximately

65� to each other. Spacings of subplanar fractures arehighly variable and range from <10 to 60 lm.

Backscattered electron imaging shows that in allsamples all quartz grains are entirely crossed by infinitesimalfractures (microfractures) and that, therefore, some grainboundaries are no longer readily identifiable. There is ahuge number of short, irregular, vermicular microfracturesforming a network across the whole area, especially in thesamples shocked at 7.5 GPa and higher pressures (Fig. 7a).

Fig. 5. Cross-sections through the recovered samples cut perpendicular to the impacted surface (left sides), and optical scansproduced in transmitted light of the corresponding thin section (right sides) (black arrows = propagation direction of the shockfront). a) 5 GPa; b) 7.5 GPa; c) 10 GPa; and d) 12.5 GPa. Note the increasing number of intergranular fractures and theincreasing loss of the shocked and pulverized material during preparation in the sample shocked at higher pressures.

Fig. 4. Comparison of unshocked and shocked Seeberger sandstone samples with optical micrographs (transmitted light, left side,a–e) and BSE images (right side, f–j). Unshocked sandstone (a, f), and sandstone shocked at 5 GPa (b, g), 7.5 GPa (c, h), 10 GPa(d, i), and 12.5 GPa (e, j). Note the total closure of pore space as well as the difficulty to discriminate individual quartz grains evenat 5 GPa on BSE images. Quartz grains are surrounded by grayish regions that increase with shock pressure in amount and width(black arrows = propagation direction of the shock front).

Shock effects in dry, porous Seeberger sandstone 7

Fig. 7. BSE images of the sample shocked to 10 GPa. a) Network of short, irregular, vermicular microfractures in quartz. b)Microfractures occurring together with microbands (upper microband indicated by an arrow) in quartz. The smaller microfracturesare oriented at about 65� (NW–SE) in relation to the shock front and seem to be displaced by larger homogeneous microbands,oriented at about 10� (NNW–SSE). c–d) Enlargements of (b). c) Microbands with internal subplanar microstructures that areoriented at about 34� (NE–SW) and about 65� (NW–SE) in relation to the shock front. d) A third internal subplanarmicrostructure (white bar) within the homogenous microbands is oriented at about 10� (WNW–ESE) to the shock front; blackarrows = propagation direction of the shock front.

Fig. 6. Optical micrographs (transmitted light) of subplanar, intragranular fractures within single quartz grains displaying differentorientations in relation to the shock front as indicated by the arrows parallel to the propagation direction of the shock front.Fracture orientation at a) about 50�, NW–SE, 7.5 GPa, b) about 0�, E–W, 5 GPa, c) about 35�, NE–SW, 12.5 GPa. d) Directionalfracture sets with a larger one (orientated about 50�, NW–SE) and shorter ones (orientated at about 22�, ENE–WSW) enclosing anangle of about 65�, 5 GPa.

8 A. Kowitz et al.

These fractures are similar to those described by Reimoldand Horz (1986), in Hospital Hill Quartzite experimentallyshocked to 8 GPa, who termed these features ‘‘shockextension fractures.’’

Microfractures also occur together with microbands(Fig. 7b). The smaller microfractures seem to bedisplaced by the larger microbands. These microbandsare not open fractures but represent homogeneous,subplanar, approximately 2 mm wide features that arecut only sporadically by younger fractures, although theinterspaces between them are strongly affected bymicrofracturing. In BSE images these homogeneous,subplanar microbands contain at least three internalsubplanar microstructures (Figs. 7c and 7d) appearingdarker than the surrounding crystalline quartz. Thisindicates that they have a lower density and, hence, arediaplectic or silica glass (Langenhorst and Deutsch2012). Therefore, these fine microstructures show strongsimilarities to planar deformation features (PDF). It isnoticed that they are often oriented parallel to thesurrounding open fractures.

Tensional fractures were observed in all shockexperiments. For example, Fig. 8a shows a quartz grainbroken into two fragments due to tensional fracturing.At pressures of 7.5 GPa, additional networks of fracturesare sporadically observed that form angles of 60�,respectively, 120�, to each other (Fig. 8b), and thereforehave a strong similarity to Riedel shears (Katz et al.2004).

To obtain quantitative data for the density offractures close to the surface of the sandstone cylinders,the intersections with fractures were counted on SEMimages, along three different profiles of 1000 lm lengtheach. Averages of the three individual measurementswere calculated and plotted with their standarddeviations which show that the number of fracturesvaries considerably over short distances. However, ingeneral, the number of fractures (f) per millimeterincreases with increasing shock pressure (Fig. 9).

Melt Provenance

Four different types of melt were generated at poresand fractures in the shocked samples, of which the firstthree are melts that are generated in situ and the fourthis a result of the response of the experimental assemblyto shock. The following characterization is based on BSEimages and semiquantitative EDX analyses.1. The first type of melt is highly vesicular and occurs in

pockets. In BSE images it is much darker than thesurrounding quartz. It shows a flow texture withschlieren. Sizes of the melt pockets vary from <1 toapproximately 5 lm (Fig. 10a). The approximateaverage composition of the melt is 54 wt% SiO2, 26wt% Al2O3, 2.1 wt% MgO, 1.7 wt% FeO, 1.3 wt%K2O, 0.4 wt% TiO2, 0.3 wt% Na2O, 0.5 wt% CaO,and 0.04 wt% SO3. This dark melt was formed in allshock experiments, but the proportion of meltincreases considerably with increasing shock pressure.

2. The second type of melt is identical in appearance tothe first one and displays a similar chemicalcomposition, with the exception of a typically higher

Fig. 8. a) Tensional fracture (about 40 lm in width), along which the whole upper part of the sample has lifted up and the largecentral quartz grain has been broken into two fragments (BSE image, 5 GPa). b) Two systems of microfractures forming an angleof 60�, respectively, 120� (BSE image, 7.5 GPa); (black arrows = propagation direction of the shock front).

Fig. 9. Mean fracture densities and standard deviations versusshock pressure. The average number of fractures (f) permillimeter increases from 134 f ⁄mm in the sandstone shockedto 5 GPa, via 171 f mm)1 (7.5 GPa) to 203 f ⁄mm and 208f ⁄mm at 10 and 12.5 GPa, respectively. Note the apparentattainment of a plateau at 10 and 12.5 GPa.

Shock effects in dry, porous Seeberger sandstone 9

iron content resulting in a comparatively light colorin BSE images (Fig. 10b). This melt was not observedin the 5 GPa experiment but in all experiments athigher shock pressures. The abundance of this melttype increases with higher pressure as well.

3. The third type of melt has a similar chemicalcomposition to types (i) and (ii) and shows also aflow texture with schlieren. However, type 3contains additional iron particles with grain sizes of0.5–2.0 lm (Fig. 10c), and rarely displays vesicles.This melt is present only near the surface of asample. It appears rarely in the sandstone shockedto 7.5 GPa but is more abundant in the experimentsat 10 and 12.5 GPa. The iron particles probablyoriginate from the ARMCO iron cylinder.

4. The fourth type of melt (Fig. 10d) represents ironinjected from the ARMCO iron driver plate intofractures at the surface of the samples shocked to10 and 12.5 GPa. This melt is completelycrystallized.Image analysis shows that the total combined

amount of melt types i–iv increases from approximately1.6 vol% in the sandstone shocked to 5 GPa toapproximately 2.4 vol% (7.5 GPa), and approximately4.8 vol% (both 10 and 12.5 GPa).

Diaplectic Quartz Glass

Isolated quartz grains at the surface of the sandstonecylinders shocked to 7.5, 10, and 12.5 GPa appear partlyblurred in the optical microscope under plane polarizedlight (Fig. 11a); they are isotropic when viewed withcrossed polarizers. Such areas are homogeneous with nofractures, although the rest of the grain is microfractured(Fig. 11b). The number of fractures within single grainsincreases with increasing distance from the surface butdoes not show any fractures in this particular area.Furthermore, there is a slight grayscale difference,especially at the margins of the homogeneous zones,between the crystalline quartz (slightly lighter) and theamorphous phase (slightly darker due to lower density,Langenhorst and Deutsch 2012). Raman spectra(Fig. 11c) indicate the presence of crystalline quartz withits well-developed peak at 463 cm)1 (spectrum a5) at theouter perimeter, followed by a spectrum (a4) with thispeak shifted to 458 cm)1, which is characteristic forshock-deformed crystalline quartz (Ferriere et al. 2009).In spectrum (a1) this particular peak is absent, butseveral smaller peaks occur (e.g., at 495 cm)1, and604 cm)1) that are characteristic for diaplectic quartzglass (Ferriere et al. 2009). The Raman spectra clearly

Fig. 10. Microprobe BSE images of different types of melt generated in shocked sandstone (black arrows = propagation directionof the shock front). a) Dark, highly vesicular melt (type i) at 10 GPa. b) Comparison of dark (type i, left side) and light (type ii,right side) vesicular melt (12.5 GPa). c) Al-Fe-Mg-rich melt (type iii) with large iron particles. One feldspar grain is molten at theedge in contact to melt (gray arrow), causing enrichment of Al2O3 in this region (12.5 GPa). d) Injection of molten ARMCO iron(melt type iv) into fractures in sandstone (12.5 GPa).

10 A. Kowitz et al.

document a transition from totally crystalline quartzover deformed crystalline quartz to diaplectic quartzglass with decreasing distance to the sample surface. Thisphenomenon could be observed in quartz of the samplesshocked to 10 and 12.5 Gpa, and a transition fromunshocked quartz to only deformed quartz was noted inquartz grains of the sample shocked to 7.5 Gpa.

Localized Shock Effects

In addition to the above described deformationfeatures, several shock-induced local phenomena aredistributed locally within the entire sandstone cylinder(e.g., Fig. 12a). They include PDF, cataclasticmicrobreccia, the presence of diaplectic quartz glass, andSiO2 melt formation. Most of these features developexclusively in the vicinity of large intergranular fracturesor shear zones (described in the Fracturing Phenomenasection).

Figure 12a shows exemplary the occurrence oflocalized shock effects in the sandstone cylinder shockedat 10 GPa. Relatively close to the initiation points of shearzones, starting from the upper edges of the samplecylinders in contact with the ARMCO iron container, halfa dozen quartz grains display PDF parallel to (10�13) and

(10�12) with spacings of <1 to approximately 3 lm(Fig. 12b). Interestingly, they are slightly curved in directcontact to the shear zone. BSE images (Figs. 12c and 12d)show quartz grains with two sets of PDF. In Fig. 12d bothsets are nearly perpendicular to each other. Both BSEimages show a slightly curved nature of the subplanarfeatures in some areas near the contact to the shear zone,which is not typical for PDF, but has been observedoccasionally in nature (Trepmann and Spray 2005).

The shear zones locally show a zonation. The outerpart consists of a cataclastic microbreccia, followed by azone of partly fused quartz grains, and a SiO2-rich melt inthe center. In many parts of the shear zones this sequenceis incomplete. Figure 13a displays microbrecciated quartzat the rim of such a shear zone and a SiO2-rich melt in thecenter including slightly elongated vesicles indicative oflateral movement. There are also schlieren visible in themelt due to material contrast indicating inhomogeneousmixing of the different molten minerals of the sandstone.Figure 13b shows a melt vein filled with a fine-grainedSiO2-rich matrix including elongated vesicles andschlieren of molten rutile and iron minerals that arederived from the adjacent areas. At the rim of the meltvein adjacent quartz grains are partly molten. Theselocalized SiO2-rich melts consist—in contrast to the

Fig. 11. Formation of diaplectic quartz glass in the sandstone shocked to 12.5 GPa (black arrows = propagation direction of theshock front). a) Transition of quartz to diaplectic quartz glass in a single grain (optical micrograph, plane polarized light); a1–a5annotations correspond to Raman measurements displayed in (c). b) BSE image of the area shown in (a) with diaplectic quartzglass (homogeneous zone without fractures) close to the upper sample surface. c) Raman spectra of crystalline quartz (a5), shockedquartz (a4), and diaplectic quartz glass (a1) demonstrating the transition from crystalline quartz to diaplectic quartz glass withdecreasing distance to the upper sample surface.

Shock effects in dry, porous Seeberger sandstone 11

previously described melts (see the Melt Provenancesection)—more or less of pure SiO2 and displaysignificantly fewer vesicles. The localized SiO2 melts occurat shock pressures of 10 and 12.5 GPa, exclusively withinintergranular fractures of variable size, and at their

intersections in the central part of a sample where largermelt pockets were generated. In addition, some quartzgrains within these intergranular fractures show, asconfirmed by Raman spectroscopy, a transition todiaplectic quartz glass.

Fig. 12. a) Sketch of the distribution of localized deformation effects in the sandstone sample shocked to 10 GPa. Note that mostof these effects are restricted to large curved intergranular fractures and shear zones (black arrows = propagation direction of theshock front). b) Three quartz grains with one or two sets of PDF (optical micrograph; transmitted light). Orientations in relationto the shock front are approximately 58� (NW–SE) in the upper left grain (PDF parallel to (10�13)), about 45� (NE–SW) in theupper right grain (PDF parallel to (10�12), and about 60� (NNE–SSW) and 70� in the upper and lower parts, respectively, of thelower middle grain (PDF parallel to (10�13). c) Quartz grain with two sets of PDF, the first one orientated ±90� (N–S) to the shockfront, and the second about 52� (NNE–SSW) to the shock front (BSE image). d) Quartz grain with two perpendicular sets of PDForientated at about 62� (NE–SW) and about 43� (NW–SE) in relation to the shock front (BSE image).

Fig. 13. a) SiO2-rich melt with flow texture containing elongated vesicles, partially molten quartz, and cataclastic microbreccia atthe margins of a wide shear zone (10 GPa, BSE image). b) A melt vein trending at NW–SE, which is filled with a fine-grained SiO2-rich matrix including schlieren and droplets of molten rutile and iron minerals (lighter phases), elongated vesicles, and partlymolten quartz grains along the edges of the vein (12.5 GPa, BSE image); (black arrows = propagation direction of the shockfront).

12 A. Kowitz et al.

DISCUSSION

Influence of the Experimental Set-Up

Although the time–distance plots for all four shockexperiments suggest that the initial shock wave does notreach the bottom of the sandstone cylinders, the entiresamples have undergone shock deformation. Forexample, microscopic and BSE observations of the entiresandstone cylinders have demonstrated a total closure ofpores and formation of melt and fractures. This could beexplained by plastic waves, unloading after propagationof the initial shock pulse, or additional spherical shockwaves that originate from the ARMCO iron-sandstoneinterface at the sandstone cylinder jacket. The shockwave propagates much faster through the ARMCO ironand releases, therefore, such spherical shock waves atevery point of the interface before the initial planarshock wave reaches these points.

In addition, the shock pressure caused by the initialplanar shock wave generally decreases significantly withdepth due to pore crushing that consumes energy fromthe shock wave and attenuates shock pressure(Guldemeister et al. 2013). Therefore, we can use onlythe uppermost part of the shocked sandstonecylinders for the investigation of regular (i.e.,compression ⁄decompression-related) shock effects. Thespherical shock waves originating from the cylinderjacket were also reflected at the bottom of the sandstonecylinder at contact to ARMCO iron, leading to furtherinterferences in the lower parts of the sample cylindersuntil the arrival of the rarefraction wave. This causesstrong fragmentation and at higher shock pressurespulverization (cf. Fig. 5).

Another effect appears at the upper edge of thesandstone cylinders. In these ring-like areas the shockwave propagating from the ARMCO iron into thesandstone is reflected both forward and backward severaltimes, so that pressure peaks are generated. Also shear isstronger in these areas due to the large difference inshock impedance between ARMCO iron and the sandstone,which enhances both pressure and postshock temperature.

In addition to the effect of shock pressureattenuation in lower parts of the sample cylindersreferred to above, the interaction of the initial planarshock wave in the sandstone cylinder with additionalspherical shock waves originating from the cylinderjacket makes shock pressure determination for themiddle and lower parts of the sample cylinder difficult.Generally, the experimental set-up cannot generate idealuniaxial compression. Moreover, the set-up leads to aslight lateral extension of the sample cylinder. Thisextension leads to the formation of large, nonplanar,intergranular fractures observed in all experiments

(Figs. 5 and 12a). Also, this lateral extension has causedthe formation of fractures in single quartz grains, whichare oriented at approximately 45� to the shock front(Figs. 6a and 6c).

Progressive Shock Metamorphism

Shock compression of porous sandstone is distinctlydifferent from that of nonporous rocks, especially at lowshock pressures. This is obvious from the differencebetween the Hugoniot curve of quartz or quartzite andsandstone (e.g., Kieffer 1971; Kieffer et al. 1976;Guldemeister et al. 2013), which for the low pressureregime strongly depends on porosity. The large contrastin the shock impedances of quartz and pores leads to adistinctly heterogeneous distribution of shock pressuresand temperatures until the pores are completely closed.This causes heterogeneous distribution of shock featuresat the microscopic scale, as observed in nature (e.g.,Kieffer 1971; Kieffer et al. 1976; Grieve et al. 1996;Osinski 2007) and in experimentally shocked samples(e.g., Hiltl et al. 1999, 2000; this work).

The shocked Seeberger sandstone samples displayprogressive shock metamorphism with increasing shockpressure as indicated by decreasing pore volume,characteristic changes in optical extinction of quartz,increased fracturing, increased melting, and theformation of diaplectic quartz glass. For pressurecalibration an average shock pressure is used in thiswork, which is based on the Hugoniot curve of porousCoconino sandstone. This average shock pressureneglects the strong differences in shock pressure betweenthe different minerals, and especially between mineralgrains and pores.

Pores are entirely closed or filled with melt in allshock experiments. Modeling of the effects of porecrushing by Guldemeister et al. (2013) resulted in totalclosure of pores at a shock pressure of 6 GPa—inexcellent agreement with our observation that at 5 GPapore space had been experimentally closed completely.The amount and the width of brownish and partiallyopaque regions (also described by Kieffer [1971] innaturally shocked Coconino sandstone) that surroundsingle quartz grains and represent fully or partially moltenareas strongly increases with increasing shock pressuredue to pore collapse and the resultant high temperatures.Calculations of such pressure peaks by Guldemeister et al.(2013) show that shock pressures at pores locally can betwo to four times higher than elsewhere in the samesample.

The extinction of quartz grains changes from mainlysharp in the unshocked sandstone over undulatoryextinction in the sandstone shocked to 5 GPa to weakbut still prominent mosaicism at 7.5 GPa and above.

Shock effects in dry, porous Seeberger sandstone 13

This observation ought to be checked against naturallyshocked sandstone samples.

The number of all kinds of fractures (intergranular insandstone and intragranular in individual quartz grains)also increases with higher shock pressure. At a pressureof 10 GPa, a saturation of fracture density is reached.Above 10 GPa fracturing is most likely replaced bymelting due to higher shock pressures and significantlyhigher postshock temperatures. The orientation of bothtypes of fractures depends on the location of a quartzgrain within the sample and its orientation to the shockfront. Within individual quartz grains the sequence offeatures produced is (1) formation of shortmicrofractures (Fig. 7a), (2) microbands containing PDF(Fig. 7b), and, finally (3) extension fractures (Fig. 8) thatoriginate at the surface of all samples.

The combined amount of all kinds of melt increaseswith increasing shock pressure, due to enhancedpostshock temperatures, up to 10 GPa. The ternaryMgO + K2O + Na2O ) Al2O3 ) FeO diagram (Fig. 14)shows that the analyzed unshocked phyllosilicate and thedark vesicular melt both plot into the Al2O3-rich areaaround the montmorillonite locus. The lighter meltphases are mixtures of Al2O3 and FeO, i.e., ofphyllosilicate and iron oxide minerals. Figure 14demonstrates that the dark vesicular melts, the lightvesicular melt, as well as the melt containing the largeiron particles, can all be derived from the melting ofphyllosilicates and phyllosilicate-limonite ⁄goethite-mixtures. The dark and light vesicular melt most likelywere generated in situ, due to the high temperaturesassociated with pore collapse, at so-called ‘‘hot spots.’’Both vesicular melt types occur already at low shockpressures, the dark vesicular melt as of 5 GPa, and thelight vesicular melt as of 7.5 GPa. Both types of melt fillmainly pores and display mobilization only over shortdistances into their surroundings.

At higher shock pressures, additional melt with largeiron particles occurs, which is injected into alreadyexisting fractures. It is reasonable to assume that the ironis derived from the driver plate. The enrichment of meltwith molten iron, in contrast to the dark, vesicular melt,and its increased mobility with increasing shock pressurecan be related to the higher shock temperatures caused byinitial higher shock pressure. At shock pressures above10 GPa, nearly all the phyllosilicates and phyllosilicate-goethite ⁄ limonite mixtures of the sandstone are molten.Therefore, the amount of melt reaches a plateau until thequartz portions of the sandstone begin to melt. Thetemperatures generated by shock pressures up to12.5 Gpa do not cause melting of quartz within the entiresandstone sample; this would require temperatures above1610 �C (Weast 1976) that are only reached locally atshear zones (see below).

The melting of ARMCO iron at the top of thecontainer and the cylinder jacket is most likely caused bysmall cavities and the collapse of pores located at orclose to this interface, as well as plastic work. Thesefeatures lead to shock pressure and temperature peaksthat can cause the melting of ARMCO iron within thecontact area to the sandstone sample.

Diaplectic quartz glass could be observed locally inthe samples shocked to 10 and 12.5 GPa, near the top ofthe sandstone cylinders where highest shock pressure isexperienced. Therefore, the formation of diaplecticquartz glass within the upper part of these shockedsandstone cylinders is interpreted as a real pressure-dependent shock feature. The shock pressure necessaryfor the formation of diaplectic quartz glass in theseporous samples is obviously much lower than thatproduced in shock experiments with quartz singlecrystals and quartzite (e.g., Langenhorst and Deutsch1994; Stoffler and Langenhorst 1994; Grieve et al. 1996),but locally, due to pore crushing, pressure peaks aregenerated reaching up to 2 to 4 times of the initial shockpressure and also much higher temperatures. A strongtemperature dependency for the formation of diaplecticquartz glass has also been recognized in shockexperiments with preheated quartz, quartzite, and granitesamples (Langenhorst and Deutsch 1994; Huffman et al.1993; Huffman and Reimold 1996).

Localized Shock Effects

The localized shock effects are not a directconsequence of the initial shock compression, but rathera corollary of the experimental set-up. Nevertheless, theappearance of these localized shock effects also increaseswith increasing shock pressure.

Fig. 14. Ternary MgO + K2O + Na2O ) Al2O3 ) FeO-diagramdisplaying the chemical compositions of phyllosilicates andphyllosilicate-goethite ⁄ limonite-mixtures of unshocked Seebergersandstone, selected phyllosilicate minerals (calculated from theirstoichiometric formulae), and melt compositions from the sampleshocked at 10 GPa. Note that the iron-rich unshocked phyllosilicatelikely represents analyses of mixtures of montmorilloniticphyllosilicate+Fe-oxides ⁄hydroxides.

14 A. Kowitz et al.

PDF observed in the samples shocked to 10 and12.5 GPa are restricted exclusively to the upper edges ofthe sandstone cylinders. Their crystallographicorientations indicate that the sandstone in these areas ismoderately (>10 GPa) to strongly (>20 GPa) shocked(Stoffler and Langenhorst 1994). Locally, pressure peaksproduced by multiple reflections of the shock wave at thecontainer-sample interface occur. Therefore, exactpressure determination within these regions is difficult.The slightly curved nature of some of these PDF isprobably the result of (1) a rotation of the individualquartz grain during their formation, which is caused bythe simultaneous formation of the large shear zone fromthe edges of the sandstone cylinder, or (2) a later offsetof the individual quartz grain caused by movement alongthe large shear zone.

The large nonplanar intergranular fractures andshear zones host microbreccia, diaplectic quartz glass,and SiO2 melt. Cataclastic microbreccias are generatedwithin these shear zones due to high mechanical strain.They are, therefore, only indirectly related to shockpressure. Diaplectic quartz glass and SiO2 melt occurwithin these shear zones in the 10 and 12.5 GPa samples.Both features could be the result of significantly highertemperatures within these shear zones in comparison tothe surrounding sandstone. These high temperaturesoriginate from the combination of shock heating andadditional frictional heating effective only in the shearzones. Indicators for extremely high temperatures,locally occurring within the shear zones, are SiO2 meltsincluding schlieren of molten titania after rutile, whichhas a melting point of 1835–1840 �C (Weast 1976).

Shock Classification

Based on the shock classification system for poroussandstone, our experimentally shocked samples belong toshock stages 1b (5 GPa) and 2 (7.5, 10, 12.5 GPa)(Table 1) (Kieffer 1971; Kieffer et al. 1976). For theseshock stages average shock pressures of approximately3–5.5 GPa and approximately 5.5–13 GPa wereestimated by these authors. Our 5 GPa experiment showsa near-complete closure of pore space, which is the basiccriterion for shock stage 1b, thus confirming the originalpressure calibration. Nevertheless, the commonoccurrence of PDF and up to 10 vol% of quartz glass,and small amounts of high-pressure phases, observed inshock stage 2 samples of Coconino sandstone (Kieffer1971; Kieffer et al. 1976), are not seen in our 7.5 to12.5 GPa samples. In contrast, only a small amount ofdiaplectic quartz glass was noted in our 10 and 12.5 GPasamples. This discrepancy, especially the absence of thehigh-pressure phases, between the shock experiments andthe naturally shocked Coconino sandstone samples

might be an effect related to differences in pressure-pulseduration and postshock temperature between experimentand nature. Nevertheless, a first step for the calibrationand improvement of the shock classification system forporous sandstone is achieved with these new shockexperiments.

CONCLUSION

The numerous local effects, caused by theexperimental set-up, include formation of PDF and shearzones associated with cataclastic microbreccias,diaplectic quartz glass, and SiO2 melt.

Our first shock experiments with dry, porousSeeberger sandstone have produced shock features inquartz as known from naturally shocked poroussandstone and shock experiments with quartz singlecrystals and quartzite at low shock pressures (e.g.,Kieffer 1971; Kieffer et al. 1976; Reimold and Horz1986; Langenhorst and Deutsch 1994). The progressiveshock deformation stages for the Seeberger sandstoneare:1. At 5 GPa, pores are totally closed or filled with melt

and quartz grains show undulatory extinction. Onaverage, 134 fractures per mm are observed. Darkvesicular melt occurs and its amount is, on average,1.6 vol%.

2. At 7.5 GPa, quartz grains show weak mosaicismand the number of fractures per mm increases up to171. Two additional kinds of melt, a lighter,vesicular melt and a melt containing large ironparticles, occur. The volume of all types of meltamounts to 2.4 vol%.

3. At 10 and 12.5 GPa, quartz grains also show weakmosaicism and the number of fractures permillimeter increases to a mean value of 200. Thevolume of melt averages at 4.8 vol%. Diaplecticquartz glass is present.In summary, effects exclusively characteristic for

shock loading are rare, and are at the moment restrictedto the formation of diaplectic quartz glass at shockpressures of 10 GPa and above.

Acknowledgments—This work is supported by DeutscheForschungsgemeinschaft (DFG) grants FOR 887 and Re528 ⁄8-1 and 8-2. High-quality polished thin sections of theshocked samples were prepared by U. Heitmann, WWUMunster. The Raman investigations were carried out withthe expert assistance of J. Fritz. Modeling of theexperimental set-up was carried out by Nicole Guldemeister,which led to many fruitful discussions. We appreciatetechnical assistance for sample and container preparationand the subsequent investigations of the shocked samples byK. Born, P. Czaja, A. Ueno, H.-R. Knofler, H. Schneider,

Shock effects in dry, porous Seeberger sandstone 15

K. Krahn, C. Radke, A. Yener, and H. Gotz. We want tothank the reviewers, F. Langenhorst and G. Osinski, and theeditor A. Deutsch for their very constructive comments thatresulted in significant improvement of the manuscript.

Editorial Handling—Dr. Alex Deutsch

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