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Hydrogen in “anhydrous” minerals
Nuclear Instruments North-Holland
HYDROGEN George
and Methods
in Physics
IN “ANHYDROUS”
Research
B45 (1990) 41-44
MINERALS
*
R. ROSSMAN
Divisionof Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, lJsA
Trace concentrations of water and hydroxide are commonly bound in minerals which are normally considered anhydrous. These hydrous species influence the physical and chemical properties of the host mineral to an extent far disproportionate to their low concentration. These propertiesinclude mechanical strength, internal structural order, optical properties, dielectric constant, response to external ionizing radiation, and internal diffusion. Low concentrations of hydrous species are conveniently detected by infrared spectroscopy,but the IR method is not self-calibrating. Nuclear reaction analysis has proven to be one of the more sensitive methods to determine the absolute concentration of the hydrous species, but in practice, reduced the accuracy of analyses of samples with water contents below 0.1%.
1. Introduction Most nominally anhydrous minerals actually contain trace amounts of hydrous species either in fluid inclusions or as structurally bound, crystallographically oriented components [1,2]. The most important hydrous species usually are the hydroxide ion (OH-) and water (H,O), although in some cases hydrogen is present in the ammonium ion (NH:). Low concentrations of these hydrous species can modify the physical and chemical properties of the host mineral to an extent far out of proportion to their actual concentration. The most prominent role of minor hydrous components has been in the phenomenon of hydrolytic weakening which associates a large decrease in the mechanical strength of crystals with small amounts of internal water [3-51. Most of these studies have been concerned with synthetic quartz [6-81 although the role of water in deformation and flow of olivine, a major component of the earth’s mantle, has also been of current interest [9]. Other phenomena influenced by trace hydrous components are exsolution and cation interdiffusion. These effects have been most recently discussed in regard to the feldspar minerals [lo]. At high temperatures of crystallization, complete solid solutions exist in between the cation pair K and Na. At low temperature, K and Na are thermodynamically more stable in separate phases and slowly diffuse in the solid state into distinct bands of potassium feldspar and sodium feldspar. Hydrous components are highly effective in increasing the
* Research supported Grant EAR-86-12000. 0168-583X/90/$03.50 (North-Holland)
by the National
Science
0 Elsevier Science Publishers
Foundation
B.V.
is subject
to a variety
of limitations
which
have
rate of cation diffusion which leads to ordering and exsolution. Small water contents likewise are of importance for transport experiments in glasses [ll]. Hydrous species in minerals also greatly modify the susceptibility of the host mineral to damage by ionizing radiation [12,13]. In many systems, trace water is effective in quenching or minimizing the effects of such radiation. Frequently color changes are associated with the radiation damage. Amethyst color in quartz is one of the more familiar examples. Hydrous components, especially water, influence the dielectric and other electronic properties of minerals and their technologically important synthetic counterparts. Although early studies focused on quartz, recent studies have been concerned with a wider range of materials such as berlinite (AlPO,) and cordierite (Mg,Al,Si,AlO,s) [14]. The trace element, hydrogen, is also a probe which records the water activity of the growth environment of the host mineral. Initial interest focused on the hydrogen in minerals from the Earth’s mantle because of the suggestion that this hydroxyl might be a significant reservoir of hydrogen in the mantle [15] with its consequent effect on the rheology of mantle minerals. Significantly, in many instances, mantle minerals are the most hydrogen-rich of their species [16-181. Water in rocks and melts is also important. The initial melting temperature of rocks is strongly depressed by water [19], and the viscosity of melts is dependent upon the extent of silicate polymerization which is, in turn, depends on the water content of the melt. Many recent studies have been concerned with the concentration and speciation of water in vitrified melts [20,21] because they provide samples of the volatile chemical components in magma chambers. I. NRA
G.R. Rossman / Hydrogen in “anhydrous” minerals
42
2. Quantification at low H concentrations As these examples show, the presence of hydrous components is established in many mineral systems, and their association with a variety of properties is amply demonstrated. Still there are a variety of analytical problems which remain to be solved. One of the more important is the difficulty surrounding the quantification of low concentrations of hydrogen. Precise hydrogen contents lead to insight into the c~stallograp~c details of hydrogen incorporation and the associated chemical substitutions. A variety of methods have
proven successful for the determination of water contents in minerals at the 0.1 wt.% or higher level such as nuclear methods, hydrogen manometry, proton NMR and ion microprobe analysis [22-241, but precise and reproducible analyses of low concentrations (-z 0.01 wt.% as H,O) of hydrogen in single crystals has proven to be surprisingly difficult in our experience. In collaboration with research groups at Caltech, Frankfurt, Brookhaven and Albany, we have been concerned with the deter~nation of the OH content of natural garnets using nuclear analytical methods. Garnets span a range of chemical compositions and
0.10
a.3
).
a
Grosaular-andradite Garnets
,
Pyrope-almandine Garnets
b :
0.2 x i
*
.
3
.
z
2
z?
‘I
0.04 -
.
* 0.1
0.02-
.
*
0.
.
. 0.0 0
2
4
6
8
IO
Spessartite Garnets
12
0.05
0.10
0.15
Infrared Peak Intensity
Infrared Peak Intensity
c
Infrared Peak Intensity
Fig. 1. Correlationsbetween the hydrogencontent (expressed as H,O) of suites of garnets and the infrared peak
intensityin the OH region. Tb.e IR intensities should be proportional to the OH concentration. Circles: “N analyses; triangles: “F analyses; diamonds: elastic recoil analysis. (a) The grossular to andradite solid solution series: Ca,A12(Si0,),-Ca3Fe2(Si0,),. (b) The pyrope to ahnandine solid solution series: Mg3A12(Si04),-Fe,Al,(SiO,),. (c)The spessartite to almmdine solid solution series: ~~~~(SiO~)~-Fe~~~(SiO~)~.
G. R Rossman / Hydrogen in “anhydrous” miner&
come from a wide variety of geologic environments. They can incorporate varying, but usually minor, amounts of OH- in part in substitution as 4 OHgroups replacing SiO:- . Such traces of OH- in garnets estimated to have concentrations down to 0.001 wt.% {as H20) have been readily identified with infrared (IR) spectroscopy. IR spectroscopy is a sensitive technique for detecting the OH content of garnets, but is not intrinsically self-calibrating. Each type of garnet, and each other type of miaeral as well, requires its own calibration of the IR spectrum. In principle, the nuclear analytical methods should be an ideal way of establishing the absolute calibration of the IR spectra. Garnets chosen for hydrogen analysis by nuclear methods were usually of gem or near-gem quality with a minimum of cracks or internal defects. They were usually prepared by grinding and polishing two opposite and parallel surfaces through which the infrared spectra were obtained [17]. Nuclear analyses were run with 19F, 15N and elastic recoil analyses. Only a few of these rest&s have been previously published [22]. Figs. la-c summarize our c~ulative experience with the nuclear methods. In these figures the hydrogen content is expressed as H,O and plotted against the intensity of the infrared OH- absorption band in the 3600-3400 cm-’ region. These plots are based on the assumption that the IR intensity is directly proportional to the OH- content of the garnet. Although this assumption has never been proven for very low OHcontent garnets, it has been established with high OHcontent garnets where the OH- content can be established by independent methods. The results of the nuclear analyses show considerable scatter. In some instances, the same sample has been submitted for multiple analyses, often with comparable scatter. Of particular concern is the occasionally high hydrogen content for samples with very little infrared absorption. As fig. lb shows, in some cases when such samples have been reanalyzed they have yielded much lower hydrogen contents. The garnets chosen for these studies are among the best available for such study. They are 4 mm or larger, homogeneous, and of comparatively high OH- content. Most natural mineral samples which would need analysis are smaller as a result of natural grain size or internal heterogeneity. They also would typically have less than one-third the l&O content of the highest water content samples on the figures. The results to date indicate that the nuclear analyses for hydrogen have the potential to address these mineralogical problems, but are not yet routinely obtainable at the accuracy needed. In some instances, careful work has produced reliable calibration data with acceptable limits of uncertainty 1221. Several problems are evident. First there are the problems of preparing a suitable mineral surface which
43
is representative of the bulk sample. Very few samples are both large enough and free from internal cracks, fluid inclusions and solid inclusions which themselves may contain hydrous components. Future requirements will clearly go in the direction of ever smaller beam-spot sizes. Generally, a flat surface cannot be cleaved, so the sample must be ground and polished to expose the interior of the crystal. There is the risk that the preparation method may either hydrate the new surface or induce microscopic imperfections which could retain water or hydrous alteration products. Although some of the anomalously high hydrogen values might have resulted from such surface contamination, we have obtained enough analyses which indicate that once the analyzed region reaches about 1000 A below the surface, preparation-induced hydration is not normally a problem. There are also problems associated with the nuclear analytical methods working with very low levels of hydrogen. Very low instrument background is important for these analyses, but has been difficult to obtain in practice. Background cont~butions can arise from materials of construction in the sample chamber and unshielded ambient radiation. For applications to a variety of mineralogical problems, extremely low background count rates will be needed in future work.
References [l] RD.
Aines and G.R. Rossman, J. Geophys. Res. 89 (1984) 4059. [2] G.R. Rossman, Rev. Mineral. 18 (1988) 193. [3] D.T. Griggs, J. Gcophys. Res. 79 (1974) 1653. [4] S.H. Kirby and J.W. McCormick, Bull. Mineral. 102 (1979) 124. [5] P. Cordier and J.C. Doukhan, Chem. Geol. 70 (1988) 158. [6] R.D. Aines, S.H. Kirby and G.R. Rossman, Phys. Chem. Mineral 11 (1984) 204. [7] A.K. Kronenbcrg, S.H. Kirby, R.D. Aines and G.R. Rossman, J. Geophys. Res. 91 (1986) 12, 723. [S] A.C. McLaren, J.D. Fitz Gerald and J. Gerretsen, Phys. Chem. Mineral. 16 (1989) 469. [9] S. Mackwell, D.L. Kohlstedt and MS. Paterson, J. Geephys. Res. 90 (1985) 11319. [lo] J.R. Goldsmith, Contrib. Mineral. Petrol. 95 (1987) 311. [ll] H. Schafer, Z. Anorg. Alg. Chem. 543 (1986) 217. [12] A.M. Hofmeister and G.R. Rossman, Amer. Mineral. 70 (1985) 794. 1131 R.D. Aines and G.R. Rossman, Amer. Mineral. 71 (1986) 1186. [14] I.M. Sil’vestrova, Y.V. Pisarevskii, O.V. Zvereva and A.A. Shtemberg, Sov. Phys. Crystallogr. 32 (1987) 467. [15] L. Ackermann, L. Cemic and K. Langer, Earth Planet. Sci. Lett. 62 (1983) 208. 1161 G.H. Miller, G.R. Rossman, and G.E. Harlow, Phys. Chem. Mineral. 14 (1987) 461. 1171 R.D. Aines and G.R. Rossman, Geology 12 (1984) 720. I. NRA
44
[18] A. Beran and M.A. G&zinger,
G.R Rossman / Hydrogen in “anhydrous” minerals
Mineral Petrol. 36 (1987) 41. [19] C.R. Stem and P.J. Wyllie, Earth Planet. Sci. Lett. 28 (1975) 189. [ZO] A.T. Anderson, S. Newman, S.N. Williams, T.H. Druitt, C. Skirius and E. Stolper, Geology 17 (1989) 221. [21] S. Newman, E.M. Stolper and S. Epstein, Amer. Mineral. 71 (1986) 1527.
[22] G.R. Rossman, F. Rauch, R. Livi, T.A. Tombrello, C.R. Shi and Z.Y. Zhou, Neues Jahrbuch fiir Mineralogie, Mh. (1988) 172. f23] V.I. Kovalenko, R.L. Hervig and M.F. Sheridan, Amer. Mineral. 73 (1988) 1038. [24] J.P. Yesinowski, H. Eckert and G.R. Rossman, J. Amer. Chem. Sot. (1988) 1367.
HYDROGEN George
and Methods
in Physics
IN “ANHYDROUS”
Research
B45 (1990) 41-44
MINERALS
*
R. ROSSMAN
Divisionof Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, lJsA
Trace concentrations of water and hydroxide are commonly bound in minerals which are normally considered anhydrous. These hydrous species influence the physical and chemical properties of the host mineral to an extent far disproportionate to their low concentration. These propertiesinclude mechanical strength, internal structural order, optical properties, dielectric constant, response to external ionizing radiation, and internal diffusion. Low concentrations of hydrous species are conveniently detected by infrared spectroscopy,but the IR method is not self-calibrating. Nuclear reaction analysis has proven to be one of the more sensitive methods to determine the absolute concentration of the hydrous species, but in practice, reduced the accuracy of analyses of samples with water contents below 0.1%.
1. Introduction Most nominally anhydrous minerals actually contain trace amounts of hydrous species either in fluid inclusions or as structurally bound, crystallographically oriented components [1,2]. The most important hydrous species usually are the hydroxide ion (OH-) and water (H,O), although in some cases hydrogen is present in the ammonium ion (NH:). Low concentrations of these hydrous species can modify the physical and chemical properties of the host mineral to an extent far out of proportion to their actual concentration. The most prominent role of minor hydrous components has been in the phenomenon of hydrolytic weakening which associates a large decrease in the mechanical strength of crystals with small amounts of internal water [3-51. Most of these studies have been concerned with synthetic quartz [6-81 although the role of water in deformation and flow of olivine, a major component of the earth’s mantle, has also been of current interest [9]. Other phenomena influenced by trace hydrous components are exsolution and cation interdiffusion. These effects have been most recently discussed in regard to the feldspar minerals [lo]. At high temperatures of crystallization, complete solid solutions exist in between the cation pair K and Na. At low temperature, K and Na are thermodynamically more stable in separate phases and slowly diffuse in the solid state into distinct bands of potassium feldspar and sodium feldspar. Hydrous components are highly effective in increasing the
* Research supported Grant EAR-86-12000. 0168-583X/90/$03.50 (North-Holland)
by the National
Science
0 Elsevier Science Publishers
Foundation
B.V.
is subject
to a variety
of limitations
which
have
rate of cation diffusion which leads to ordering and exsolution. Small water contents likewise are of importance for transport experiments in glasses [ll]. Hydrous species in minerals also greatly modify the susceptibility of the host mineral to damage by ionizing radiation [12,13]. In many systems, trace water is effective in quenching or minimizing the effects of such radiation. Frequently color changes are associated with the radiation damage. Amethyst color in quartz is one of the more familiar examples. Hydrous components, especially water, influence the dielectric and other electronic properties of minerals and their technologically important synthetic counterparts. Although early studies focused on quartz, recent studies have been concerned with a wider range of materials such as berlinite (AlPO,) and cordierite (Mg,Al,Si,AlO,s) [14]. The trace element, hydrogen, is also a probe which records the water activity of the growth environment of the host mineral. Initial interest focused on the hydrogen in minerals from the Earth’s mantle because of the suggestion that this hydroxyl might be a significant reservoir of hydrogen in the mantle [15] with its consequent effect on the rheology of mantle minerals. Significantly, in many instances, mantle minerals are the most hydrogen-rich of their species [16-181. Water in rocks and melts is also important. The initial melting temperature of rocks is strongly depressed by water [19], and the viscosity of melts is dependent upon the extent of silicate polymerization which is, in turn, depends on the water content of the melt. Many recent studies have been concerned with the concentration and speciation of water in vitrified melts [20,21] because they provide samples of the volatile chemical components in magma chambers. I. NRA
G.R. Rossman / Hydrogen in “anhydrous” minerals
42
2. Quantification at low H concentrations As these examples show, the presence of hydrous components is established in many mineral systems, and their association with a variety of properties is amply demonstrated. Still there are a variety of analytical problems which remain to be solved. One of the more important is the difficulty surrounding the quantification of low concentrations of hydrogen. Precise hydrogen contents lead to insight into the c~stallograp~c details of hydrogen incorporation and the associated chemical substitutions. A variety of methods have
proven successful for the determination of water contents in minerals at the 0.1 wt.% or higher level such as nuclear methods, hydrogen manometry, proton NMR and ion microprobe analysis [22-241, but precise and reproducible analyses of low concentrations (-z 0.01 wt.% as H,O) of hydrogen in single crystals has proven to be surprisingly difficult in our experience. In collaboration with research groups at Caltech, Frankfurt, Brookhaven and Albany, we have been concerned with the deter~nation of the OH content of natural garnets using nuclear analytical methods. Garnets span a range of chemical compositions and
0.10
a.3
).
a
Grosaular-andradite Garnets
,
Pyrope-almandine Garnets
b :
0.2 x i
*
.
3
.
z
2
z?
‘I
0.04 -
.
* 0.1
0.02-
.
*
0.
.
. 0.0 0
2
4
6
8
IO
Spessartite Garnets
12
0.05
0.10
0.15
Infrared Peak Intensity
Infrared Peak Intensity
c
Infrared Peak Intensity
Fig. 1. Correlationsbetween the hydrogencontent (expressed as H,O) of suites of garnets and the infrared peak
intensityin the OH region. Tb.e IR intensities should be proportional to the OH concentration. Circles: “N analyses; triangles: “F analyses; diamonds: elastic recoil analysis. (a) The grossular to andradite solid solution series: Ca,A12(Si0,),-Ca3Fe2(Si0,),. (b) The pyrope to ahnandine solid solution series: Mg3A12(Si04),-Fe,Al,(SiO,),. (c)The spessartite to almmdine solid solution series: ~~~~(SiO~)~-Fe~~~(SiO~)~.
G. R Rossman / Hydrogen in “anhydrous” miner&
come from a wide variety of geologic environments. They can incorporate varying, but usually minor, amounts of OH- in part in substitution as 4 OHgroups replacing SiO:- . Such traces of OH- in garnets estimated to have concentrations down to 0.001 wt.% {as H20) have been readily identified with infrared (IR) spectroscopy. IR spectroscopy is a sensitive technique for detecting the OH content of garnets, but is not intrinsically self-calibrating. Each type of garnet, and each other type of miaeral as well, requires its own calibration of the IR spectrum. In principle, the nuclear analytical methods should be an ideal way of establishing the absolute calibration of the IR spectra. Garnets chosen for hydrogen analysis by nuclear methods were usually of gem or near-gem quality with a minimum of cracks or internal defects. They were usually prepared by grinding and polishing two opposite and parallel surfaces through which the infrared spectra were obtained [17]. Nuclear analyses were run with 19F, 15N and elastic recoil analyses. Only a few of these rest&s have been previously published [22]. Figs. la-c summarize our c~ulative experience with the nuclear methods. In these figures the hydrogen content is expressed as H,O and plotted against the intensity of the infrared OH- absorption band in the 3600-3400 cm-’ region. These plots are based on the assumption that the IR intensity is directly proportional to the OH- content of the garnet. Although this assumption has never been proven for very low OHcontent garnets, it has been established with high OHcontent garnets where the OH- content can be established by independent methods. The results of the nuclear analyses show considerable scatter. In some instances, the same sample has been submitted for multiple analyses, often with comparable scatter. Of particular concern is the occasionally high hydrogen content for samples with very little infrared absorption. As fig. lb shows, in some cases when such samples have been reanalyzed they have yielded much lower hydrogen contents. The garnets chosen for these studies are among the best available for such study. They are 4 mm or larger, homogeneous, and of comparatively high OH- content. Most natural mineral samples which would need analysis are smaller as a result of natural grain size or internal heterogeneity. They also would typically have less than one-third the l&O content of the highest water content samples on the figures. The results to date indicate that the nuclear analyses for hydrogen have the potential to address these mineralogical problems, but are not yet routinely obtainable at the accuracy needed. In some instances, careful work has produced reliable calibration data with acceptable limits of uncertainty 1221. Several problems are evident. First there are the problems of preparing a suitable mineral surface which
43
is representative of the bulk sample. Very few samples are both large enough and free from internal cracks, fluid inclusions and solid inclusions which themselves may contain hydrous components. Future requirements will clearly go in the direction of ever smaller beam-spot sizes. Generally, a flat surface cannot be cleaved, so the sample must be ground and polished to expose the interior of the crystal. There is the risk that the preparation method may either hydrate the new surface or induce microscopic imperfections which could retain water or hydrous alteration products. Although some of the anomalously high hydrogen values might have resulted from such surface contamination, we have obtained enough analyses which indicate that once the analyzed region reaches about 1000 A below the surface, preparation-induced hydration is not normally a problem. There are also problems associated with the nuclear analytical methods working with very low levels of hydrogen. Very low instrument background is important for these analyses, but has been difficult to obtain in practice. Background cont~butions can arise from materials of construction in the sample chamber and unshielded ambient radiation. For applications to a variety of mineralogical problems, extremely low background count rates will be needed in future work.
References [l] RD.
Aines and G.R. Rossman, J. Geophys. Res. 89 (1984) 4059. [2] G.R. Rossman, Rev. Mineral. 18 (1988) 193. [3] D.T. Griggs, J. Gcophys. Res. 79 (1974) 1653. [4] S.H. Kirby and J.W. McCormick, Bull. Mineral. 102 (1979) 124. [5] P. Cordier and J.C. Doukhan, Chem. Geol. 70 (1988) 158. [6] R.D. Aines, S.H. Kirby and G.R. Rossman, Phys. Chem. Mineral 11 (1984) 204. [7] A.K. Kronenbcrg, S.H. Kirby, R.D. Aines and G.R. Rossman, J. Geophys. Res. 91 (1986) 12, 723. [S] A.C. McLaren, J.D. Fitz Gerald and J. Gerretsen, Phys. Chem. Mineral. 16 (1989) 469. [9] S. Mackwell, D.L. Kohlstedt and MS. Paterson, J. Geephys. Res. 90 (1985) 11319. [lo] J.R. Goldsmith, Contrib. Mineral. Petrol. 95 (1987) 311. [ll] H. Schafer, Z. Anorg. Alg. Chem. 543 (1986) 217. [12] A.M. Hofmeister and G.R. Rossman, Amer. Mineral. 70 (1985) 794. 1131 R.D. Aines and G.R. Rossman, Amer. Mineral. 71 (1986) 1186. [14] I.M. Sil’vestrova, Y.V. Pisarevskii, O.V. Zvereva and A.A. Shtemberg, Sov. Phys. Crystallogr. 32 (1987) 467. [15] L. Ackermann, L. Cemic and K. Langer, Earth Planet. Sci. Lett. 62 (1983) 208. 1161 G.H. Miller, G.R. Rossman, and G.E. Harlow, Phys. Chem. Mineral. 14 (1987) 461. 1171 R.D. Aines and G.R. Rossman, Geology 12 (1984) 720. I. NRA
44
[18] A. Beran and M.A. G&zinger,
G.R Rossman / Hydrogen in “anhydrous” minerals
Mineral Petrol. 36 (1987) 41. [19] C.R. Stem and P.J. Wyllie, Earth Planet. Sci. Lett. 28 (1975) 189. [ZO] A.T. Anderson, S. Newman, S.N. Williams, T.H. Druitt, C. Skirius and E. Stolper, Geology 17 (1989) 221. [21] S. Newman, E.M. Stolper and S. Epstein, Amer. Mineral. 71 (1986) 1527.
[22] G.R. Rossman, F. Rauch, R. Livi, T.A. Tombrello, C.R. Shi and Z.Y. Zhou, Neues Jahrbuch fiir Mineralogie, Mh. (1988) 172. f23] V.I. Kovalenko, R.L. Hervig and M.F. Sheridan, Amer. Mineral. 73 (1988) 1038. [24] J.P. Yesinowski, H. Eckert and G.R. Rossman, J. Amer. Chem. Sot. (1988) 1367.