The temperatures induced in crystalline calcite upon planar shock compression (95–160 GPa) are reported from two-stage light gas-gun experiments. The temperatures are obtained fitting 6-channel optical pyrometer radiances in the 450 to 900 nm range, to a Planck radiation law temperature varied from 3300 to 5400 K. Calculations demonstrate that the temperatures are some 400 to 1350 K lower than if either shock-induced melting and/or disproportionation of calcite behind the shock front was not occurring. Here calcite is modeled as disproportionating into a molecular liquid, or a solid CaO plus CO2 gas. For temperature calculations, specific heat at constant volume for one mole of CO2 is taken to be 6.7R as compared to 9R in the solid state; whereas calcite and CaO have their solid state values (15R and 6R). Calculations also suggest that the onset of decomposition in calcite to CaO and CO2 during loading occurs at ~75±10 GPa, along the Hugoniot whereas decomposition begins upon unloading from 18 GPa. The 18 GPa value is based on comparison of VISAR measurements of particle velocity profiles induced upon isentropic expansion with one-dimensional numerical simulation

Ahrens, Thomas J.

Gupta, Satish C.

Love, Stanley G.

English

Caltech Authors - Main

CP505, Shock Compression of Condensed Matter - 1999 
edited by M. D. Furnish, L. C. Chhabildas, and R. S. Hixson 
0 2000 American Institute of Physics l-56396-923-8/00/$17.00 
SHOCK TEMPERATURES IN CALCITE (CaC03): IMPLICATION 
FOR SHOCK INDUCED DECOMPOSITION 
Satish C. Gupta, Stanley G. Love and Thomas J. Ahrens 
Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125. 
Abstract. The temperatures induced in crystalline calcite upon planar shock compression (95 160 GPa) 
are reported from two-stage light gas-gun exp.eriments. The temperatures are obtained fitting 6-channel 
optical pyrometer radiances in the 450 to 900 nm range, to a Planck radiation law temperature varied 
from 3300 to 5400 K. Calculations demonstrate that the temperatures are some 400 to 1350 K lower 
than if either shock-induced melting and/or disproportionation of calcite behind the shock front was not 
occurring. Here calcite is modeled as disproportionating into a molecular liquid, or a solid CaO plus 
CO2 gas. For temperature calculations, specific heat at constant volume for one mole of CO2 is taken to 
be 6.7R as compared to 9R in the solid state; whereas calcite and CaO have their solid state values (15R 
and 6R). Calculations also suggest that the onset of decomposition in calcite to CaO and CO2 during 
loading occurs at - 75+10GPa, along the Hugoniot whereas decomposition begins upon unloading from _ 
18 GPa. The 18 GPa value is based on comparison of VISAR measurements of particle velocity 
profiles induced upon isentropic expansion with one-dimensional numerical simulation. 
INTRODUCTION carbonates, raised possibility of an extinction A a 
Research on the shock-induced devolatilization 
behavior of calcite is motivated by a need to 
understand the role the greenhouse gas CO;! played 
in the formation and evolution of atmosphere of 
Earth, Mars and Venus. These planets have long 
thought to contain substantial amount of carbonates 
in their crusts. In the case of Mars, defination of the 
shock pressure required to volatilize substantial 
quantities of CO% from carbonates can be used to 
define the cratering rate and the period over which 
Mars operated an efficiently CO2 greenhouse, such 
that a liquid Hz0 reservoir became available on the 
surface which might host early life. 
Shock devolatilization is also important to 
evaluate the mechanisms of impact induced 
extinction of marine and terrestrial biota for 
example at the WI’ boundary. The discovery of the 
Chicxulub crater as the impact site and studies that 
indicated -70% of the 3-km-thick section are 
mechanism involving a long duration ( lo”- 10” 
years) global greenhouse event with surface land 
temperature increasing by - 10 degrees at K/T times 
due to injection of impact-induced CO2 into 
atmosphere [ 11. 
Temperature measurements along the Hugoniot 
are useful for detecting and characterizing phase 
transitions including molecular dissociation [ 21. In 
the present work, we measured shock temperatures 
in single-crystal calcite using optical pyrometery. A 
two-stage light gas-gun launched impactors shock 
compressed the samples. Comparison of 
measurements and calculations demonstrates that 
temperatures are lower than if either melting and/or 
devolatilization of calcite are not occurring. Since 
deficit we have is much more than that expected 
from melting suggests that shock-induced 
decomposition of calcite occurs behind the shock 
front. 
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EXPERIMENTS 
Experiments were performed on the two-stage 
light-gas gun at our laboratory. The projectile 
bearing flyer was accelerated to the velocities 5.0 to 
6.9 km/s. The impact of the 0.5-mm-thick flyer 
plate induces a shock wave in the 0.5~mm-thick 
driver plate, which then propagates through the 3 
mm thick, 12mm diameter sample (Fig. 1). The 
optical radiation from the shock-heated sample is 
directed into a six-channel (450 to 900 nm) optical 
pyrometer [3]. The photodiode (#1801, New Focus, 
Inc.) has a built in preamplifier with a 125 MHz 
bandwidth. Digital oscilloscopes recorded each 
channel on the optical bandwidth of -5.Onm. 
Temperature and emissivity are determined by 
fitting the radiance at six wavelengths to the gray- 
body Planck spectrum. The results of the six 
experiments in the 95 to 160 GPa pressure range 
are shown in Fig. 2. Shock pressures were 
determined by performing impedance-match 
calculations employing measured flyer plate 
velocity and known equation of state of tantalum 
and calcite. 
MODEL TEMPERATURE CALCULATIONS 
Calculations are presented below for two cases: 
one in which calcite does not decompose in the 
shocked state up to the highest shock pressure of 
the experiment, and other in which calcite 
decomposes according to the chemical reaction 
CaC03 * CaO(s) + CO2 (g). 
No Decomposition of Calcite (Model -1) 
Temperature increase upon shock compression in 
a material is calculated using the relation, 
r& dT=EH-E -U 
T V S tr (1) 
S 
where Tn and Ts are temperatures along Hugoniot 
and isentrope at the same compression and E, is 
phase transition energy. Temperature Ts, Hugoniot 
energy, En, and isentropic energy, Es, are given by 
the following relations: 
Ts= Toexp[F( l-V/v,)] 
1 
EH =-PH(v, -vH) 
2 
Figure 1. Schematic diagram of experimental setup for shock- 
temperature measurements. 
E = 
S 
-j$ P dV 
S 
0 
where Pn and Ps are pressures along Hugoniot and 
isentrope at the same specific volume Vn; the 
specific volumes Vo and V, correspond to low 
pressure phase and high-pressure phase (on release) 
at ambient pressure; & and &’ are the bulk 
modulus and its derivative. The analytic form 
assumed for the isentrope is the Birch-Murnaghan 
equation. The entropy increase along the Hugoniot 
has been determined using the relation, 
C 
AsH =AS +&+dT tr s T (2) 
where AS, 
transition. 
1s the entropy change of phase 
With Chemical Decomposition of Calcite 
For chemical decomposition of calcite, the 
entropy criterion is employed. The decomposition 
begins at pressure for which shock-entropy equals 
Si, entropy of incipient decomposition at normal 
pressure. The decomposition gets completed at 
pressure for which shock-entropy equals Sv, the 
entropy of complete decomposition at normal 
pressure. We have estimated the values of Si and Sv 
for calcite to be 1.47 kJ(kg.K)-’ and 2.91 kJ(kg.K)-’ 
respectively, from the available thermodynamical 
data at normal pressure (Robie et al. [4]). 
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The shock-induced chemical decomposition of 
calcite is modeled in two ways. In Model-2, it is 
modeled to occur at constant shock temperature, 
and in Model-3, it is treated to occur over a range 
of temperatures. 
Decomposition Occurring at Constant 
Temperature (Model-2) 
Upon shock compressions down to specific 
volume Vo, for which ASH 2 Si the material does 
not decompose. Temperature and entropy along the 
Hugoniot are calculated using equations 1 and 2, as 
in Model-l. At higher compressions, the 
decomposition of calcite is modeled to occur at 
constant temperature Tno. The entropy in the 
compressed state is calculated using relation, 
(E -E )-(E 
AS =s + 
H S HO -ESO) 
H I T 
HO 
where En0 and Es0 are Hugoniot energy and energy 
along isentrope at same specific volume VO. At 
successively higher shock pressures, temperature 
remains constant but entropy increases, resulting in 
decomposition of additional material until 
compression to specific volume VI, entropy value 
reaches Sv, and calcite is completely decomposed. 
At still higher shock pressures, the additional 
Hugoniot energy is used up in compressing and 
increasing temperature of the mixture of reaction 
products. Hugoniot temperature is given by 
CEH 
TH =THo +TS -TSl + 
- ES I- @HI - Es1 > 
%2 
where Tsi is the isentropic temperature, En1 is 
Hugoniot energy and E s1 is isentropic energy at 
specific volume Vi. The heat capacity, Cvz, of the 
mixture of the reaction products is assumed to be 
constant. 
Decomposition Over Range of Temperature 
(Model-3) 
Once decomposition of calcite sets in at shock- 
entropy Si, upon higher compression, the fraction a 
of decomposed calcite is determined from the 
entropy of shocked solid calcite using relation, 
CI= (ASH -s&v -SI>. 
In the shocked state, material consists of a moles of 
each CaO and CO2 and (l- a) moles of calcite. The 
temperature for this mixture is given by 
T 
= T + EHO - ESO - Etr + EH - ES - EHO + ESO - &sub 
H S C C 
Vl Va 
where Esu,, is the energy required for decomposing 
one mole of calcite. Cvcl is the mass weighted heat 
capacity of the mixture 
products, and it is given by 
of calcite and reaction 
Cva = (1- “)CVI + acv2. 
These equations are applicable even when calcite is 
completely transformed to CaO and CaCOz. 
Material parameters assumed for calcite are 
taken from Tyburczy and Ahrens [5]. Since no 
specific heat Cv data is available on CaC03 and 
CaO, the Dulong Petit values of 15R and 6R per 
mole of molecules have been used. For gaseous 
CO2 we have estimated it from the data of specific 
heat at constant pressure. The estimated value of 
6.7R is smaller than solid state value of 9R. 
RESULTS AND DISCUSSIONS 
Experimental results and model calculations are 
plotted in Fig. 2. Comparison of measurements with 
calculations using Model-l (curve a in Fig. 2a) 
indicates that the measured temperatures are some 
400 to 1350 K lower than if shock-induced melting 
and/or decomposition is not occurring. 
Melting of calcite has been observed in high 
pressure experiments [6]. We have assessed the 
influence of melting on shock temperatures. We 
estimated the entropy of fusion and assumed it is 
approximately equal to R per mole of atoms, the 
same as those of simple elements [7]. The value 
0.41 kJ(kg.K)-’ for calcite is about 28% of the value 
of entropy of vaporization. Temperature deficit 
estimated for calcite is -450 K. This is too small to 
account for the observed temperature deficit. 
In figure 2a, the curve b shows the calculations 
using Model-2 in which calcite decomposition 
begins at shock entropy Si ( 1.47 kJ(kg.K)-’ ) and 
further decomposition at higher pressures occurs at 
constant temperature. The curve c displays 
calculation using Model-3 in which decomposition 
begins at shock entropy St but it occurs over a 
range of temperatures on further compressi on. The 
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(a) 
~ 
0 50 
Pm&% (GPa) 
150 200 
8000 
6000 
g 
0 50 100 
Pressure (GPa) 
150 200 
Figure 2. Experimental and calculated shock-temperatures in 
calcite. Solid circles with error bars are measurements. Solid 
lines are calculations with different models: a- no 
decomposition; b-decomposition at constant temperature; c- 
decomposition over range of temperatures; d-with Sr=l.97 and 
Sv=3.41; e-with S1=2.27 and Sv=3.9 1; A, B, C are same as a, b 
and c; D- with S1=1.97 and Sv=2.91, E- with Sr=2.27 and 
Sv=2.9 1, All entropies are in units of kJ(kg.K)-’ 
fact that these curves lie below the experimental 
data suggests that the decomposition does not begin 
when shock entropy equals the theoretical estimates 
of Si but sets in when system is overdriven. Thus, 
the decomposition process shows hysterisis. Curves 
d and e indicate calculations done with Model-3 in 
which the values of Si and Sv are increasd by same 
amount keeping (Sv-Si) constant. For calculation d, 
Si is 1.97 kJ(kg.K)-’ and Sv is 3.41 kJ(kg.K)-’ , 
whereas for calculation e, Si is 2.27 kJ(kg.K)-’ and 
Sv is 3.41 kJ(kg.K)-‘. The curves D and E of figure 
2b show calculations using Model-3 where the 
value of Si is increased but the value of Sv is kept 
constant. These curves correspond to St values of 
1.97 kJ(kg.K)-’ and 2.27 kJ(kg.K)-’ respectively. 
Considering the uncertainty in the experimental 
data, the comparison through visual inspection of 
all the calculations with these show that the 
calculation d and e are much closer to the 
measurements. This suggests that the chemical 
decomposition of calcite behind the shock front 
begins around 75&l 0 GPa. This is higher than 18 
GPa pressure at which calcite begins to decompose 
on unloading. The 18 GPa value is based on the 
comparison of VISAR measurements of particle 
velocity profile induced upon isentropic exapansion 
with numerical simulation [8]. The higher pressure 
for the onset of chemical decomposition at the 
shock front could be due to kinetic effects, as the 
time available for the reaction to occur at shock 
front is much smaller than in experiments involving 
isentropic release subsequent to loading. 
CONCLUSIONS 
Shock temperatures in single crystal calcite 
have been measured at 95 to 160 GPa in two-stage 
light-gas gun using optical pyrometery. Model 
calculations have been performed for shock 
temperatures in this material. Comparison of the 
measured and calculated temperatures suggests that 
the onset of decomposition in calcite to solid CaO 
and gaseous CO2 occurs at the shock front around 
75flO GPa. This decomposition pressure is higher 
than 18GPa from which pressure calcite begins 
decomposing upon unloading. 
ACKNOWLEDGEMENTS 
Research supported by NASA. We thank K.G. 
Holland for helpful comments and M.D. Furnish for 
useful suggestions. Contribution # 8670 of Division 
of Geological and Planetary Sciences, (Caltech). 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
REFERENCES 
Pope, K.O., Baines, K.H., Ocampo, A.C., and 
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Radousky, H.B., Nellis, W.J., Ross, M., Hamilton, 
D.C., and Mitchell, A.C., Phys. Rev. Lett., 57,2419 
(1986). 
Yang, W., Ph. D. theses, Caltech (1996). 
Robie, R.A., Hemingway, B.S. and Fisher J.R., 
U.G.S.G. Bulletin, 1452,456 (1979). 
Tyburczy, J.A., and Ahrens, T.J., J. Geophys. Res. 
91,473O (1986). 
Irving, A. J., and Wyllie, P.J., Geochim. 
Cosmochim. Acta, 39,35 (1975) 
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Shock temperatures in calcite (CaCO3): Implication for shock induced decomposition

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Shock temperatures in calcite (CaCO3): Implication for shock induced decomposition

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