Shock-Induced Transition of Quartz to Stishovite

Vg)I,. 78, NO. 26 •[OURNAL OF GEOPHYSICAL RESEARCH SEPTEMBER 10, 1973

Shock-InducedTransition of Quartzto Stishovite

JOHN D. KLEEMAN 2 AND THOMAS J. AHRENS

SeismologicalLaboratory, California Institute o• Technology Pasadena, California 91109

The transformation of quartz to stishovite has been studied by X-ray and optical examination of a series of experimentally shock-loaded specimens of a quartz-copper mixture. Shock pressuresof 68 to 260 kb and peak temperatures of 320ø to 870øK were achieved. Stishovite was identified from quartz shock-loaded above 90 kb; the quantity increases with increasingpressure, but is not dependent on temperature. The formation of stishovite under shock conditionsappears to be intimately related to a short-range order phase.

Stishovite is a high-pressurepolymorph of and Rosenberg [1968] indicate an increasing silicon dioxide, having the same structure as proportionof high-pressurepolymorphs above rutfie and cassiterite. It is found in nature as a 120 kb. DeCarli and Milton [1965] suggested minute componentof shockedquar•tz-bearing that the major proportionof stishoviteat equi- rocks associatedwith meteorite impact. Stisho,v librium reverts to an amorphous, or short- and PoTova [1961] first produced this phase range order (SRO), phase upon releasefrom in static high-pressure experiments. Shortly the high-pressureshock state. Our first experi- afterward, Chao et al. [1962] found small quan- ments revealedno significantquantities of SRO tities of stishovite in the intensely shocked phase coexistingwith the minute fraction of quartz sandstoneat. Meteor Crater, Arizona. stishovite recovered from shock pressureso,f Subsequentwork [Chao and Littler, 1963] has about 180 kb. We wish to characterize the demonstrated the natural occurrence of this occurrenceof SRO phasein relationto stisho- mineral at only one other impact crater (Ries vite formation. basin, Germany). In 1965 stishovite was first The third objectiveof this study was to in- isolated from experimentally shocked quartz vestigatethe parameterslikely to reflecton the [DeCarli and Milton, 1965], although it was quantity of stisho,viteproduced. Factors such not possibleto compare,in detail, the pressure as temperature (both in equilibrium shock and temperatureconditions required to produce state and in postshockstate), rate of cooling stishovite dynamically with the static results. duringshock pressure release, duration of shock, One impo•ant objectiveof the present study as well as the amount of pressureoverdriving, was to determine the lowest shock pressure were thought to be potentially effective in required to produce stishovite from quartz. varyingthe yield of recoveredstishovite. The Measurementso,f the Hugoniot equation of resultsof this aspectof the study will help in state of quartz indicate that stishovite,with or selectingthe optimumconditions for producing without coesite,will form a major part of the other high-pressureminerals in the laboratory. phase assemblageat pressuresabove 140 kb BAS•S OF EXPERIMENTS [Wackerle,1962; McQueenet al., 1963]. Ahrens The impactof a metal flyer plate,imbedded at the front of a Lexan plastic projectile,in- x Contribution 2203, Division of Geological and ducesa shockwave in a targetassembly (Figure Planetaw Sciences,California Institute of Tech- 1). The shockstate producedin the target is nology, Pasadena, California. determinedfrom the measuredflyer-plate veloc- •Permanent address: Department of Geology, University of New England, Armidale, N.S.W., ity and the Itugoniotsof the targetand flyer- Australia. platematerials by theimpedance match method [Rice eta/., 1'958].In theseexperiments the Copyright ¸ 1973 by the American GeophysicalUnion. sampleconsisted of a mixtureof quartzand

5954 KLEEMANAND AHRENS' QUARTZTO STISHO¾ITE 5955 copperpowders (quartz, 6% by weight). The Hugoniots for tungsten alloy (the usual pressure-volumeHugoniot for the mixture (as- striker plate material' 16.8 g/cmS), the copper- sumed to retain no porosity) was constructed quartz aggregate, and the stainlesssteel encas- fro.m the data for the individual components ing the sample are shown schematically in usingthe relation [McQueen et al., 1970] Figure 2. Experimental data for constructing

2 the Hugoniots shown in Figure 2 are taken V(p) = • m•V•(p) (1) from McQueen et al. [1970], Ahrens and Ga#- i--1 hey [1971], and Wackerle [1962]. where V{ is the specificvolume of the ith com- The duration of the shock pulse was deter- ponentat pressurep and m{ is the massfraction mined by the time taken for the rarefaction of this component.The assumption,implicit in originating from the rear of the striker plate Figure 2, is that the quartz and copper are to pass the same point as the shock front. By both shocked to pressure-volumestates along calculating the velocities of shock wave and their individual Hugoniotscentered at standard rarefaction, a time-distanceplot can be con- conditions.However, the internal energy in- structedfor one geometry[Gibbons and Ahrens, crease of the mixture depends upon initial 1971]. For the 5-mm thick flyer plates used in porosity. Before shocking, the mixture was these experiments, the shock pulse duration is compactedinto a hard pellet usinga reproduci- about 2 ysec.There is also a reflectedcompres- ble pressure.The density of the compressed sion originating from the back of the sample material is also reproducible.Since the sample cavity, from the nonporous denser stainless was porous before shock loading, the appro- steel. This reflection will enhancethe pressure priate Hugoniot for use in the impedance in the rear of the sample cavity, and fo•; this match method was calculated from the relation reason no quartz was mixed in the copper powder in this volume (see Figure 1). There u = [p(r• - r)] •/2 (2) shouldnot be a significantpressure gradient in where u is the particle velocity of the mixture, the quartz-copper portion of the target. V• is the specificvolume of the porousmixture The use of coppermixtures has somedistinct at zero pressure,and V is the specificvolume advantages.The Hugoniot of the sample mixture is closer to that of stainless steel than at pressure p, as calculated from Figure 2. Equation 2 amountsto the 'ideal locking' model quartz (see Figure 2), especially when com- of Linde and Schmidt [1966]. pared with porous quartz aggregates (e.g., pressedquartz powder or sandstone).This re-

NICHROME sults in lessperturbation of the shock front as THERMOCOUPLE LEADS F HEATINGELEMENT it passesthrough the sample,giving more even / SAUREISENCEICERAMIC"7 distributionO f pressure,especially at the edges, o-•o •ooooooo o r and more favorable conditions for recovery intact. In addition, higher pressurescan be CAP obtained with the same projectile velocity. An- other important advantage is temperature con- trol. Since the thermal equation of state of copper is well determined [McQueen et. al., 1970], the shocktemperature can be calculated. LEXAN PROJECTILE The postshock temperature achieved in the 3 0 0 0 0 ) 0 0 0 SiO• experimentsis largely controlled by the

STRIKER PLATE- propertiesof the copper.In addition,the copper STEEL conductsheat rapidly into the stainlesssteel block, which acts as a heat sink, quicklyreduc- • QUARTZ-COPPERMIXTURE ing postshocktemperatures. •. COPPERALONE SHOCK AND POSTSHOCK TEMPERATURES Fig. 1. Diagrammatic cross section of the shock recovery experiment. Projectile velocity and The copper-quartzmixture is relatively in- thermocouple voltage are recorded before impact. compressible,and shockpressures are relatively 5956 KLEEMANAND AHRENS' QUARTZTO STISHOVITE modest,so that the entropyproduction is not great, and the shock and postshocktemperatures are largely controlledby the preshock temperature. Nevertheless,it is useful to at- tempt to approximately calculate shock and postshocktemperatures for these experiments. If, upon shock compressionthe quartz frag- ments remain at their initial maximumgrain size of 0.01 to 0.025 cm in diameter, thermal equilibriumwith the surrounding copper matrix takes place in the order of 10-5 sec. Since the high-pressureshock duration is 2 x 10-6 sec, onlythermal equilibrium in the postshock state will be achievedwithin the copper matrix. On the other hand, if a significantnumber of quartz grainsare crushedby the shockand the effective particle size is significantly reduced, thermal equilibrium (largely controlled by the shock state in the copper) shouldbe achievedduring passageof the stress wave. For the present (Uo,Po) calculation,the latter case is assumed.This 0 0.5 1.0 1.5 assumption is not critical, as the postshock P(:]rf•cle velocity, km/sec and shock temperatures are nearly equivalent, Fig. 2. Pressure-particlevelocity plane indi- sincethe shock-inducedtemperature rise is caringshock state (u•, px) produced by impact largely accountedfor by the irreversiblework of flyer plate with velocityuo into stainlesssteel done on the sample in crushingout initial cap. Transmittedstress wave producesshock porosity. statein copper-quartzmixture (p•, u•). Two estimatesof shockand postshocktem- peratureswere obtained. Asa firstapproxima- ing the entropy increases [Ahrens, 1972] for tion,the actual quartz content ofthe samples the two cases outlined above and comparing wasneglected. A lower temperature wascalcu- the entropies with thermodynamic tabulation latedby assumingthat the samples were en- of entropyversus temperature at standard tirelycopper and the density (6.45 to 6.55 pressure[Robie and Waldbaum, 1968], it was g/cm3) arosewholly from initial porosity of foundthat the calculated postshock tempera- 27% in the copper matrix. The Hugoniot tem- tures differ by about 30øC at most from the peraturesforporous aggregate were calculated shock temperature. The near equivalence of usingthe formulas given by Ahrens [1972] and thesetemperatures occurred because shock theGruneisen parameter andtemperature along heating was largely the result of irreversible theprincipal Hugoniot of copper reported by crushing-outofthe initial sample porosity. McQueenet al. [1970]. Slightlyhigher values EXPERIMENTALMETHOD forthe shock temperatures wereobtained by A singlecrystal of Brazilianquartz was calculatingtheinternal energy increase versus crushed, andthe fraction -60 to +160mesh temperaturefora purecopper sample having was mixed with pure copper powder in the thesame porosity asthe sample (17%). In both proportion6%quartz by weight. This mixture calctfiations,theshock temperatures (Table 1) (theoreticalnonporous density of 7.82g/cm 3) wereobtained by correlatingthe increases in wasthen pressed into the cavity in the stainless internalenergy for thepure copper case with steelblock (alloy 304) with a reproducible thecorresponding energy increase for the actual pressureequivalent toabout 2.5 kb. Pellets made sample.The latter wasobtained by construct- underthe same conditions have a densityof 6.45 ing a porousquartz and copperHugoniot as to 6.55g/cm 3 or a porosityranging from 16.2 describedin the precedingsection.' By calculat- to 17.5%. A cap of stainlesssteel was then KLEEMANAND AHRENS' (•UARTZTO STISHO¾ITE 5957

TABLE 1. Results of Recovery of Stishovite from Shocked Quartz

Calculated Preshock Shock No. of Laboratory Pressure, Temp., Temp., Stishovite Quality of Yield ,Run kb øK øK Lines Identification Estimatet

IPG-22 188 295 380 to 420 9 Good 3 23 170 520 625 to 660 4 Fair 2 24 160 295 363 to 395 5 Fair 2 34 194 740 830 to 870 7 Good 3 35 200 295 390 to 435 17 Excellent 4 36 216 529 673 to 723 16 Good 4 43 145 770 833 to 858 4 Fair 2 44 9O 770 806 to 818 1 Equivocal ? 45 9O 295 328 to 340 4 Fair 1 46 100 295 333 to 348 9 Good 2 47 72 295 320 to 330 None Negative None 48* 166 295 369 to 403 18' Excellent 5 49 68 295 317 to 328 None Negative None 50 234 295 410 to 470 9 Good 2 51 260 295 433 to 500 11 Good 3

*Fused SiO 2 starting material. ton a scale of 0 to 5. pressedinto the coppermixture and faced off would be obtained from minute amounts of so that the front was fiat. The target was shock- stishovite.The samplecould not be rotated. An loadedby the techniquesdescribed by Kleeman internal standard of NaC1 was used in some [1971] and Gibbonsand Ahrens [1971]. The exposuresfor calibrationof film and camera. velocity can be measured to ñ1% accuracy, Five runs were made at high starting tem- but uncertainty in the Hugoniotsmay increase perature.The stainlesssteel blockswere coated the systematicerror of pressureestimates to with synthetic porcelain,wound with heating ---+5%.However, internal precisionwill be rather element wire, then insulated with more porce- better than this. lain. A thermocouplewas inserted in a hole After shock-loading,the stainlesssteel cap drilled to just contact the sample cavity at its was machinedaway to exposethe copper-quartz middepth. Chromel-alumel and copper-con- mixture. The block was placed in 10% HNO8 stantin thermocoupleswere used to measure at room temperature for I to 3 days, during temperaturesup to 770øK. These heatedshots which the copperdissolved and SiO2was freed. were quenchedby arrangingthe target to fall Usually 100% recoveryof the SiO, wasobtained into cold water immediatelyafter impact. (300-400 mg). About 50 mg of SiO• was re- •:•ESULTS tained for other observations,and the rest was leachedin 48% HF at 23øC for 6 hours. The $tishovite recovery and identification. The acid was then diluted to less than 5% strength white plastic Millipore filter became colored and was filtered through a Millipore filter of after the diluted hydrofluoricacid was filtered. 0.22-/zmhole size.In many shotsthe stishovite Runs 47 and 49 had minimal coloration,whereas amounted to little more than a coloration on higher-pressureshots were medium brown or the paper and couldnot be scrapedoff. greenishbrown. The colorwas not evenlydis- A samplewas taken from this paper by roll- tributed and appearedvaguely spotty under ing a tiny ball of rubber cement (•0.2-mm low-powermagnification. X ray samplestaken diameter) over its surface. The ball was then from the filters indicated the presence of put on the end 'of a glasscapillary (~100-/zm stishovite in all but runs 47 and 49. The num- diameter), which was used in place of the ber of reflections of stishovite with Fe-Ka radia- collimatorin a 57.3-mm Debye-Scherrercamera tion is limited to 18 (d spacingsgreater than so that optimum peak to background ratios 1.03 A) by the constructionof the special 5958 I•LEEMANAND AHRENS' QUARTZTO STISHO¾ITE collimator. Only one sample, an almost pure TABLE 2. X Ray Powder Diffraction Data from Shot 48 recovery of stishovitefrom run 48, gave all 18 Chao et al. 57.3-mm 114.6-mm reflections,and on this specimena Cu-Ka ex- [1962] Camera Camera posureproduced 24 lines (d spacingsto 0.88 A). Reflection d, d, d, This specimencontained sufficient stishovite to hk I A œ/œ1 A œ/œ1 A œ/œ1 yield a satisfactory powder pattern when 11o 2 959 lOO 2 940 lOO 2 951 lOO mounted on a rotated spindle in a normal 114.6- lol 2 246 18 2 241 20 2 244 20 200 2 09 1. 2 082 5 2 086 2 mm Debye-Scherrercamera. However, certain of 111 1 981 35 1 974 40 1 977 30 the weaker reflections were not found in this 21o 1 870 14 1 864 15 1 866 lO 211 1 530 50 1 531 50 1 528 5o exposureowing to a higherbackground. Results 220 1 478 18 1 476 20 1 476 15 002 1 333 lO 1 333 15 1 332 lO of all lines measuredfrom Cu-Ka exposureby 31o 1 322 4ñ 1 321 5 1 321 5 221 1 291 2 1 295 2 1 291 2 using the modified 57.3-mm camera and the 3Ol 1 235 25 1 235 25 1 233 20 112 1 215 lO 1 216 15 1 214 lO large normal camera are shown on Table 2. 311 1 185 2 1 184 5 1 183 3 The measurementsof Chao et al. [1962] (see 320 1 159 8ñ 1 158 2 202 1 123 2ñ 1 125 4 1 123 1 ASTM file card 15-26) are included for com- 212 1 084 2 1 086 5 1 085 5 321 1.o62 2 1 063 5 1 063 5 parison. Unit cell dimensionscalculated from 400 1.o45 2 1 047 5 1 045 8 41o 1.o13 2 1 o15 4 1 o14 5 the large cameraare ao-- 4.179 A, co-- 2.665 A, 222 0.9900 6 o 991 15 o 990 12 which are consistent with the values ao -- 330 0.9850 2ñ o 987 5 o 985 lO 411 0.9475 4 o 949 15 4.1790 A and co -- 2.6649 A reported by Chao 312 0.9382 4 o 939 lO 421 0.8824 1 o 885 5 et al. Lines previously listed as being found only in syntheticallyproduced stishovite were *Observed only on synthetic material in the past. present, as were lines previously found only in ñNot observed on synthetic material in the past. natural material. Table 1 summarizes all the data obtained no more than 50% stishoviteby volume.) Other after the hydrofluoric acid treatment of re- sampleshave notably lessstishovite, and total covered quartz. Lines on the powder patterns yield may be as low as 5 /•g or less. The shot were attributed to stishoviteonly if they did using SiO, glassas starting material produced not overlap with reflectionsof any other im- the greatestamount of stishovite.Ignoring this purity present. In only one case,shot 44, was run and consideringonly runs with quartz as there an equivocal result. Only the most in- starting material, we see that increasingthe tense line was found. The position where the pressurefrom 90 to 220 kb increasesthe yield secondmost intenseline shouldhave appeared and that increasingthe preshock temperature was clear of any interferenceby impurities,but (and therefore also equilibrium state tempera- no line was observed. In this case the identifica- ture) does not. Yield is reduced above 230 kb. tion must remain equivocal, although we are Runs 50 and 51 also produceda phase with biasedtoward interpreting the result as a par- large d spacings,at presentnot identified. ticularly low yield. Optical examination oi shocked quartz and Yield estimateswere made by comparingthe silica glass. Aliquots of the samplewere kept intensitiesof lines on the powderpatterns. Ap- aside for optical examination. When viewed under a binocular viewer, the shockedquartz proximately equal sectionsof the filter paper ranged from clear to turbid white. If hand were swept by the rubber ball during each X picked on this basis, the clear grains gave X ray samplepreparation, and they were exposed ray powder patterns with only slight loss of to consistent X radiation intensities for known definition and intensity of back reflections.The durations.The scale0-5 is an order of magni- turbid grainsalso gave quartz powder patterns, tude scale on approximately2 base (i.e., 2ø-•). but with more pronounced loss of definition, Visual examinationsuggests that yield 5 prob- especiallyin back reflections. ably co.rrespondsto no more than 100 /•g of Grains were also immersed in oils of known stishoviteand that the powder patterns from refractive index and were examined with a shot 48 were obtainedfrom X ray specimens polarizing microscope.The results of the re- of the order of 1-5/•g. (The rubberball is 0.1- to fractive index measurements are shown in 0.2-mm diameter and is estimatedto comprise Table 3. KLEEMANAND AHRENS' QUARTZ TO STISHOVITE 5959 TABLE3. Optical Examination of Shocked Quartz shocktemperature). This is at or slightlybelow the thermodynamicallypredicted pressure.At Shock Preshock Normal Refractive Separate Pressure, Temp., Quartz, Index Range, Glass, 350øK a similar point shouldbe 73 kb. Run kb øK % • % The most significantresult is that the formation of stishovite from quartz is consistentwith 49 68 295 99 1.542 to 1 544 47 72 295 99 1.542 to 1 544 the calculated quartz-stisho.viteboundary of 45 90 295 98 1.542 to 1 544 46 100 295 98 1.542 to 1 544 Holm et al. [1967] and the experimentalcoesite- 24 160 295 95 1.540 to 1 544 22 188 295 90 1.538 to 1 544 stishovitephase line of Ostrovsky[1967] and S0 234 295 70 1.510 to 1 544 Akirnoto and Syono [1971] (see Figure 3). The S1 260 295 60 1.496 to 1 544 44 90 770 80 1.540 to 1 544 data are also consistent with the 75-kb value 23 170 520 95 1.538 to 1 544 36 216 520 60 1.516 to 1 544 inferred by Ahrens and Gregson [1964] from 34 194 740 60 1.530 to 1 544 48* 166 295 * 1.459 to 1 530 study of the P-¾ Hugoniot of Coco.ninosand- stone. Increasing yields of stishovite were re- *Fused Si02 starting material. covered as pressuresincreased from 90 kb, until shots 50 and 51 actually produced less, Optical examinationwith plane-polarizedwith the accompanimentof whole grains of light alsoshows a rangeof shockeffects within shock-formedglass. Temperature does not affect each shockedsample. A large proportionof yield. (An increasedyield with increasingtem- grainsare clear and, althoughsome cracking peraturemay have indicatedthat the rate of and undulose extinction are observed, show transformation to stishovite is kinetically con- little evidence of shock loading. A variable trolled.) proportionof grains are cloudy to varying The increasedyield of stishovitewith increas- degreesand have lower refractive indices. Table ing pressureis accompaniedby a greater pro- 3 showsproportions of quartz with unchanged portion of quartz with diminishedrefractive and lower refractive indexes. The total range index. These lower refractive indices may well of m refractive index is shown and the propor- be causedby submicroscopicinclusions of glass tion remainingat m -- 1.544.The data are con- in the quartz. In this case,the increasedyield sistent with those of HSrz [1968]. Planar fea- of stishovite is accompanied by increasing tures were observed in most samples shock quantitiesof submicroscopicglass, •This associa- loaded to more than 140 kb. They were rare tion may be an important indicationof the way below 210 kb, but no more than 1% or 2% stishovite forms during shock loading. This of the grainshad planar featuresin any sample. submicroscopicglass is the zero-pressureequiv- The three shots above 210 kb produced a alent of either a crystalline phase, such as separateglass phase, that is, wherewhole grains stishovite,or collapsedquartz of SRO. All shots were essentiallyisotropic. The refractiveindices achievedtemperatures of lessthan 900øK, and are higher than for fused silica (7 -- 1.459 [Gibbons and Ahrens, 1971]). (This shock- formedglass is the sameas is variouslytermed • o NOSTISHOVITE RECOVERED Ld ß Equivocal identification 'thetomorphic'or 'diaplectic.') Z:) ß STISHOVITE PRESENT Data from the shot with fused silica are also <• ß FusedSlO 2 starhngmatehal rr' I000 . STISHOVlTE& SEPARATEGLASS L,J I shownin Table 3. The range of refractive index Aklmoto and I I is consistentwith measurementsreported by • Syono(1968)I'1 Gibbonsa•d Ahre•s [1971]. 0 - Cl/•,•S•Ostrovsky(1967) DISCUSSION Quartz co• 600 / Quartz + Figure 3 illustrates the results of stishovite 5LJ_J 400 Holmetal-•, ii/I/ Sfishovde Q) oo ß ß recovery in the pressure-temperatureplane. _•D - (I967)c?J Shocktemperatures are plotted, althoughpost- c) 2000 510 I•O I•O 2•)0 2•0 500 shocktemperatures are probablynot more than SHOCK PRESSURE (kb) 30øC lower. Stishovite is found at shock pres- Fig. 3. Shock recovery products shown in re- suresof 90 kb and greater. An equivocalidenti- lation to calculated shock temperature and shock fication resulted from shot 44 (90 kb,--825øK pressure. 5960 K•,•,•,MA• ANDA•R•,•s: QuaRTzTo STxS•OVIT•, were quenchedimmediately, so that reversion Holm, J. L., O. J. Kleppa, and E. F. Westrum, of stishovite to a SRO phase is unlikely. The Thermodynamics of polymorphic transforma- inferred submicroscopicglass is therefore more tions in silica: Thermal properties from 5 to 1070øK and pressure-temperature stability fields likely the zero.-pressureequivalent of a dense for coesite and stishovite, Geochim. Cosmochim. SRO phase, which is an essentialpart of an Acta, 31, 2289, 1967. equilibrium assemblageinvolving shock-pro- HSrz, F., Statistical measurements of deformation duced stishovite. The shot for which silica structures and refractive indices in experimentally shock-loaded quartz, in Shock Metamor- glass was the starting material produced by phisin o• Natural Materials, edited by B. M. far the greatest yield of stishovite.It may well French and N.M. Short, Mono, Baltimore, be that a denseSRO phase from which stisho- Md., 1968. vite forms is more easily producedfrom silica Kieffer, S. W., Shock metamorphism of the glass. Coconino sandstoneat Meteor Crater, Arizona, J. Geophys. Res., 76, 5449-5471, 1971. 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