The Hydrothermal Diamond Anvil Cell (HDAC) and Its Applications

Home , Diamond anvil cell

Mineral Spectroscopy: A Tribute to Roger G. Burns © The Geochemical Society, Special Publication No.5, 1996 Editors: M. D. Dyar, C. McCammon and M. W. Schaefer

The hydrothermal diamond anvil cell (HDAC) and its applications

I 2 2 WILLIAM A. BASSETTI, TZy-CHUNG WU , I-MING CHOU , H. T. HASELTON , JR., JOHN FRANTZ3, 3 4 5 6 BJORN O. MYSEN , WUU-LIANG HUANG , SHIV K. SHARMA , DAVID SCHIFERL 'Department of Geological Sciences, Snee Hall, Cornell University, Ithaca, NY 14853, U.S.A. 2MS 959 U.S. Geological Survey, Reston, VA 22092, U.S.A. 3Geophysical Laboratory, 5251 Broad Branch Rd. NW, Washington, DC 20015, U.S.A. "Exxon Production Research Company, 3319 Mercer St. Houston, TX 77027, U.S.A. 5Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822, U.S.A. 6Materials Research, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.

Abstract-A new style of diamond anvil cell, which is called the hydrothermal diamond anvil cell (HDAC), has been developed to make accurate measurements on samples under pressure and at temperatures between -190°C and + 1200°C. It is especially suitable for fluid samples and for observing reactions between solids and fluids. The techniques for making observations and determinations used to date include optical microscopy, x-ray diffraction, Raman spectroscopy, luminescence, and infrared absorption. Three studies described in this paper deal with pressure-temperature calibra-

tion. They are (1) H20 equation of state, (2) rapidly reversible second order phase transitions, and (3) shift of the Raman spectrum of diamond. Three applications of the HDAC described here include (1) luminescence and infrared absorption analyses of kerogen, lignite, and crude oil, (2) Raman spectroscopy of silicate melts, and (3) x-ray diffraction studies of dehydration and rehydration of montmorillonites using synchrotron radiation.

INTRODUCTION results from these techniques applied to a variety THE HYDROTHERMALdiamond anvil cell (HDAC) of samples held at high pressures and temperatures is a new style of diamond anvil cell that was de- in the HDAC. signed and built to meet a specific set of require- ments (BASSETT et al., 1993). It has been used to THE HYDROTHERMAL DIAMOND ANVIL CELL (HDAC) subject samples to a very uniform (±0.5°C) temperature over the range from -190 to + 1200°C, Until recently, very few studies of hydrothermal hold the temperature constant within ±2°C, and reactions or of equations of state of fluids had been measure the temperature by thermocouple with an made in the diamond anvil cell (e.g., VAN VALKEN- accuracy of ± 1.SOC. A rhenium gasket properly BURG et al., 1987). The hydrothermal diamond prepared is capable of holding a constant volume anvil cell (HDAC), which is described in detail by within ±2% or better over most of the temperature BASSETT et at. (1993), consists of two platens with range. If a fluid such as water is used as the sample diamond anvils and heaters mounted at their cen- or pressure medium, its equation of state can be ters (Fig. I). The two platens are drawn together used to determine the pressure from the measured by tightening three nuts, thus applying pressure to temperature. When the equation of state is well a sample held between the diamond anvils. Sam- known and the density of the fluid is accurately ples are typically placed in a rhenium gasket con- measured by homogenization temperature or freez- sisting of a hole 200 to 500 µm in diameter and ing temperature, the accuracy of such pressure 75 to 500 µm thick. A rocker under one anvil measurements is significantly greater than by other allows for orientational alignment and a sliding pressure measuring methods formerly used with disk under the other allows for translational align- diamond anvil cells. This and two other new pres- ment. Holes through the centers of the platens sure measuring techniques are described in this allow access to the sample for viewing and for paper. various forms of electromagnetic radiation. Molyb- Optical microscopy can be used to make direct denum wires wrapped around the tungsten carbide in-situ observations of changes in the solid, liquid, seats which support the diamond anvils serve as and vapor phases of a sample as a function of heaters (Fig. 2). These can heat the anvils and the pressure and temperature. Raman spectroscopy, lu- sample very uniformly and very constantly to tem- minescence, infrared absorption, x-ray diffraction, peratures as high as 1200°C. By introducing liquid as well as other techniques can also be used for nitrogen the temperature can be reduced to making in-situ observations in the new diamond - 190°C. Electrical leads for the heaters and the anvil cell. In this paper we describe some new 262 W. A. Bassett et al.

Electric connectors

Lower platen

f------4 ~Brassbase lcm

FIG. 1. Plan and elevation of the new diamond anvil cell. The diameter of the cell is 7.62 ern. The height is 5.72 ern. The base is constructed of brass; the platens, posts, and cylinder are constructed of stainless steel.

ume containing the heaters, the anvils, and the sample remains constant during isochoric experiments. ple can be enclosed and completely surrounded Fringes produced by laser light reflected from the with a gas to prevent oxidation. We have found top and bottom of a doubly polished solid sample that a constant flow of a mixture of Ar with 1% provide a very sensitive means for observing vol- H2 is very satisfactory for this purpose. ume and refractive index changes in samples due Laser light with any wavelength in the visible to transitions and reactions. spectrum can be introduced into the top of the diamond cell to produce interference fringes in the PRESSURE MEASUREMENT BY EQUATION sample chamber as well as the solid samples. Inter- OF STATE OF A FLUID AND HYDROTHERMAL STUDIES USING VISUAL ference fringes produced by laser light reflected OBSERVATION from top and bottom anvil faces. provide a means for monitoring the volume of sample chamber. This When a sample is loaded in a gasketed diamond is made possible by assuming that the refractive anvil cell surrounded by a fluid of known equation index of the homogeneous fluid portion of the sam- of state such as water, the pressure at which an Hydrothermal diamond anvil cell 263

Ceramic Rhenium Sample Tungsten Cement Gasket Chamber Carbide Seats

Chromel-Alumel (K-type) Thermocouples

nOVAC

Digital Thermometers

o 1 2 3 4

Variable /' I Resistors

~llg nOVAC

Isolation Variable Transformer Transformer

FIG. 2. Schematic diagram showing details of the sample, diamond anvils, heaters, and the electric circuits for heating, controlling, and measuring the temperature of the sample.

observation is made can be ascertained by the fol- during the cooling part of the cycle during which lowing steps: (1) determine the density of the fluid there is much less change of the gasket thickness. by measuring the temperature of a suitable transi- The path that is typically followed through the P- tion such as homogenization or melting using the T-p diagram is illustrated in Fig. 4. equation of state or P- T -p diagram of the fluid (Fig. 3); (2) monitor the sample chamber size to see Independent Check of the Equation of State that it does not change significantly by observing of H20 interference fringes caused by reflected laser light from the upper and lower anvil faces; (3) measure At Cornell University we have used the HDAC to the temperature at which you make an observation compare the equation of state of H20 with the P-T on your sample; (4) using the equation of state or boundary of the a-f3 transition in quartz. The sample P-T-p diagram of the fluid, solve for the pressure. for these experiments consisted of a doubly-polished In practice the most accurate results are obtained quartz platelet, distilled, deionized water, and an air 264 W. A. Bassett et al.

rapid solution of the quartz crystal took place be- 100 .60 fore the a-f3 transition in the quartz could be observed. These techniques are described in more detail in BASSETTet at. (1993) and SHENet al. (1992) . .55 Precision and Accuracy .50 The thermocouples are not subject to any pressure-induced error because they are external and therefore not pressurized. Temperature can be measured with a precision of ±O.l°C over the range of

30 temperature from -110°C to 380°C. The accuracy of our measurements is ±O.SOC over the same 20 range. At temperatures from - 190°C to - 110 1°C and above 380°C, the precision is ±0.5°C and the l:t I Ll~ accuracy is ± 1.SOC. 50 100 150 200 250 300 350 400 450 The accuracy of a pressure determination de- T,oC pends on the accuracies of the temperature mea-

surements and the equation of state of H20 (Fig. FIG. 3. Pressure-temperature plot of the equation of 3) or other fluid being used as pressure medium. state of H20. LV represents the liquid-vapor coexistence curve and CP represents the critical point. The isochores The accuracy for our homogenization temperature are labeled by density (p) in g/cnr'. measurements in H20 is believed to be ±O.soC, which corresponds to a maximum density uncer- tainty of ±0.004 (g/cnr'), When all of the uncer- tainties are collected, this method is considered to bubble (Fig. 5). The sample was then heated until the bubble disappeared (homogenization). As the sample provide pressure determinations with an accuracy was further heated, both the temperature and pressure of 1% in the range of temperatures and pressures up to 1000°C and 0.5 GPa respectively. of the homogeneous fluid increased. This was contin- ued until sudden motion of interference fringes was NEW SOLID PRESSURE CALIBRANTS USING observed indicating the a-f3 transition. The sample VISUAL OBSERVATION was then cooled until a vapor bubble reappeared. If the interference fringes produced by reflection from Three new pressure calibrants have been charac- the upper and lower anvil faces and the outline of the terized in the HDAC using H20 as the pressure chamber hole indicated insignificant change during medium. They are BaTi03 (CHOU et al., 1993), cooling, the volume of the sample chamber was as- Pb3(P04)2 (CHOU and NORD, 1994) and PbTi03 sumed to have remained constant and, therefore, to (CHOU and HASELTON, 1994), and their applicable have been isochoric. The sample was then slowly temperature ranges are below about 120, 180, and reheated until the vapor bubble disappeared, and the 490°C, respectively (Fig. 6). The common features temperature at the moment of bubble disappearance for these calibrants include: (1) they are all based was measured. If there was no loss of fluid during on ferroelastic phase transitions that are rapid and the experiment, this new homogenization temperature can be easily and precisely determined visually; is typically found to be lower than the starting ho- (2) the transition temperatures decrease as pressure mogenization temperature indicating an increase in increases, such that the phase boundaries in P- T density due to relaxation of the gasket during the space intersect isochores of common geologic flu- heating. The isochore density was determined from ids at high angles (Fig. 6) and, thus, facilitate their this homogenization temperature and the transition application; and (3) that they are inert in most pres- pressure was determined from the temperature along sure media in a wide P- T range. the isochore at the time fringe movement in the quartz To apply these calibrants in the HDAC, a doubly was observed. Our results (SHEN et al., 1992) show polished platelet or a cleavage fragment (typically 200 that the equation of state of H20 of HAARet al. (1984) x 200 x 40 µm) is loaded with a fluid pressure medium is in good agreement with the a-f3 quartz boundary in the HDAC. The tetragonal-cubic phase transition of MIRWALDand MASSONNE(1980) for several dif- for BaTi03 and the monoclinic-trigonal transition for ferent isochores. PblP04)2 can both be detected by the disappearance Isochores of increasing density were studied this and reappearance of twin boundaries during heating way until temperatures required were so high that and cooling, respectively. The hysteresis is less than Hydrothermal diamond anvil cell 265

1000

900 ~ ro e, 800 - - ~ 700 '-" D il.) If)' I-< 600 :::l C/J C/J il.) 500 [ d:: 400 300 I :.·E 200

100

0 0 500 600 700 800 Tee)

FIG. 4. This diagram shows a typical pressure-temperature path of an experiment to examine the a-fJ transition in quartz. Insets show the appearances of the sample chamber at various steps along the path. CP is the critical point, LV is the liquid-vapor coexistence curve, ICh is the isochore followed during heating, IC, is the isochore followed during cooling, Thh is the homogenization temperature during heating, The is the homogenization temperature during cooling, T" is the transition temperature, a is an air bubble, w is water, scf is supercritical fluid, Q is a sample of quartz. The steep straight line represents the a-fJ transition in quartz.

1°C and 0.2°C for BaTi03 and Pb3(P04)2, respectively. EVANS(1961), have different r,values (113-120°C We can determine the transition temperatures (Ttr) with at 1 atm.); these differences may be due to varia- a precision of ±0.2°C and an accuracy of ±0.5°C. The tions in impurity content or local strain. For a spe- tetragonal-cubic phase transition in PbTi03, which is cific region of the crystal in our heating experi- indicated by the movements of phase fronts, was de- ments, we observed T; = 115.3°C at 0.17 MPa

tected optically at a heating or cooling rate of about (1.7 bar) along the liquid-vapor curve of pure H20 l°C/min. The temperature interval in which the phase (SAUL and WAGNER, 1989) and 49.2°C at 1404.8 transition occurs ranges from' 0.8°C to 2.8°C, de- MPa (14048 bar) along the ice VI-water boundary pending on pressure. The temperature at which the (WAGNERet al., 1994). On the basis of these mea- movement of phase fronts starts (or ends) on heating surements, we obtained the relation (or cooling) is taken as the transition temperature [Ttr.h (or Ttr.,)]; these temperatures can be determined with a precision of ±OAoC and an accuracy of ± I.SOC. Results of our calibration are shown in Fig. 6 and where LV represents the liquid-vapor pressure of described in the following sections. pure H20. Observations at intermediate pressures, when compared with H20 equation of state calcula- BaTi03 tions, are consistent with a linear relationship. The Different regions of our BaTi03 sample, synthe- slope of -21.25 MPa/°C (-212.5 bar/X') agrees sized by MATTHIAS (1948) and characterized by very well with the value reported by SAMARA 266 W. A. Bassett et al.

Air bubble

Quartz specimen with The sample used in our calibration experiments interference fringes was synthesized and characterized by BING-NAN SUN (personal communication, 1994). The tetragonal-cubic phase transition temperatures were de-

termined at 1 atm. and along six isochores of H20 in our heating and cooling experiments (CHOU and HASELTON, 1994). Least squares regression of our data give

Ptr.h(MPa) = 7021.7-14.235 Ttr.h("C), (2)

Ptr.,(MPa) = 6831.3-14.001 Ttr.,("C), (3)

with the R-squared values of 0.9966 and 0.9987, respectively. For clarity, only the first equation is plotted in Fig. 6. The slopes for the phase boundary are - 14.235 and - 14.001 MPa/OC (- 142.35 and Quartz specimen with interference fringes -140.01 bar/X'), respectively. They are more neg- Interference fringes ative than the -11.91 MPa/°C (-119.1 bar/'C) re- in the fluid ported by SAMARA (1971) for uranium-doped FIG. 5. Typical appearance of a solid sample in liquid PbTi03; the variation may result from the composi- water with an air bubble in a circular rhenium gasket hole tional difference. 500 µm in diameter. The portion of the sample illuminated with laser light shows coarse fringes due to interference between light reflected from the two anvil faces. The fine RAMAN SPECTROSCOPY OF l3C DIAMOND fringes in the solid quartz sample are caused by interfer- AS PRESSURE SENSOR AT HIGH ence between light reflected from the top and bottom of TEMPERATURES: MELT STUDIES OF the platelet. HYDRATED SILICATES

Raman spectroscopy of !3C diamond has been developed as a new method for measuring pressure

(-21.0 MPa/°C with (Ttr)ol MPa = 122°C (-210 barf "C with (Ttr)lbar = 122°C) cited in RAMIREZet at. (1990). The data of DECKERand ZHAO (1989) indicate that the equation given above is probably valid to the tricritical pressure for BaTi03 of about 3.5 GPa (35 kbar). 1.5 P (GPa) 1.0 In our heating experiments, the monoclinic-trigonal transition occurs at 180.2°C and 1.01 MPa 0.5 (10.1 bar) along the L-V curve of H20 and at 49.1°C and 1402.7 MPa (14027 bar) along the ice VI and water boundary. These two P-T points de- fine a linear phase boundary having a slope of 100 700 900 1100 -10.62 MPa/°C (-106.2 bar/'C). Observations at intermediate pressures (along several isochores of FIG. 6. A P- T plot showing the phase relations in pure H20), when combined with H20 equation of state H20 and four pressure calibrants (quartz, PbTi03, calculations, are consistent with this linear relation. Pb3(P04)2, and BaTi03). I, III, V, and VI are ice poly- Our measured T; value agrees very well with the morphs; LV is the liquid-vapor curve of H20; CP is the value of 180AoC at 1 atm. reported by BISMAYER critical point of H20; the long dashed and dotted curves are isochores of H20 derived from equations of state for- and SALlE (1981). This calibrant has a pressure mulated by HAAR et al. (1984) and SAUL and WAGNER resolution of ±2.1 MPa (21 bar), which is twice (1989), respectively. The numbers in the rectangles give the sensitivity of BaTi03 described above. the densities of the isochores in g/cm". Hydrothermal diamond anvil cell 267

at high temperatures in diamond anvil cells (SCHIF- 1332.5 cm " under ambient conditions. This prob- ERL et al., in preparation). When combined with lem persists to pressures above 14 GPa (140 kbar), Raman spectroscopy of 12C diamond, the method where the Raman signal from the sensor chip under becomes a powerful means for measuring pressure hydrostatic pressure moves (at 2.90 ± 0.05 cm-I/ at high temperatures, especially in samples that are GPa) higher and becomes well separated from the corrosive to oxide pressure sensors such as ruby Raman signals of the anvils. and Sm:YAG. We have used the HDAC as well as the Merrill- At the University of Hawaii, experiments are Bassett and Mao-Bell cells for measuring pressure being conducted at simultaneous high P and T con- dependence of the first-order Raman line of syn- ditions in diamond anvil cells for studying minerals thetic 13C-enriched (91 atomic %) diamond film. and hydrated silicate melts. The HDAC style of The pressure dependence of the spectra of small cell as well as an externally heated Merrill-Bassett (20 µm X 20 µm X 20 µm) fragments of the poly- DAC have been adapted for use with a multichan- crystalline diamond film was measured with ruby nel micro-Raman spectrometer with a fiber-optics RI and R2 fluorescence, using either methanol-etha- based technique to acquire pressure sensitive fluo- nol (4:1) mixture or water as pressure transmitting rescence spectra of Sm: YAG simultaneously with medium. At room temperature, 23°C (296 K), the Raman spectra of the sample. High-frequency Ra- Raman line of the 13C-enriched film appears at man spectra of forsterite have been measured to 1287.79 cm ", and a fit of our data and that of 650°C (923 K) and pressures above 8 GPa (80 Muxrov er al. (1994) for the same 13C diamond kbar) (SCHIFERLet al., 1993). film yields a shift of 2.8294 ± 0.047 cm-I/GPa The HDAC is easily mounted on the stage of (SCHlFERL et al., in preparation). The temperature the Raman microscope for optical observation and dependence of the Raman line of the 13Cdiamond Raman spectral measurements of samples pressur- has been measured to 1056°C (1329 K) and is given ized to 10 GPa (100 kbar) and simultaneously by: heated to 900°C (1173 K). With the HDAC we 6 measured Raman spectra of hydrous potassium sili- V(T) = 1294.5 - 0.022 T - 3.8 X 10- T2, cate and hydrous albite to 10 GPa (100 kbar) at (4) room temperature. We have also measured spectra of hydrous potassium silicate melts to 550°C (823 where V is the frequency in cm " and T is the K) and at pressures of about 0.7 GPa (7 kbar) but temperature in Kelvin (K = "C + 273.13). Using encountered technical difficulties. One of the major these calibrations, the 13C-diamond chip provides problems with hydrous silicate systems is that these the first means of measuring pressure up to 16 GPa are extremely corrosive, even at moderate tempera- (160 kbar) accurately (±0.3 GPa or ±3 kbar) at ture, and would invariably react with either ruby or temperatures as high as 927°C (1200 K) for sam- Sm:YAG pressure calibrants. Furthermore, silicate ples in corrosive environments. Above 14 GPa glasses and melts are very weak Raman scatterers. (140 kbar), the Raman spectrum from the 12Cdia- In order to overcome these technical difficulties, mond chip under hydrostatic pressure comes out we have refined the confocal micro-Raman tech- from under the diamond-anvil Raman line. Re- nique so that it can be used with diamond anvil cently, the temperature dependence of the Raman cells for recording spectra of very weak Raman line of natural' diamonds has been measured to scatterers (SHARMAetat., 1994), and developed the 1627"C (1900 K) (HERCHEN and CAPPELLI, 1991; 13C/12Cdiamond Raman sensor system for measur- ZOUBOULISand GRIMSDITCH, 1991). The 13C/12C ing pressure for all samples under any simultaneous Raman pressure sensor system is now ready for P- T conditions that can be achieved in externally measuring simultaneous high temperature and high heated diamond-anvil cells (SCHIFERL et al., in pressure samples in diamond anvil cells. preparation). The use of the Raman spectra of natural diamond IN SITU MONITORING OF ORGANIC (99% 12C and 1% 13C) for measuring pressure has TRANSFORMATION USING LUMINESCENCE AND FTIR SPECTROSCOPY been suggested by a number of authors (SHERMAN, 1985; HANDFLAND et al., 1985; SHARMA et al., In the research lab of the Exxon Production Re- 1985; ALEKSANDROVet al., 1987; TARDIEU et al., search Company we have developed techniques to 1990). However, even with the micro-Raman tech- monitor in situ organic transformation using visual nique it is very difficult to separate the Raman observation, fluorescence, and infrared spectros- spectrum of the sensor chip from the strong Raman copy. The responses of organic materials to heating signal of diamond anvils both of which appear at were observed in real-time using a microscope 268 W. A. Bassett et al. equipped with a charge couple device (CCD) video min. The temperature was then held at 450°C for monitoring system, and fluorescence was moni- 45 minutes. The results show that (1) a progressive tored using a luminescence spectrometer. Progres- decrease in the absorption band ratio of carbonyl sive changes of functional groups were monitored or carboxyl C=O stretching (~1710 cm ") to aro- during transformation by micro-fourier transform matic C=C stretching (~1630 crn "); (2) the peak infrared spectroscopy (micro-FTIR). Each spec- intensities for both aliphatic and aromatic groups trum accumulated from 50 scans was recorded with progressively decrease with maturity, suggesting a spectral resolution of 4 cm " in the 700-4000 the cracking of oil to gas; (3) the aliphatic/aromatic 1 em -I range. Experiments were conducted in open band ratio (1460 cm- 11630 cm ") increases sig- and closed systems at anhydrous or hydrous condi- nificantly (about two-fold) during the early and tions for kerogen, lignite, and crude oil samples. middle stages of maturation then decreases slightly The merit of the HDAC over conventional hot- with increasing maturity; (4) the methane absorp- stage methods is its capability of conducting exper- tion band at ~3020 crn " was detected by probing iments at hydrous and high-pressure conditions. the bubble space of the sample during cooling. The Preliminary results illustrate the capabilities of the progressive variation of the functional groups in technique to examine reaction pathways and the the samples during' pyrolysis suggests that FTIR timing and rates of organic transformations. The spectra can be used to follow the kinetics of oil visual experiments provide direct observations and cracking. estimations of the temperature (or timing) and yields of liquid and gas generation at a variety of Monitoring timing of gas generation during experimental conditions. pyrolysis of coals

A low maturity coal (lignite with vitnmte re- Pyrolysis of kerogen flectance (Ro) = 0.31 and hydrogen index (HI) = 130) was heated at a rate of 2SOC/min. to 380°C A concentrated kerogen from shale of the Green and the temperature was then held for 22 hours River Formation (Eocene) was heated at a rate of before cooling. The vapor generated within the 25°C/min to 380°C. The temperature was then held sample chamber has been probed. The vapor spe- at 380°C for 118 hours. The results show a succes- cies were identified from their designated func- sive change in spectrum distribution in the range tional groups. The results (Fig. 7) show that gas of 1700 to 3100 cm ". Deconvolution of the spec- generation occurring from early to late follows the trum in this range using curve fitting has been per- sequence: CO , carbonyl-carboxylate vapors, hy- formed to calculate five aliphatic C-Hx stretch- 2 droxyl-vapor, hydrocarbon wet gases and carbon ing bands (CH3(asym), CH2(",ym), CH, CH3(sym) and monoxide. This technique can be an effective tool CH2(sym)) and one aromatic C-H stretching band. for studying the timing of generation of gases from The dramatic increase in the CH3(asym/CH2(asym)in- source rocks. This technique is particularly useful tensity ratio of asymmetric aliphatic stretching for identifying the type of coals or kerogens which bands at the late stage of pyrolysis indicates that can thermally generate hydrocarbon gases at low the cleavage of aliphatic chains occurs during the maturity levels or for quantifying the effect of geo- kerogen transformation (LIN and RITZ, 1993). The logical catalysts on generating premature gases trend of the aromatic/aliphatic stretching band ratio, which first increases then dramatically de- from coals or kerogens. creases, is probably attributable, respectively, to ION-PAIRING IN MgS04 SOLUTIONS BY the formation of the alkyl-substituted aromatic hy- RAMAN SCATTERING drocarbons and the rapid cracking of aliphatic chains in the aromatic compounds at the late stage Gaskets lined with platinum were used to contain of pyrolysis (BENKHEDDAet al., 1992). This prelim- chemically reactive samples of MgS04 (FRANTZet inary study shows that in situ FTIR spectroscopic al., 1994). A 0.5 m solution of MgS04 was ana- analysis during HDAC experiments has a potential lyzed using a micro-Raman spectrometer while to monitor the reaction pathways or derive kinetic conditions were changed along an isochore from information during kerogen transformation. room temperature to 500°C and 1.1 GPa (11 kbar). Relative abundances of species and the extent of ion pairing of MgS04 were measured along an iso- Cracking of crude oil chore of ~ 19/cm3. Ion pairing was observed to A crude oil sample from Gwydr Bay (API grav- increase with increasing pressure and the mass- ity = 31) was heated to 450°C at a rate of 25°C/ action constants for the reaction Mg+2 + SO~- Hydrothermal diamond anvil cell 269

2 Q) o s::: m .c o"- en 1 «.c

4000 3000 2000 1000 Wavenumbers (cm-t)

FIG. 7. Infrared spectra showing the generation of CO2, carbonyl-carboxylate vapors, hydroxyl- vapor, hydrocarbon wet gases and CO during heating of a lignite sample.

= MgS04 were found to range from 35 at 20°C tive charges on the sheets, charges that are balanced and 0.1 MPa (1 bar) to 1.25 x 104 at 500°C and by cations filling the spaces between the sheets. 1.14 GPa (11.4 kbar). These results are in good Because of the low charges only a small number agreement with those obtained by RITZERT and of cations fill the spaces and when they fill the FRANCK (1968) using electrical conductance spaces, they carry with them envelopes of H20 measurements. molecules. These H20 molecules reside between This trend of MgS04 to become a weaker elec- the sheets forming layers of water resulting in d- trolyte under hydrothermal conditions indicates a spacings along the c-axis of 12.5A, 15A, and 19A. greater disruption of the relatively weak ion-water These cations and their accompanying water mole- dipole bonds thus allowing a higher frequency of cules are readily exchanged when other cations are ion-ion interactions that result in ion pairing. This made available in solution. study of the effect of pressure and temperature on Combining the HDAC with the very intense syn- ion-water dipole bonds provides information that chrotron radiation available on the white x-ray helps us to better understand viscosity as a function beamline at the Cornell High Energy Synchrotron of pressure and temperature, a relationship that Source (CHESS) has enabled us to make real-time plays an important role in heat and mass transfer measurements of changes in the hydration states in the Earths crust and mantle by flow of aqueous of montmorillonites in response to changes in pres- fluids. sure and temperature. The sample consists of a grain of montmorillonite, distilled H20, and an air bubble. Pressure can be determined from the equa- HYDRATION AND DEHYDRATION OF CLAYS USING SYNCHROTRON RADIATION tion of state of the water as described in an earlier section. Energy dispersive x-ray diffraction pat- The montmorillonite members of the clay family terns of our sample were obtained by passing a of minerals contain large quantities of water and white beam of x-rays through the diamond anvils are abundant in sediments. Therefore, they are as the sample was heated and compressed. X-rays among the most important hydrous phases to un- diffracted by our sample were collected by an in- derstand. These are sheet silicates with low nega- trinsic germanium detector placed at a 2(J angle of 270 W. A. Bassett et al.

30 (Fig. 8). Less than a minute was required to dOOl collect enough information to measure d-spacings. l8.808A This allows us not only to observe the changes in the hydration states of water but also the response rate when temperature and pressure are changed. The three prominent hydration states of montmorillonite are easily recognized from the c-axis d- spacings. Dehydration and rehydration experiments have been performed on montmorillonite with Ca, Mg, and Na cations in the exchange positions. Our results show that the 19A hydrate is stable up to 200-380°C when the water pressure is greater than

that along the H20 liquid-vapor (LV) curve. The 15A hydrate is stable up to 450-600°C. At pressures greater than the LV curve the dehydration temperatures are not very sensitive to pressure but are sensitive to the interlayer cation species (Fig. 9). Increasing ionic potential (charge/radius) is cor- related with a decrease in the temperature of the 19A-15A transition and an increase in the temperature of the 15A-12.5A transition. The effect of pressure and temperature on the hydration states of montmorillonite based on our measurements are compared in Fig. 10 with a recently published thermodynamic model proposed by RANSOMand HELGESON(1995). In their model Ransom and Helgeson propose a starting hydration state of 15A and a 50% dehydration by approximately 180°C during burial. In contrast, our measurements indicate that the starting hydration state

3.0

2.5

.~c;] B 0 0... ·2 1.5 ...... 0

o 10 20 30 40 50 60 Energy (Ke V) 100 200 300 400 500 600 700

FIG. 8. A series of diffraction patterns of Ca-montmoril- Temperature (0C) lonite as the sample is heated and then cooled along the 1.053 g/cnr' isochore. FIG. 9. Plot of the dependence of the 19A-15A and 15A-12.5A transitions in montmorillonite as a function of ionic potential (ionic charge/radius). Hydrothermal diamond anvil cell 271

250 CONCLUSIONS 3 to 2 start 3 to 2 middle 3to 2 end The hydrothermal diamond anvil cell (HDAC) 0 2 to 1 (K&G) has found a wide variety of applications in the ~1 study of geological samples employing a variety of observational techniques where there is need to 150 make observations and measurements at simultaneous high pressure and high temperature. A new P (MPa) paper by CHOUet al. (1995), reports the use of the o 100 HDAC to study eutectic melting of the assemblage

Ca(OHh + CaC03 with excess H20 and lack of evidence for' 'portlandite II" phase. Another study 50 o o is under way at the Bayerisches Geoinstitut in Bayreuth, Germany by Spetzler, Shen, and Angel to develop techniques for making ultrasonic veloc-

100 200 300 400 500 600 ity measurements as a function of pressure and temperature in the HDAC with gigahertz sound Temperature (Oe) waves. Judging from the interest in the HDAC, FIG. 10. The two dotted lines bracket our measured many more novel applications will be developed dehydration from 19A to IsA hydrates and the open cir- in the years ahead. cles indicate dehydration from IsA to 12.sA hydrates in montmorillonites (KOSTER VANGROOSand GUGGENHEIM, 1987). These data are compared with the recently pro- Acknowledgements-We wish to thank the Exxon Pro- posed 50% dehydration (dashed line) assuming an initial duction Research Company, Houston, TX for supporting IsA hydrate (RANSOM and HELGESON, 1995). Heavy solid the research on clays and organic samples. Some of the lines show two versions of geotherms from BURST (1969) research was conducted at the Geophysical Laboratory of and HOWER et al. (1976). the Carnegie Institution of Washington and in US Geolog- ical Survey (USGS) research labs. Most of the rest of the technique development and research reported in this paper was supported by diverse grants from the National Sci- is 19A and that dehydration to 15A does not occur ence Foundation over a period of several years. We wish until approximately 200-380°C and the dehydra- to thank the staff of the Cornell High Energy Synchrotron tion to the 12.5A hydrate does not occur until 450- Source for their help in carrying out synchrotron radiation 600°C. Thus, our observations indicate that mont- experiments. morillonite retains more water to higher temperatures than previously thought. These observations REFERENCES are important for sedimentary basin and petroleum ALEKSANDROVI. V., GONCHAROVA. F., ZISMAN A. N. generation modeling. Our results indicate that for and STISHOVS. M. (1987) Diamond at high pressures: any water-releasing reaction shallower than 5 km, Raman scattering of light, equation of state and high the 19A hydrate probably dominates and may be pressure scale. Soviet Phys. JETP 66, 384-390. responsible for the release of 44% more water than BASSETT W. A., SHEN A. H., BUCKNUM M. and CHOU I.-M. (1993) A new diamond anvil cell for hydrother- previously thought. Another consequence of our mal studies to 2.S GPa and from -190 to 1200°C. Rev. observations concerns the mechanical properties of Sci. Instrum. 64, 2340-234S. montmorillonite clay. Our results show that those BENKHEDDAZ., LANDAIS P., KISTER J., DEREPPE J.-M. clays, if they can escape chemical alteration to illite and MONTHIOUSM. (1992) Spectroscopic analysis of by potassium, are expected to remain ductile down aromatic hydrocarbons extracted from naturally and ar- tificially matured coals. Energy and Fuels 6, 166-172. to considerable depths where they may be the best BISMAYERU. and SALJE E. (1981) Ferroelastic phases in candidate to form a good seal for deep oil Pb3(P04)2-Pb3(As04)2; X-ray and optical experiments. reservoirs. Acta Cryst. A37, 14S. Our observation that the 15A hydrate of mont- BURST J. F. (1969) Diagenesis of Gulf Coast clayey sediments and its possible relation to petroleum migration. morillonite is stable up to 450-600°C at pressures Amer. Assoc. Petrol. Geol. Bull. 53, 73-93. higher than the H20 LV curve suggest that clays CHOU I.-M., HASELTONH. T., JR., NORD G. L., JR., SHEN might be transported down subducting slabs before A. H. and BASSETT W. A. (1993) Barium titanate as the remaining water is released. The dehydration a pressure calibrant in the diamond anvil cell. Amer. in subducting slabs is considered to be an important Geophys. Union Trans. 74, l70. CHOU I.-M. and HASELTONH. T., JR. (1994) Lead titanate cause of shallow earthquakes as well as the source as a pressure calibrant in the diamond anvil cell. Abstr., of volatiles that create volcanic activity along some 16th Gen. Mtng., IMA, 74-7S. of the boundaries of tectonic plates. CHOU, I.-M. and NORD G. L., JR. (1994) Lead phosphate 272 W. A. Bassett et al.

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