72
J . Phys. Chem. 1985, 89, 72-76
77 K, and the low effective Debye temperature all indicate that the iron(II1) oxide is highly dispersed. (2) After reduction in H2at 675 or 740 K the Fe/C-1 catalysts contain about 60 and 77% metallic iron, respectively. At 295 and 77 K this a-Fe is predominantly in a superparamagnetic state. At 4 K, however, all the a-Fe exhibits magnetic splitting, with a magnetic hyperfine field which is about 7 kOe larger than the 338-kOe field of bulk a-Fe. The difference is in agreement with the influence expected from the demagnetizing field in singledomain a-Fe particles. (3) The Occurrence of superparamagnetic a-Fe in Mossbauer spectra at 77 K indicates that the distance between the a-Fe particles is such that dipole-dipole interactions between the magnetic moments of the particles are negligible. Also, the total magnetic anisotropy energy of the particles must be small. (4) The reduced Fe/C-1 catalysts contain an Fez+phase which in the Mossbauer spectra appears as a broadened magnetic sextet at 4 K, a doublet at 77 K, and an unresolved doublet at 295 K. The unusual occurrence of a single line for Fez+ is explained by
electronic and lattice contributions to the electric field gradient which are opposite in sign and compensate each other at 295 K. ( 5 ) All iron in the reduced Fe/C-1 catalyst becomes oxidized upon exposing the Fe/C-1 catalyst to air at 295 K, in agreement with the small particle size (2 nm) which follows from C O chemisorption, and the occurrence of superparamagnetism at relatively low temperatures. ( 6 ) Treatment of reduced Fe/C-1 catalysts at 510 K in CO and H2 converts all a-Fe into iron carbides which are superparamagnetic at 295 K, whereas the Fez+-phase remains unaffected. The various interesting aspects of small-particle behavior could only be revealed after application of Mossbauer spectroscopy in situ at liquid nitrogen and helium temperatures. The present investigation illustrates that equipment which permits measurement of spectra in situ at cryogenic temperatures is an absolute necessity for complete Mtissbauer studies of highly dispersed catalysts. Registry No. Fe, 7439-89-6.
Intracrystalline Site Preference of Hydrogen Isotopes in Borax Trinetra M. Pradbananga* and Sadao Matsuo Department of Chemistry, Tokyo Institute of Technology, 0-Okayama, Meguro-Ku, Tokyo 152, Japan (Received: August 24, 1984)
The total hydrogen involved in borax synthesized at 25 OC in aqueous solution is enriched in deuterium by 5.3960 compared with the mother liquor. There is no change in the value of the D/H fractionation factor between the hydrogen in borax and those in the mother liquor with changes in the degree of supersaturation. The fractionation factor changes slightly with a change in the crystallization temperature of borax in the range from 5 to 25 OC. The D/H ratio in the different sites of borax was estimated by a fractional dehydration technique. The results show that hydrogen atoms of the polyanionic group [B405(OH)4]are much more enriched in deuterium than those of the cationic group [Na2-8H20].The 6D values, referred to the mother liquor from which the borax was crystallized, for the cationic group (site A) and the polyanionic group (site B) are -35 f 3 and 167 & 13960, respectively, based on the fractional dehydration results obtained at -21 OC. At -21 "C, isotopic exchange between different sites during dehydration is assumed not to occur. The mechanism for dehydration of borax is discussed.
Introduction The difference in the isotopic composition of oxygen between free water and water molecules in the hydration sphere of cations and anions in aqueous salt solutions was first discussed by Taube.' The effects of ions on the isotopic fractionation between aqueous solutions were investigated and expanded by a number of res e a r c h e r ~ . ~ -In~ addition, information on the fractionation of hydrogen and/or oxygen isotopes between crystals and their saturated solutions has also been a c c ~ m u l a t e d . ~ -Bigeleisen" ~~ and Matsuo et a1.I2 advocated that the temperature dependence (1) Taube, H. J . Phys. Chem. 1954,58, 523. (2) Sofer, 2.;Gat, J. R. Earth Planet. Sci. Lett. 1972,15, 232. (3) Truesdell, A. H. Earth Planet. Sci. Lett. 1974,23, 387. (4) Stewart, M. K.; Friedman, I. JGR J. Geophys. Res. 1975,80, 3812. (5) Barrer, R. M.; Denny, A. F. J. Chem. SOC.1964,4677. (6) Tanaka, H.; Negita, H. Bull. Chem. SOC.,Jpn. 1970,43,3079. (7) Gonfrantini, R.; Fontes, J. C. Nature (London) 1963,200, 644. (8) Fontes, J. C.; Gonfiantini, R. C. R . Acad. Sci., Ser. D 1967,265,4. (9) Johansson, M.; Holmberg, K. E. Acta Chim. Scand. 1969,23,765. (10) Matsubaya, 0.;Sakai, H. Geochem. J. 1973,7, 153. (1 1) Bigeleisen, J. J. Chem. Phys. 1961,34, 1485. (12) Matsuo, S.; Friedman, I.; Smith, G. I. Geochim. Cosmochim. Acta 1972,36,427.
0022-3654/85/2089-0072$01 SO10
of the hydrogen isotope fractionation factor between hydrates and aqueous solution can be used as a geothermometer. However, the water of crystallization in a hydrate crystal is not always in the same geometric and/or energetic site, so that there should be an isotopic site preference in the crystal of hydrates as was pointed out by Heinzinger and Rao.13 Heinzinger and Maiwald14 conducted a fractional dehydration experiment on CuSO4-5H20 and verified such a site preference for this compound. Heinzinger and GOtzl5 further estimated a temperature dependence for the site preference. Based on their results, Heinzinger16 suggested that even a single mineral with isotopic site preference can be used as a geothermometer. Kita and Matsuo" concluded that water molecules in the coordination sphere of Cu2+were depleted in through deuterium by 32% and water molecules bound to Sod2hydrogen bonding were enriched by 22.6% compared with the mother liquor. Hamza and Epsteinls found that there is a significant difference in the ls0/l6O ratio between the OH groups (13) Heinzinger, K.; Rao, T. S . Z. Naturforsch., A 1967, 22A, 2111. (14) He!nzinger, K.; Maiwald, B. Bull. Chem. SOC.Jpn. 1972.45,2237. (15) Heinzinger, K.; GBtz, D. Earth Planet. Sci. Lert. 1975, 27, 219. (1 6) Heinzinger, K. J . Radioanal. Chem. 1976,30, 227. (17) Kita, I.; Matsuo, S . J . Phys. Chem. 1981,85,792. (18) Hamza, M. S.; Epstein, S. Geochim. Cosmochim. Acta 1980,44,173.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 1, 1985 73
Site Preference of H Isotopes in Borax
HIGH VACUUM PlRANl
K
"II
Liq. N2
THERMOSTAT 0
water molecule
Sample
Figure 1. Projection of Na octahedra in borax on the (100) plane. 0 0
( S i t e Bi
0 0
@OH
1
\
(Site 6)
Figure 2. Projection of the borax structure on the (010) plane. Only half of the unit cell is shown. Broken line represents the hydrogen bonding connecting the independent group.
and the rest of the aluminosilicate oxygen in rock-forming hydrous silicates. In this paper, we will discuss the intracrystalline site preference of hydrogen isotopes in borax crystals based on a fractional dehydration technique. The structure of borax was determined precisely by M o r i m ~ t o . ~ ~ Levy and LisenskyZ0redetermined it by a neutron diffraction method. Borax contains two structural units: a chain consisting of octahedra of water molecules around Na+ (Figure 1) and a chain formed by the polyanionic group. The polyanionic group consists of two tetrahedra formed by 02-and OH- around a boron atom and two triangles also formed by 02-and OH- around a boron atom by sharing some 02-so as to make the composition B405(OH)4. The polyanionic groups are connected to each other through hydrogen bonding as shown in Figure 2. The structure of borax suggests that there is a significant difference in geometry and energetics with respect to hydrogen atoms in the crystal. There should be a hydrogen isotope site preference between the water molecules bound to Na+ and OH in the polyanionic group. The water molecules surrounding Na+ are taken as site A [Na2-8H20]and the polyanion [B405(OH),] as site B.
Experimental Section Crystallization of Borax. Borax, Na28Hz0.B405(OH)4 (reagent grade), was dissolved in deionized water for the preparation of a supersaturated solution (10% excess with respect to borax). The solution was filtered into a stoppered conical flask and a minute amount of borax seed crystals was added to it. The flask was kept immersed in a thermostated bath at 25 & 0.1 OC for more than 15 days. The borax formed in this way was not taken from the solution before use in order to prevent dehydration in air to form tincalconite, Naz3Hz0.B405(OH)4. Bulk Water of Crystallization. A small crystal of borax (10-20 mg) was taken from the solution kept at 25 O C and was dried by pressing between filter paper. The crystal was put into a dehy-
0 Glass Ball m Glass Wool ( G ) 00 Ring Joint
0 Stopcock 0 Stopcock ( S )
Figure 3. System used for the fractional dehydration of borax.
dration tube (Pyrex tube), the weight of which was previously measured. Procedures such as taking the crystal from the solution, drying it with fiiter paper, and weighing were done as quickly as possible in order to prevent any loss of water of crystallization on exposure to the atmosphere. N o measurable weight loss was found. The dehydration tube, cooled externally by liquid nitrogen, was connected to a high vacuum line and evacuated. The sample was then warmed gradually and finally heated to 300 O C for about 1 h for complete dehydration. The dehydrated water was condensed at liquid nitrogen temperature. The dehydration system used was the same as that of Kita and Matsuo.17 The dehydrated water was converted into hydrogen gas by the method of Bigeleisen et a1.,21i.e., passing the water vapor through uranium metal at about 750 OC. The amount of hydrogen gas was measured manometrically. The D / H ratio of this gas was measured with a double-inlet and double-collector type mass spectrometer, Hitachi-RMD. The data are presented in the 6D notation referring to the D / H ratio of the mother liquor by the following equation:
The error in the 6D measurements is &lYk Fractional Dehydration. In order to prepare powdered samples of borax for fractional dehydration, we used a special device for pulverization under vacuum at liquid nitrogen temperature. Powdering of borax in the air gives tincalconite quite easily because loss of water takes place to a remarkable extent during pulverization. A schematic diagram of the dehydration system used for the fractional dehydration is shown in Figure 3. About 200 mg of freshly prepared borax was taken from the solution kept at 25 O C for fractional dehydration a t 0, 10, and 25 O C . About 500 mg of freshly prepared borax was taken for fractional dehydration at -21 O C . The borax was dried quickly by pressing between filter paper and put in an L-shaped tube (L) containing a glass ball, the weight of the tube and ball being measured in advance. The weight of the tube was measured again after loading the borax sample. A small amount of glass wool (G) was also put in the neck of the tube to prevent any loss of sample due to decrepitation during the dehydration process and the weight was measured again. The tube (L) was connected to the upper part (B) of the dehydration system in which the stopcock (S)was closed. The dehydration system was then removed after complete evacuation at liquid nitrogen temperature, and the sample was pulverized by moving the glass ball while cooling the tube (L) in liquid nitrogen to prevent any loss and exchange of hydrogen atoms in different sites during pulverization. The powdered crystal was distributed thinly in the L shaped tube so as to give a minimum temperature gradient in the powder during dehydration.
(19) Morimoto, N. Mineral. J . 1956, 2, 1.
(20) Levy, H. A.; Lisensky, G. C. Acta Crystallogr., Sect. B 1978, B34, 3502.
(21) Bigeleisen, J.; Perlman, M. L.; Prosser, H. C . Anal. Chem. 1952,24, 1356.
14
Pradhananga and Matsuo
The Journal of Physical Chemistry, Vol. 89, No. I , 1985
TABLE I: Variation of the Hydrogen Isotopic Fractionation Factor ( a ) between the Bulk Water of Crystallization and the Mother Liquor with Different Degree of Supersaturation at 25 OC degree of (Y suDersaturation % 1.0058 5 1.0053 10 1.0055 20 TABLE II: Variation of Hydrogen Isotopic Fractionation Factor, a (between the Bulk Water of Crystallization and the Mother Liquor), of Borax Synthesized at Different Temperatures, t t , oc a 5 1.0019 15 1.0028 25 1.0053
The size distribution of the borax powder could not be measured before dehydration. The dehydration system was connected again to the evacuation line through a ground joint. The glass tube (L) was once more evacuated at liquid nitrogen temperature. Then the tube (L) was immersed in a thermostated bath at a designated temperature (25, 10, 0, and -21 "C). The water collection was made without interruption by using the two-way system shown in Figure 3, so that a sample tube prepared for the collection of dehydrated water in one limb can be evacuated while the condensation of dehydrated water in the other limb was going on. The conversion of the water collected in each fraction into hydrogen gas and the measurement of the amount of evolved hydrogen were done as mentioned previously. The measurement of dD of the hydrogen gas obtained was made as mentioned earlier. The starting time was taken when the glass tube (L) was immersed in the thermostat. Fractional dehydration at -21 "C was repeated twice. The duplicate results agree well.
Results and Discussion Bulk Water of Crystallization. Barrer and Denny5 and Matsuo et a l l 2 reported that the D / H ratios of hydrated salts are mostly lower than those of the mother liquor in equilibrium with the salts. A study made by Matsubaya and SakaiIo for the gypsum-water system showed that gypsum is also depleted of deuterium compared with the mother liquor. On the other hand, in the icewater,22 mirabilite (Na2S04.10H20)-water,23 and natron (Na2CO3-1OH20)-water24 systems, D / H ratios are higher in the solid phase than in the mother liquor. The 6D value for the bulk water of crystallization of borax synthesized at 25 O C is 5.3 f 1.07~(average of four measurements). The 6D value of the bulk water of crystallization for CuS04.5H20 and NiS04-7H20is -21.1 and -3.0%0, respectively.'7925 The fractionation factor for the borax-water system is defined as
where BW stands for the bulk water of crystallization and ML denotes the mother liquor. Since bDML was taken to be zero in this study, the value of CY obtained at 25 "C is 1.0053, in agreement with the value obtained by Matsuo et a1.12 In order to check whether or not there is a diffusion effect at the interface of a growing crystal on isotopic fractionation, we measured the variation of CY for the borax synthesized from solutions with different degrees of supersaturation at 25 "C. The results are given in Table I. There is no significant change in the value of CY due to a change in the degree of supersaturation, indicating that an isotopic exchange equilibrium can be assumed between the borax and the mother liquor during the growth of the crystal. A similar study (22) O'Neil, J. R. J. Phys. Chem. 1968, 72, 3683. (23) Stewart, M. K. Geochim. Cosmochim. Acta 1974, 38, 167 (24) Matsuo, S . , personal communication. (25) Hossain, L.; Kita, I., personal communcation.
1.01
1
I
I
-
o'88 0
0.6
i 0°C .-21'c I
I
I
0
5
0
10
0
250
10 20 500
30 750
15
20 lO"C,25*C 40 0°C 1000 -21OC
1, Hours Figure 4. Relation between the fraction of water dehydrated (F), referred to 8 of the 10 stoichiometric water molecules of borax and time ( t ) at different dehydration temperatures.
Figure 5. Relation between ( F - F*)/(l- P)and ( t - t*)/(t - t*)o.s where F* and t* are, respectively, the fraction of water dehydrated and time when the slope changes in the F vs. t curve (Figure 4), and the subscript 0.5 denotes 50% decomposition. P and t* are taken to be zero at temperatures other than -21 'C.
done by Matsuo et al.I2 for the same degree of supersaturation at 25 O C with different D / H ratios for the starting solution gave the same value of CY. Dependence of a! on the Crystallization Temperature. The variation of the fractionation factor with crystallization temperature is given in Table 11. The empirical relationship between a! and T for the borax-water system is given by the following equation: In
CY
+
= -20.2 X 102T2 0.03
(3)
The change in CY with temperature found in this study is similar to that of the trona (NaHCO3-NazCO3-2H20)-watersystem,I2 Le., there is a decrease in CY (closer to unity) with a decrease in temperature (crossover temperature of -4.7 "C), though it is not significant in the borax-water system. Kinetics of Dehydration. Fractional dehydration at 25 "C at a pressure of torr results in the removal of only 8 of the stoichiometric 10 molecules of water after 2 days. It was necessary to raise the temperature to 300 "C to dehydrate all the water molecules within a reasonable period of time. The formation of dihydrate Na2B405(OH)4under vacuum at 19.1 to 46.5 "C was indicated by the kinetic study of Thomas and Soustelle.26 The dehydration mechanism from crystalline hydrates has been suggested to be either an interface reaction or a diffusion process.*' The relation between the fraction of water dehydrated and the dehydration time at different dehydration temperatures for borax is shown in Figure 4. The fraction of water dehydrated, F,in Figure 4 is referred to 8 molecules of water. In other words, F = 1 means 8 molecules of water dehydrated, and 8 of the stoichiometric 10 molecules correspond formally to the total water in sodium polyhedra, [Na2.8H20]. As seen in Figure 4, there is (26) Thomas, G . ;Soustelle, M. Bull. Soc. Chim. Fr. 1970, No. 12, 4202. (27) Brown, M. E.; Dollimore, D.; Galwey, A. K. "Reaction in the Solid State, Comprehensive Chemical Kinetics"; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier Scientific Publishing Company: The Netherlands, 1980, Vol. 22, p 117.
Site Preference of H Isotopes in Borax
The Journal of Physical Chemistry, Vol. 89, No. 1, 1985 75 0
F 0.25
-10
0.75
0.50
-4
33.0 350 37.0 390 41.0
vT x104
Figure 6. Relation between In k and the reciprocal of the dehydration temperature.
no or a very short induction period at dehydration temperatures above 0 O C . The fraction of water dehydrated at -21 O C is linear with respect to the dehydration time (zeroth-order rate process) up to F = 0.1, and there is a change in slope at this point. Hence, ( F - Fc)/(l - Fc) is plotted against ( t - t * ) / ( t - t*)o.5 in Figure 5, where, F* and t* are respectively the fraction of water dehydrated and the time when the slope changes,2ssubscript 0.5 denoting 50% decomposition. Fc and r* may be taken to be zero a t temperatures other than -21 O C . The dehydration curves at different temperatures superimpose on each other as seen in Figure 5, suggesting the same dehydration mechanism over the studied range of dehydration temperature^.^^ Figure 5 indicates that the dehydration of borax over the studied range of temperatures can be treated as a first-order rate process except for the earliest fractions at -21 O C , i.e. dF/dt = k(1 - F) (4)
-bo -50
r1
I
I
I
I
I
-
--2l0C -. 0 ° C 4 10°C 25'c Figure 7. Relation between bD of fractionally collected water at different temperatures and F. TABLE III: Change in 6D of the Fractions Obtained by Dehydration under Vacuum at 25 OC and That on Heating the Residual Product at 300 O C bD, % yield, % bulk water of crystallization +5.3 100 fractional dehydration at 25 OC (42 h) -10.7" 79.9 dehydration on raising the temperature +65.9 20.1 to 300 OC (a single fraction) integral mean +4.7
" Integral mean of 6 0 values obtained for eight fractions.
temperature, dehydration through dislocation must have been enhanced at least in the initial stage. A plot of the fraction of water dehydrated vs. the 6D value of -In (1 - F) = kt (5) each fraction at different temperatures, using identical borax, is shown in Figure 7. For runs with different dehydration temThe first-order dehydration process is the case when the size peratures, there is practically no change in the 6D value of the of crystalline grains is sufficiently small and the rate of reaction fractions collected in the range of F from 0.1 to 0.5 except for is controlled by nucleation in an assemblage of identical reactant the run at -21 "C. The constancy of the 6D value in the range fragments.30 In other words, random nucleation of the product of F from 0.1 to 0.5 can be explained in the following way: when on a large number of crystallites takes place, and this model is extensive nucleation occurs in that range, both the amount of water a special case of the Mampel treatment.j' As seen in Figure 5, left in the crystals and the isotobic fractionation factor between a deviation from the master plot (the curve for the first-order rate the water driven off and the water remaining are controlled only process, after Sharp et al.29) was observed in the later stage of by the dehydration temperature. Since the isotopic fractionation dehydration ( F > 0.5). The deviation from the first-order dehfactor is determined by nucleation, the 6D value of the dehydrated ydration curve is dependent on the temperature of dehydration, water should be kept constant during the growth of the nucleation Le., the deviation seems to take place earlier (smaller F) when product, and the dehydration due to nucleation stops at around the dehydration temperature is higher. This can be explained by F = 0.5. In the fraction of F higher than 0.5, the water remaining an increase in the number of nucleation sites in the amorphous in the nucleation product is forced to dehydrate randomly to give material with a rise in the dehydration temperature in the initial a larger isotopic fractionation and the water dehydrated from site period of dehydration. The rate constant, k, at temperatures other B (polyanionic group) may even be mixed. After reaching F = than -21 O C was calculated by using eq 5. The activation energy 0.5, the nucleation process declines due to the overlap of nuclei for dehydration calculated from the Arrhenius plot (Figure 6) or the growth process overwhelms the nucleation process. In other is 20.1 kcal/mol. This value is, however, greater than that of 12 words, once the surface of the crystal is covered by the product kcal/mol obtained by Thomas and Soustelle.26 The dehydration nuclei, further nucleation becomes difficult and isotopic frackinetics is influenced by the pretreatment of the reactants, such as irradiation, cold working, and the conditions of d e h y d r a t i ~ n . ~ * - ~ ~ tionation becomes conspicuous in the dehydrated water. During fractional dehydration at -21 OC, the 6D value increases with time The difference in the activation energy obtained in this study and in the range F < 0.1. A zeroth-order process based on random in that by Thomas and Soustelle26is due to the differences in dehydration caused by dislocation may be responsible for this pretreatment of the reactant and also the conditions of dehydration. earliest range. Since pulverization was done in this study at liquid nitrogen Isotopic Site Preference. The average 6D value of -10.7% of the water dehydrated at 25 O C on decomposition from decahydrate (28) Tang, T. B. Thermochim. Acta 1980,41, 133. to dihydrate [Na,B,O,(OH),] is much lower than the value (29) Sharp, J. H.; Brindley, G. W.; Narahari Achar, B. N. J . Am. Ceram. SOC. 1966,49, 379. +65.9%0measured for the remaining water dehydrated at 300 O C (30) Brown, M.E.; Dollimore, D.; Galwey, A. K. "Reaction in the Solid from dihydrate as given in Table 111. The overall weighted mean State, Comprehensive Chemical Kinetics"; Bamford, C . H., Tipper, C. F. H., of the 6D value of +4.7%0 calculated by mass balance (-10.7 X Eds.; Elsevier Scientific Publishing Company: The Netherlands, 1980; Vol. 0.799 + 65.9 X 0.201) is practically the same as +5.3%0 measured 22, p 59. for the bulk water. This remarkable difference in 6D for the two (31) Mampel, K.L. Z . Phys. Chem., Abt. A 1940,187, 43, 235. (32) Delmon, B. "Introduction B La Cinttique Hbttrog*ne", Technip Ed; fractions indicates that deuterium is relatively depleted in site A Paris, 1969. (Na polyhedra) and relatively enriched in site B (polyanionic (33) Broadbent, D.; Dollimore, D.; Dollimore, J. J . Chem. SOC.A . 1966, group). However, we are not confident that isotopic exchange 1491. (34) Jach, J.; Griffel, M. J . Phys. Chem. 1964,68, 731. occurs between different sites during fractional dehydration at or
76 The Journal of Physical Chemistry, Vol. 89, No. I , I985 ,
:9;0;
~
-4'0
rAIn FA - , -0.150
,
- -0130 -25 -3 5
6DA -45
?"
Figure 8. Relation between 6DA and r A In F A at -21 OC.
25 O C . Therefore, a fractional dehydration was made for borax at -21 O C and the results were interpreted with the assumption that no isotopic exchange occurs between different sites during fractional dehydration at -21 OC. Lattice defects or dislocations in crystals increase during cooling and p u l v e r i z a t i ~ n . Since ~ ~ ~ ~the ~ dehydration process at -21 OC under vacuum is a zeroth-order rate process in the range 0 < F < 0.1 (Figure 4), we can assume that the dehydration takes place quite randomly from the crystal grain mainly through dislocations in this range. Hence, a Rayleigh process can be applied for the change in the isotopic composition of the fractions collected." The relationship between the change in isotopic ratio of dehydrated water and the fraction of water remaining is given in general by 6D = 6Do
+ 103r(l - p) In FR
(6)
where 6D is the integral mean value of 6D for the dehydrated water up to the measured fraction (1 - FR), 600 is the 6D of the water of crystallization before dehydration, r = F R / (1 - FR),FR is the fraction of remaining water in the crystal, and @ is the kinetic fractionation factor of hydrogen isotopes for dehydration (and is equal to the ratio of the D / H for the dehydrated water to the D / H for the remaining water of the crystal). As mentioned earlier, dehydration of borax gives dihydrate Na2B405(OH)4after removal of all site A water. We can apply an equation identical with eq 6 for site A in the range 0 < F < 0.1 as given by 6DA = 6DAo+ 103rA(l - P A ) In
FA
(7)
where A stands for site A and FA denotes the fraction of remaining water which dehydrates through dislocation. The plot of SDA vs. rAIn FA is shown in Figure 8. The intercept on the ordinate (rA In F A 0) and the slope of this line give 6DAoand PA,respectively, which are calculated to be -35.0 f 3.3% and 0.994 f 0.001. The errors have been estimated from the results obtained from two series of measurements performed at -21 O C . The 6DAovalue of -35%~is significantly different from -1 1% (see Table 111, roughly estimated from the result of fractional dehydration a t 25 OC mentioned previously). The less negative 6D value (higher D / H ratio) of the latter indicates that isotopic exchange occurs between site A and B during dehydration at 25 OC. aho i.e., , the isotopic composition of the site B hydroxyl radical in the polyanionic group can be calculated from the following material balance equation: -+
6DBw = 0.86DA0 + 0 . 2 6 0 ~ ~
(8)
Since 6DBw is 5 . 3 7 ~and 6DAois -35.07~ as shown previously, 13.0% from the above 6DBo can be calculated to be 167
*
(35) Jach, J. Nature (London) 1962, 196, 827. (36) Thomas, J. M.; Renshaw, G. D. J . Chem. SOC.A . 1967, 2058. Matsuo, S. Geochem. J . 1982, 16, 149. (37) Matsubaya, 0.;
Pradhananga and Matsuo equation. In contrast to site A, the 6DBovalue for site B is definitely higher, Le., deuterium is much more enriched in site B. There are two types of water molecules in the chain of Na polyhedra (site A). One is the shared water molecule or water molecule which is bound to two N a atoms and the other is an unshared water molecule which is bound to only one Na atom. One can expect some change in the isotopic composition between these water molecules. The isotopic composition of the crystalline hydrate is dependent on the cation-H20 bond length.38 It is, however, difficult to distinguish the shared from the unshared water molecules with respect to Na+-H20 distances.20 Hence, we cannot expect any significant change in the hydrogen isotopic composition between shared and unshared water molecules. In nature, borax is found either with tincalconite, Na23Hz0.B405(OH)4,or kernite, Naz3H20.B406(OH)2.Borax is converted into tincalconite and vice versa under natural cond i t i o n ~(temperature ~~ of 12-25 OC and relative humidity of 55-60%). It is also known that borax changes to kernite at 58.5 OC under 1 atm pressure.40 Both of these formation conditions are far different from that in this study. Therefore, information on the kinetic dehydration process obtained in this study is not applicable to natural conditions. A coexisting pair of borax and tincalconite collected from the same horizon of L-181 core of Searles Lake, California was analyzed for D/H. The 6D value of the tincalconite was higher by 6% than that of borax. The difference of 6% is too small to be explained by the kinetic D / H fractionation factor obtained at 25 "C in this study, indicating repeated transformations between borax and tincalconite under quite different conditions from those used in this study. As described previously, the polyanion B,O,(OH), is composed of two BOz(OH) trigonals and two B03(OH) tetrahedra, sharing some of the oxygen atoms to form a compact g r o ~ p . ' ~ ,We ~ ' can expect an isotopic site preference even in the OH of the polyanionic group. However, our present study cannot give an accurate 6D value in possible subsites of site B since isotopic exchange is sure to take place at the high temperature of 300 O C necessary for the extraction of water under vacuum. There are some reports of 6D values for O H in some OHbearing These studies show that 6D for O H in these minerals is dependent on temperature, the metallic ion to which the OH is bound, the structure of mineral, and the isotopic composition of the fluid. The 6D values for the OH obtained for most clay minerals are lower than those of the coexisting aqueous p h a ~ e . ~ ~In- ,the ~ serpentine-water system, deuterium has been reported to be enriched in serpentine at temperatures lower than 100 0C.44 Our result (crystallization temperature of 25 "C), a remarkable enrichment of deuterium in the O H radical of the polyanionic group, B405(OH),, should be interpreted in view of the fact that all the O H radicals are bound to boron atoms and not to a metallic ion. Registry No. Borax, 1303-96-4. (38) Pradhananga, T. M.; Matsuo, S.manuscript under preparation. (39) Christ, C. L.; Garrels, R. M. Am. J . Sci. 1959, 257, 516. (40) Menzel, H.; Schulz, H. Z . Anorg. Allg. Chem. 1940, 245, 157. (41) Cuthbert, J. D.; Petch, H. E. J . Chem. Phys. 1963, 38, 1912. (42) Suzuoki, T.; Epstein, S. Geochim. Cosmochim. Acta 1976, 40, 1229. (43) Graham, C. M.; Sheppard, S. M. F.; Heaton, T. H. E. Geochim. Cosmochrm. Acta 1980, 44, 353. (44) Sakai, H.; Tsutsumi, M. Earth Planet. Sci. Lett. 1978, 40, 231. (45) ONeil, J. R.; Kharaka, Y . K. Geochim. Cosmochim. Acta 1976,40, 241. (46) Satake, H.; Matsuo, S. Contr. Pet. Mineral. 1984, 86, 19. (47) Savin, S. M.; Epstein, S. Geochim. Cosmochim. Acta 1970, 34, 25. (48) Lawrence, J. R.; Taylor, Jr., H. P. Geochim. Cosmochim. Acta 1971, 35, 993.
