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Chemical durability of zircon
Nuclear Instruments and Methods in Physics Research B 181 (2001) 408±412
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Chemical durability of zircon Patrick Trocellier *, Robert Delmas CEA-CNRS, Laboratoire Pierre Sue, Cente d'Etudes de Saclay, F-91191 Gif-sur-Yvette Cedex, France
Abstract Zircon ZrSiO4 exhibits a strong structural anity for uranium and thorium together with a very high chemical durability. This makes it a potential crystalline host matrix to immobilize actinides issued from separation of nuclear wastes. Irradiation induces amorphization of the crystalline structure (the metamictization process) and thus may decrease the chemical durability of the material. Leaching tests have been conducted on natural zircons from Brazil and Madagascar at 96°C for a period of 1 month, using deionized water. Leachates have been analysed by inductively coupled plasma mass spectrometry (ICP-MS) and UV-visible spectrophotometry. Zircon solid surfaces have been investigated by coupling scanning electron microscopy and X-ray microanalysis (SEM±EDX) with nuclear microprobe analysis (lPIXE; lRBS and lERDA). From the mass balance between leachates and hydrated surfaces, the probable mechanisms of zircon aqueous alteration are presented and discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 68.45.)v; 81.70.)q; 82.65.)i Keywords: Zircon; Dissolution; Amorphization; Nuclear microprobe analysis
1. Introduction Zirconium silicate (zircon) is assumed to be a potential host matrix for long duration immobilization of actinide [1]. The ®rst reason is the fact that analogue orthosilicates exist for Ce, Th (thorite or huttonite), U (conite) and Pu [2]. The two other main reasons are related, on one hand, to its high resistance to aqueous corrosion: zircons of more than 4 Gy old have been characterized [3,4] and, on the other hand, to its high resistance to radiation damage [5]. This paper describes the characterization of various zircons and discusses *
Corresponding author. Tel.: +33-169-084-530; fax: +33169-086-923. E-mail address: [email protected] (P. Trocellier).
on the ®rst leaching tests performed on natural samples. 2. Materials Natural zircons from Brazil and Madagascar were selected. A commercial zircon powder from CERAC INC.e has been chosen to monitor the data. Synthetic samples prepared using the sol±gel route based on silicon tetraethyl and zirconyl nitrate reaction according to the scheme detailed in [6] have been also considered. The characterization of all the samples has been conducted using X-ray microanalysis (EDX coupled with SEM), instrumental neutron activation analysis (INAA), nuclear microprobe analysis (NMA) and X-ray
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 7 7 - 9
P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
diraction. INAA and EDX data have been detailed in a previous paper [6]. Density and actinide contents (INAA) have also been determined [6]. 3. Leaching conditions Four natural zircons of centimetric size from Brazil and Madagascar have been sawed in two halves. One half of each sample was kept apart and the other half was submitted to static leaching tests at 96°C in demineralized water for 1 month with a solid surface over water volume of about 0:08 cm 1 . At the end of the leaching tests, silicon is analysed in the leachate by UV-visible spectrophotometry and Zr is determined by ICP-MS. Solid surfaces of the two zircons halves (leached and unleached) are then characterized using Rutherford backscattering spectrometry (RBS) [7]. Silicon and zirconium concentration values Ci measured in the leachate have been transformed in elemental release rate Ri values by taking into account the S/V ratio, the respective atomic masses of Si and Zr Mi and the duration (T) of the test, according to the following relation: Ri g cm 2 d 1 Ci mol l 1 Mi V =S cm T d. The results of the leachate analysis corresponding to the alteration test of one of the zircon from Madagascar in deionized water, respectively, give
5 10 5 < CSi < 5 10 4 mol l 1 ; CZr < 4 10 9 mol l 1 ; RSi 1:2 10 6 g cm 2 d 1 and RZr < 1:5 10 10 g cm 2 d 1 . Similar results have been obtained for Brazilian samples.
It cannot be excluded that a part of the discrepancy observed between silicon and zirconium release rates (four orders of magnitude) probably comes from the preferential leaching of silica inclusions as those found by SEM analysis before leaching. Nevertheless, the assumption of a congruent release followed by a reprecipitation of Zr has also to be considered. 4. Nuclear microprobe characterization Fig. 1 compares the typical 3:06 MeV 4 He induced elastic scattering spectrum obtained for a half of a natural zircon from Brazil with a beam
409
spot of 10 10 lm2 , leached in demineralized water with the typical spectrum of the second half maintained unleached. The ®tting procedure is conducted using the SIMNRA software [8]. A good ®tting cannot be obtained without assuming the incorporation of water by the zircon network. The same type of result was obtained on other natural zircons from Brazil or Madagascar (six analysed areas). They roughly con®rm the observations from the leachate analysis. Table 1 gives the data fom lRBS data processing, they derived from an average on ®ve dierent analysed areas. The results concerning the unleached half of zircon from Brazil are in agreement with the following composition: Si 150 mg/g, Zr 489, O 347, Hf 12 and Fe 2. Coincidently, the analysis from the unleached half of zircon from Madagascar has given the same results, except that Hf 14 mg/g and Fe is not detected. The areas analysed onto the leached samples have not been previously investigated before leaching, then the surface compositions deduced from the ®tting procedure include the chemical heterogeneity encountered in natural samples plus the eventual composition shifts due to the leaching process. The expanded uncertainty attached to each value is typically less than 10% for O, H, Si and Zr and about 20% for Fe and Hf. The H/O ratio clearly shows the hydroxylation/ hydratation of the zircon surface (incorporation of OH and H2 O species) for three of the four leached areas. The ®rst leached zone analysed on the zircon from Madagascar (®ve analysed areas) does not show any H incorporation. This result may be related to the observed Zr/Si ratio less than 1, certainly due to the presence of silica islands before and after leaching in the analysed area, as those found by SEM investigations, but the dimensions of these silica islands are not clearly known. Three leached areas give a Zr/Si ratio higher than 1 and a ratio O=Zr > 4 indicating the oxygen enrichment: incorporation of water and surface hydrolysis leading to the formation of soluble Si OH4 and Zr OH4 . Fig. 2 clearly shows the incorporation of hydrogen containing species in the leached Brazilian zircon (RIO 10) by comparing 3.06 MeV 4 He induced lERDA spectra of unleached (RIO 10B)
410
P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
Fig. 1. Comparison of the elastic scattering spectra for two halves 3:06 MeV 4 He ; 10 10 lm2 , 250 pA): (a) unleached part; (b) leached part.
of
a
natural
zircon
from
Madagascar
P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
411
Table 1 Local surface compositions of leached zircons from 4 He lRBS (mg/g) Elements and atomic ratios
Stoichiometric composition
Brazil
Si Zr Hf O H (Zr + Hf)/Si O/(Zr + Hf) O/Si H/O Fe
153 498 0 349 0 1 4 4 0 0
115 477 15 382 11 1.30 4.49 5.83 0.457 1
Madagascar 129 485 15 359 9 1.17 4.16 4.88 0.398 2
156 462 12 370 0 0.92 4.51 4.16 0 0
125 482 22 363 8 1.22 4.20 5.1 0.35 0
· a dissolution of silicon at a faster rate than zirconium due to their respective solubility, starting from a stoichiometric composition 1:1; · a congruent dissolution of the zircon together with a reprecipitation of a Zr-enriched phase, as assumed by Helean [4] and according to the mechanism proposed by Tole [9], ZrSiO4 4H2 O ! Zr OH4 H4 SiO4 Zr OH4 ! ZrO2 2H2 O
Fig. 2. Comparison of the elastic recoil detection spectra for two halves of a natural zircon: leached RIO 10A, unleached RIO 10B (3:06 MeV 4 He , 10 10 lm2 , 250 pA; h 30°).
and leached samples (RIO 10A). Moreover, on RIO 10A the hydration process is not homogeneous. The typical hydrogen content, monitored using a mica muscovite as hydrogen standard, is evaluated to 0.8% at the surface and 0.5 at.% at 0.3 lm depth in the hydrated zone, and less than 0.3 at.% in the bulk zircon (>0.5 lm). 5. Discussion A Zr/Si ratio higher than 1 in the near-surface region of the leached samples can be explained by two dierent scenarios:
Working with 4 He ions at 3.06 MeV leads us to a depth resolution of about 20 nm near the surface with an incident beam normal to the target surface. lRBS measurements show that the near surface zone aected by the leaching process has a thickness higher than the analysed depth (around 1:2 lm) and that the existence of composition gradients is not evidenced (the best ®t always assumes constant contents of Si and Zr). RBS results obtained for leached zircons have been interpreted in terms of surface material balance (molar fractions) as presented in Table 2. The most probable surface compounds are (Hf,Zr)SiO4 ; Hf; Zr OH4 and H2 O for Brazil 1 and 2 and Madagascar 2. These calculations support quite well the above comments except for Madagascar 1, for which no incorporation of water was shown. Then, two dierent hypotheses have been formulated for the analysed zone 1 on Madagascar sample because of the non-incorporation of water: zircon unaltered or simultaneous presence of zirconia and silica islands.
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P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
Table 2 Assumed surface composition of leached natural zircons (molar fraction) Compounds
Hf; ZrSiO4 Hf; Zr OH4 Hf; ZrO2 SiO2 H2 O a b
Brazil ®rst spota
0.50 0.14 0 0 0.36
Brazil second spota
0.71 0.11 0 0 0.18
Madagascar ®rst spotb First hypothesis
Second hypothesis
0.91 0 0 0.09 0
0 0 0.48 0.52 0
Madagascar second spota 0.66 0.13 0 0 0.21
A small fraction of H remains in excess. A small fraction of O remains in excess.
6. Conclusion Natural zircons from Brazil and Madagascar have been characterized by combining several analytical methods such as INAA, SEM coupled with X-ray microanalysis and NMA. Aqueous leaching tests have been carried on to study the basic alteration mechanisms of zircon. The leaching mechanism seems to involve both a selective release of silica as measured by ICP-MS and spectrophotometry in the leachate and a surface hydration as observed in lRBS and lERDA characterization of the solid. The local formation of a thick homogeneous hydrolysed/hydrated layer with a Zr/Si atomic ratio generally higher than 1 has been found. This behaviour could be interpreted either in terms of preferential silica release or congruent dissolution followed by Zr species precipitation. Natural zircons are intrinsically heterogeneous. This heterogeneity has a chemical and radiation damage origin. The volumic distribution of impurities in the crystal is non-uniform due to the events occurred during zircon genesis and radiation damage by a rays and recoil nuclei follows the distribution of U and Th. This heterogeneity may have blurred the variations of composition induced by
leaching, preventing us to conclude clearly on the dissolution mechanism. Future work will be devoted to study leaching mechanisms of synthetic zircons and to evaluate the in¯uence of the composition of the leaching solution (pH and presence of a Zr organic ligand for example). References [1] R.C. Ewing, W. Lutze, W.J. Weber, J. Mater. Res. 10 (2) (1995) 243. [2] J.A. Speer, in: P.H. Ribbe (Ed.), Zircon and Actinide Orthosilicates, second ed., Chapters 3 and 4 of Orthosilicates, Reviews in Mineralogy 5 (1982) 67. [3] D.O. Froude, T.R. Ireland, P.D. Kinny, I.S. Williams, W. Compston, I.R. Williams, J.S. Myers, Nature 304 (18) (1983) 616. [4] K.B. Helean, Zircon dissolution. Thesis, The University of New Mexico, March 1998. [5] W.J. Weber, R.C. Ewing, A. Meldrum, J. Nucl. Mater. 250 (1997) 147. [6] P. Trocellier, R. Delmas, J. Tirira, D. Massare, R. Drot, D. Gosset, Waste Manage. (submitted). [7] P. Trocellier, Microsc. Microanal. Microstruct. 7 (1996) 235. [8] M. Mayer, SIMNRA user's guide, Technical report IPP 9/113, 1997. [9] M.P. Tole, Geochim. Cosmochim. Acta 49 (1985) 453.
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Chemical durability of zircon Patrick Trocellier *, Robert Delmas CEA-CNRS, Laboratoire Pierre Sue, Cente d'Etudes de Saclay, F-91191 Gif-sur-Yvette Cedex, France
Abstract Zircon ZrSiO4 exhibits a strong structural anity for uranium and thorium together with a very high chemical durability. This makes it a potential crystalline host matrix to immobilize actinides issued from separation of nuclear wastes. Irradiation induces amorphization of the crystalline structure (the metamictization process) and thus may decrease the chemical durability of the material. Leaching tests have been conducted on natural zircons from Brazil and Madagascar at 96°C for a period of 1 month, using deionized water. Leachates have been analysed by inductively coupled plasma mass spectrometry (ICP-MS) and UV-visible spectrophotometry. Zircon solid surfaces have been investigated by coupling scanning electron microscopy and X-ray microanalysis (SEM±EDX) with nuclear microprobe analysis (lPIXE; lRBS and lERDA). From the mass balance between leachates and hydrated surfaces, the probable mechanisms of zircon aqueous alteration are presented and discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 68.45.)v; 81.70.)q; 82.65.)i Keywords: Zircon; Dissolution; Amorphization; Nuclear microprobe analysis
1. Introduction Zirconium silicate (zircon) is assumed to be a potential host matrix for long duration immobilization of actinide [1]. The ®rst reason is the fact that analogue orthosilicates exist for Ce, Th (thorite or huttonite), U (conite) and Pu [2]. The two other main reasons are related, on one hand, to its high resistance to aqueous corrosion: zircons of more than 4 Gy old have been characterized [3,4] and, on the other hand, to its high resistance to radiation damage [5]. This paper describes the characterization of various zircons and discusses *
Corresponding author. Tel.: +33-169-084-530; fax: +33169-086-923. E-mail address: [email protected] (P. Trocellier).
on the ®rst leaching tests performed on natural samples. 2. Materials Natural zircons from Brazil and Madagascar were selected. A commercial zircon powder from CERAC INC.e has been chosen to monitor the data. Synthetic samples prepared using the sol±gel route based on silicon tetraethyl and zirconyl nitrate reaction according to the scheme detailed in [6] have been also considered. The characterization of all the samples has been conducted using X-ray microanalysis (EDX coupled with SEM), instrumental neutron activation analysis (INAA), nuclear microprobe analysis (NMA) and X-ray
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 7 7 - 9
P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
diraction. INAA and EDX data have been detailed in a previous paper [6]. Density and actinide contents (INAA) have also been determined [6]. 3. Leaching conditions Four natural zircons of centimetric size from Brazil and Madagascar have been sawed in two halves. One half of each sample was kept apart and the other half was submitted to static leaching tests at 96°C in demineralized water for 1 month with a solid surface over water volume of about 0:08 cm 1 . At the end of the leaching tests, silicon is analysed in the leachate by UV-visible spectrophotometry and Zr is determined by ICP-MS. Solid surfaces of the two zircons halves (leached and unleached) are then characterized using Rutherford backscattering spectrometry (RBS) [7]. Silicon and zirconium concentration values Ci measured in the leachate have been transformed in elemental release rate Ri values by taking into account the S/V ratio, the respective atomic masses of Si and Zr Mi and the duration (T) of the test, according to the following relation: Ri g cm 2 d 1 Ci mol l 1 Mi V =S cm T d. The results of the leachate analysis corresponding to the alteration test of one of the zircon from Madagascar in deionized water, respectively, give
5 10 5 < CSi < 5 10 4 mol l 1 ; CZr < 4 10 9 mol l 1 ; RSi 1:2 10 6 g cm 2 d 1 and RZr < 1:5 10 10 g cm 2 d 1 . Similar results have been obtained for Brazilian samples.
It cannot be excluded that a part of the discrepancy observed between silicon and zirconium release rates (four orders of magnitude) probably comes from the preferential leaching of silica inclusions as those found by SEM analysis before leaching. Nevertheless, the assumption of a congruent release followed by a reprecipitation of Zr has also to be considered. 4. Nuclear microprobe characterization Fig. 1 compares the typical 3:06 MeV 4 He induced elastic scattering spectrum obtained for a half of a natural zircon from Brazil with a beam
409
spot of 10 10 lm2 , leached in demineralized water with the typical spectrum of the second half maintained unleached. The ®tting procedure is conducted using the SIMNRA software [8]. A good ®tting cannot be obtained without assuming the incorporation of water by the zircon network. The same type of result was obtained on other natural zircons from Brazil or Madagascar (six analysed areas). They roughly con®rm the observations from the leachate analysis. Table 1 gives the data fom lRBS data processing, they derived from an average on ®ve dierent analysed areas. The results concerning the unleached half of zircon from Brazil are in agreement with the following composition: Si 150 mg/g, Zr 489, O 347, Hf 12 and Fe 2. Coincidently, the analysis from the unleached half of zircon from Madagascar has given the same results, except that Hf 14 mg/g and Fe is not detected. The areas analysed onto the leached samples have not been previously investigated before leaching, then the surface compositions deduced from the ®tting procedure include the chemical heterogeneity encountered in natural samples plus the eventual composition shifts due to the leaching process. The expanded uncertainty attached to each value is typically less than 10% for O, H, Si and Zr and about 20% for Fe and Hf. The H/O ratio clearly shows the hydroxylation/ hydratation of the zircon surface (incorporation of OH and H2 O species) for three of the four leached areas. The ®rst leached zone analysed on the zircon from Madagascar (®ve analysed areas) does not show any H incorporation. This result may be related to the observed Zr/Si ratio less than 1, certainly due to the presence of silica islands before and after leaching in the analysed area, as those found by SEM investigations, but the dimensions of these silica islands are not clearly known. Three leached areas give a Zr/Si ratio higher than 1 and a ratio O=Zr > 4 indicating the oxygen enrichment: incorporation of water and surface hydrolysis leading to the formation of soluble Si OH4 and Zr OH4 . Fig. 2 clearly shows the incorporation of hydrogen containing species in the leached Brazilian zircon (RIO 10) by comparing 3.06 MeV 4 He induced lERDA spectra of unleached (RIO 10B)
410
P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
Fig. 1. Comparison of the elastic scattering spectra for two halves 3:06 MeV 4 He ; 10 10 lm2 , 250 pA): (a) unleached part; (b) leached part.
of
a
natural
zircon
from
Madagascar
P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
411
Table 1 Local surface compositions of leached zircons from 4 He lRBS (mg/g) Elements and atomic ratios
Stoichiometric composition
Brazil
Si Zr Hf O H (Zr + Hf)/Si O/(Zr + Hf) O/Si H/O Fe
153 498 0 349 0 1 4 4 0 0
115 477 15 382 11 1.30 4.49 5.83 0.457 1
Madagascar 129 485 15 359 9 1.17 4.16 4.88 0.398 2
156 462 12 370 0 0.92 4.51 4.16 0 0
125 482 22 363 8 1.22 4.20 5.1 0.35 0
· a dissolution of silicon at a faster rate than zirconium due to their respective solubility, starting from a stoichiometric composition 1:1; · a congruent dissolution of the zircon together with a reprecipitation of a Zr-enriched phase, as assumed by Helean [4] and according to the mechanism proposed by Tole [9], ZrSiO4 4H2 O ! Zr OH4 H4 SiO4 Zr OH4 ! ZrO2 2H2 O
Fig. 2. Comparison of the elastic recoil detection spectra for two halves of a natural zircon: leached RIO 10A, unleached RIO 10B (3:06 MeV 4 He , 10 10 lm2 , 250 pA; h 30°).
and leached samples (RIO 10A). Moreover, on RIO 10A the hydration process is not homogeneous. The typical hydrogen content, monitored using a mica muscovite as hydrogen standard, is evaluated to 0.8% at the surface and 0.5 at.% at 0.3 lm depth in the hydrated zone, and less than 0.3 at.% in the bulk zircon (>0.5 lm). 5. Discussion A Zr/Si ratio higher than 1 in the near-surface region of the leached samples can be explained by two dierent scenarios:
Working with 4 He ions at 3.06 MeV leads us to a depth resolution of about 20 nm near the surface with an incident beam normal to the target surface. lRBS measurements show that the near surface zone aected by the leaching process has a thickness higher than the analysed depth (around 1:2 lm) and that the existence of composition gradients is not evidenced (the best ®t always assumes constant contents of Si and Zr). RBS results obtained for leached zircons have been interpreted in terms of surface material balance (molar fractions) as presented in Table 2. The most probable surface compounds are (Hf,Zr)SiO4 ; Hf; Zr OH4 and H2 O for Brazil 1 and 2 and Madagascar 2. These calculations support quite well the above comments except for Madagascar 1, for which no incorporation of water was shown. Then, two dierent hypotheses have been formulated for the analysed zone 1 on Madagascar sample because of the non-incorporation of water: zircon unaltered or simultaneous presence of zirconia and silica islands.
412
P. Trocellier, R. Delmas / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 408±412
Table 2 Assumed surface composition of leached natural zircons (molar fraction) Compounds
Hf; ZrSiO4 Hf; Zr OH4 Hf; ZrO2 SiO2 H2 O a b
Brazil ®rst spota
0.50 0.14 0 0 0.36
Brazil second spota
0.71 0.11 0 0 0.18
Madagascar ®rst spotb First hypothesis
Second hypothesis
0.91 0 0 0.09 0
0 0 0.48 0.52 0
Madagascar second spota 0.66 0.13 0 0 0.21
A small fraction of H remains in excess. A small fraction of O remains in excess.
6. Conclusion Natural zircons from Brazil and Madagascar have been characterized by combining several analytical methods such as INAA, SEM coupled with X-ray microanalysis and NMA. Aqueous leaching tests have been carried on to study the basic alteration mechanisms of zircon. The leaching mechanism seems to involve both a selective release of silica as measured by ICP-MS and spectrophotometry in the leachate and a surface hydration as observed in lRBS and lERDA characterization of the solid. The local formation of a thick homogeneous hydrolysed/hydrated layer with a Zr/Si atomic ratio generally higher than 1 has been found. This behaviour could be interpreted either in terms of preferential silica release or congruent dissolution followed by Zr species precipitation. Natural zircons are intrinsically heterogeneous. This heterogeneity has a chemical and radiation damage origin. The volumic distribution of impurities in the crystal is non-uniform due to the events occurred during zircon genesis and radiation damage by a rays and recoil nuclei follows the distribution of U and Th. This heterogeneity may have blurred the variations of composition induced by
leaching, preventing us to conclude clearly on the dissolution mechanism. Future work will be devoted to study leaching mechanisms of synthetic zircons and to evaluate the in¯uence of the composition of the leaching solution (pH and presence of a Zr organic ligand for example). References [1] R.C. Ewing, W. Lutze, W.J. Weber, J. Mater. Res. 10 (2) (1995) 243. [2] J.A. Speer, in: P.H. Ribbe (Ed.), Zircon and Actinide Orthosilicates, second ed., Chapters 3 and 4 of Orthosilicates, Reviews in Mineralogy 5 (1982) 67. [3] D.O. Froude, T.R. Ireland, P.D. Kinny, I.S. Williams, W. Compston, I.R. Williams, J.S. Myers, Nature 304 (18) (1983) 616. [4] K.B. Helean, Zircon dissolution. Thesis, The University of New Mexico, March 1998. [5] W.J. Weber, R.C. Ewing, A. Meldrum, J. Nucl. Mater. 250 (1997) 147. [6] P. Trocellier, R. Delmas, J. Tirira, D. Massare, R. Drot, D. Gosset, Waste Manage. (submitted). [7] P. Trocellier, Microsc. Microanal. Microstruct. 7 (1996) 235. [8] M. Mayer, SIMNRA user's guide, Technical report IPP 9/113, 1997. [9] M.P. Tole, Geochim. Cosmochim. Acta 49 (1985) 453.