The redox state of iron in tektites

Journal of Non-Crystalline Sohds 67 (1984) 349-359 North-Holland, Amsterdam

349

T H E R E D O X S T A T E OF I R O N IN T E K T I T E S Henry D. S C H R E I B E R , Leslie M. M I N N I X and G. Bryan BALAZS Department of Chemtstry, Virgmta Mthtary Institute, Lexington, VA 24450, USA

Several tektites have been analyzed for the concentrations of mdtwdual ~ron redox states dissolved thereto Essentially all the iron m these tektites was determined to be Fe(II) These tekhtes were then remehed under controlled temperature and oxygen fuga ot y m order to set constraints on the prevadmg redox conditions d u n n g tekUte formation The F e ( I I I ) - F e ( I I ) and Fe(II)-Fe(0) equthbna m the resyntheslzed tektites were then determined as a function of the redox co ndmo ns necessary to sustain each. Since these e q u l h b n u m cons~deratmns indicated that tektites should possess Fe(III) or Fe(0) along with iron as Fe(ll), some process must have been operational m separating the ~ron redox states m order to achteve the final product whtch contained essenUally all iron as Fe(II) One plausible explanation for the observed redox state of ~ron m tekutes Is that the tektites were tmtially molten at sufficiently reducmg conditions so that the Fe(II)-Fe(0) eqmhbrium was estabhshed. Before the quench to the glass, however, much of the Fe(0) as metal separated due to density differences from the melt to yield a glass with essentmlly all Iron as Fe(II)

1. Introduction

One characteristic of tektites is a low ferric/ferrous ratio [4] which differentiates these natural glasses from "ordinary" terrestrial rocks. This low percentage of oxidized iron indicates a relatively uniform thermal history under relatively reducing conditions (high temperature a n d / o r low oxygen fugacity). In this regard the tektites resemble lunar material. The presence of nickel-iron metallic spherules [1,2] in tektites confirms an exposure to sufficiently reducing conditions so that some dissolved ferrous ions are reduced to iron metal. The results of Walter and Doan [3] suggest a temperature-oxygen fugacity relation of l o g f o 2 = 9.13

32 600 T(K)

(1)

for tektites. The relationship yields an oxygen fugacity of 1 0 - 1 4 6 atm at 1100 o C which is about six orders of magnitude below terrestrial rocks at this temperature [4] and is about the order of magnitude of lunar basalts [5]. See also Brett and Sato [6], who confirm Walter and D o a n [3]. Using magnetic and chemical analyses of iron in tektites, Thorpe et al. [7] determined that the ferric/ferrous ratio for bediasites is constant at about 0.05. Furthermore, their results implied an approximately constant median ratio for all tektite families, indicating a reasonably uniform thermal history and 0022-3 093/84/$03 00 © Elsevmr Soence Pubhshers B V (North-Holland Physics Pubhslung Dlwsion)

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H D Schretber et al / The redox state of tron m tektites

melting atmosphere during formation. Ostertag et al. [8] used magnetic susceptibility measurements to arrive at the same basic conclusion of a very low ferric/ferrous ratio for tektites. The ratio was duphcated by heating synthetic tektites to either high temperature (e.g., 1800 °C) or low oxygen fugacities. In fact, a possible source material for the tektites was envisioned to be a melt containing only divalent iron. Evans [9] concluded from his Mossbauer study of iron in tektites from various strewn fields that the chemical determination of the ferric/ferrous ratio produces larger values than the actual ratio. His results indicate that the ferric/ferrous ratio in tektites is well below 0.05 and points to an upper limit of 0.01 for the ratio. Once again, evidence is presented for the uniform and severe reduction of all classes of tektites. The low ferric/ferrous ratio along with low water and volatile element contents are usually cited as key arguments for a lunar origin for tektites instead of a terrestrial impact origin [4]. In order to achieve an extremely low ferric/ferrous ratio in melts of an impact origin, one would have to impose very high temperatures a n d / o r the possibility of shock reduction [10].

2. Objectives The principal objective of this research is to ascertain the conditions that were prevailing during the stage in the formation of tektites when they were molten. The redox state of a rapidly cooled glass such as a tektite is indicative of these prevailing conditions during its evolution. In this study the thermal and atmospheric conditions that the glass specimens experienced during melting will be reconstructed. Since iron is the major multivalent element in most natural glasses, measurements of ferric/ferrous ratios in tektites should be useful in estimating T - f o 2 regimes for tektite formation - and, thus, useful in setting constraints on possible modes of tektite formation.

3. Experimental approach Representative bulk amounts of Australian and S.E. Asian (Thailand) tektites were initially prepared by cutting the outer few millimeters from the specimens with a diamond saw. The purpose of this separation was to obtain glass that was free from surface contamination due to weathering. Homogeneous powders of these virgin specimens were then obtained by grinding in an alumina mortar and pestle and by mechanically mixing the powders. A glass chip of each tektite sample was retained for eventual electron microprobe analysis. The Fe(II) concentration and the total Fe concentration of each bulk sample were then determined. Individual samples were subsequently synthesized from these bulk powders by melting at various temperatures and imposed oxygen fugacities. After

H D Schrelber et al. / The redox state of iron m tektttes

351

equilibration for about 24 h, the samples were then rapidly quenched to glasses in order to preserve the redox eqmlibrium established under melt conditions. After analysis of each sample for Fe(II) and total Fe content, the ferric/ferrous ratio of the glass was determined as a function of the melting temperature and oxygen fugacity. The ferric/ferrous ratio of the tektite samples was then matched to the ferric/ferrous rauos of the experimentally synthesized samples m order to ascertain possible melting temperatures and oxygen fugacities which accompanied tektite formation. This study was divided into three parts according to the method by which Fe(II) and total Fe concentrations were measured in the glass samples. In part A chemical redox titrations on the dissolved glass monitored the Fe(II) content while electron microprobe analyses were used to obtain total Fe contents. Part B employed optical ,spectrophotometry on glasses to determine Fe(II) concentrations. Finally, o-phenanthroline was used as a complexing agent for Fe (II) m part C; the colored complexes in soluUon were determined spectrophotometrically as a funcUon of Fe(II) and total Fe contents.

4. Experimental methods 4.1. S a m p l e s y n t h e s t s -

general

For the synthesis of individual samples from the bulk powdered tektites, a specially modified high-temperature controlled-atmosphere furnace was employed [11], as it allowed precision temperature and oxygen fugacity control. Approximately 300 mg aliquots of the desired tektite were contained in Pt, graphite, or alumina capsules and suspended in the sample chamber of the furnace. Melt temperatures from 1150°C to 1550°C were momtored by a Pt/Pt90Rhl0 thermocouple immediately adjacent to the sample. Atmospheric control of the oxygen fugacity was accomplished by the introduction of various pure gases (02, air, CO2, CO) or of C O 2 / C O gas mixtures. The atmospheres were determined as a function of the absolute oxygen fugaclty by a solid ceramic electrode [11]. Individual samples were equihbrated under melt conditions for about 24 h; that this was sufficient time to establish F e ( I I I ) - F e ( I I ) - F e ( 0 ) equilibrium was shown by a prior study [12]. After synthesis, each sample was rapidly quenched to a glass in order to preserve the equilibrium under melt conditions. 4.2. P a r t A

Four separate S.E. Asian tektites identified as I-1, 1-2, 1-3, and 1-4 were the base compositions for this study. Their average composition by electron microprobe analysis was 74.3 wt% SiO2, 0.76 wt% T102, 13.0 wt% A1203, 1.9 wt% CaO, 1.8 wt% MgO, 0.10 wt% MnO, 4.44 wt% FeO, 1.3 wt% Na2 O, and 2.6 wt% K 2 0 . Individual samples were synthesized in Pt capsules at tempera-

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The redox state o f tron in tektttes

tures from 1300°C to 1550°C at a variety of known oxygen fugacmes. Samples were rapidly quenched to glass and ground to a fine powder except for the retention of several glass chips. 4.2.1. Analysis The powdered samples, either virgin tektites or synthesized glasses, were analyzed for Fe(II) content by chemical redox titrations and for total Fe content by the electron microprobe. Aliquots of the powders weighing about 100 mg were dissolved in a H 2 S O a - H F - H 2 0 digestion solution under a protective argon atmosphere. The concentration of Fe(II) was subsequently analyzed by titration with standard ceric solution with ferroin indicator [12]. This procedure has been used successfully to analyze for trace concentrations of individual redox ions in glass [13]. Standard electron microprobe procedures were employed to measure total iron contents on the mounted chips of the glasses. Fe(III) concentrations were then determined by difference. 4.3. Part B

Thailand, Austrahan, and synthetic (73.5 wt% SiO:, 0.70 wt% TiO z, 11.5 wt% AI203, 3.5 wt% CaO, 2.1 wt% MgO, 4.54 wt% Fe203, 1.3 wt% N a 2 0 , 2.3 wt% K 2 0 ) tektites as well as an obsidian were used as starting bulk compositions. The synthetic tektite was prepared with a simplified australite chemical composition. Individual samples were fused at 1150°C and 1250°C in Pt or graphite capsules under known oxygen fugacities. They were then rapidly quenched to glasses. 4.3.1. Analysm Finely polished thin sections (8 m m diameter by about 1 m m thick) of each sample were prepared from the glass charges for spectrophotometric analyses [12]. Spectra over the range of 2800 nm in the near-infrared region to 200 nm in the near-ultraviolet region were recorded on a Beckman 5240 double-beam spectrophotometer. Both sample and reference beams were adjusted with 3 m m apertures. Fe(II) dissolved in glass displays an absorption band at about 1000-1150 nm, so that this absorption should give an indication of the Fe(II) content. The remaining portion of each sample was then powdered as m part A for chemical redox titrations in order to calibrate this absorption band in terms of Fe(II) content. 4.4. Part C

Several S.E. Asian tektites (part A for base composition) were combined to be the bulk starting material. Individual samples were synthesized at 1350 o C and 1400 o C in Pt capsules under oxidizing atmosphere, in graphite capsules in a CO atmosphere, and in alumina capsules over the entire range of oxygen fugacities. The entire glass charges were powdered for analyses.

H D Schretber et al / The redox state oftron m tektttes

353

4.4.1. A n a l y s t s

The technique of Jones et al. [14] was adapted to determine the Fe(II) and total Fe contents of the same sample of powdered glass. This procedure makes use of the reaction of Fe(II) ions in solution with o-phenanthroline to produce a red complex having an absorption maximum at 512 nm. The complex is stable in solutions with p H of 2 - 9 and conforms to Beer's Law up to a concentration limit of 10 ppm. Total iron can then be determined in this procedure by the reduction of all ferric ions to ferrous ions with hydroquinone or by the digestion of all iron metal. Initially, a calibration of absorption intensity at 512 nm was made for solutions of known Fe(II) concentrations. The spectral determinations of Fe(II) and total Fe contents in the glass samples were then made on 9 to 30 mg ahquots of the powdered samples. Digestion of the glass was done at room temperature in a H 2 S O 4 - H F - H 2 0 solution under a protective argon atmosphere. After addition of o-phenanthroline, the p H of the resulting solution buffered with K H P was adjusted to about 3.7 by dropwise additions of concentrated a m m o n i u m hydroxide solutions. The spectral absorption of the red complex was then measured at 512 nm. The iron concentrations in these same solutions were subsequently determined by adding hydroqulnone and allowing them to set overnight. The spectral absorption measurement at 512 nm was repeated. The difference between the two readings also provided a value for Fe(III) concentration in the sample.

5. Results 5.1. P a r t A

Figs. 1 and 2 display the results of the F e ( I I ) - F e ( I I I ) equilibrium m resynthesized tektites on imposed oxygen fugacity and temperature. A plot of - l o g fo2 versus log([Fe(II)]/[Fe(III)]), as in fig. 1, for constant composition and temperature should yield a straight line of slope four [15]. The graph of log([Fe(II)]/[Fe(III)]) versus T-1 shown in fig. 2 can be used to illustrate the temperature dependence of the equilibrium and to provide an estimate of the A H for the reduction of Fe(III) to Fe(II) in the molten glass [15]. The relationship of the F e ( I I I ) - F e ( I I ) equilibrium as a function of imposed oxygen fugacity in fig. 1 departs from the predicted linear relaUonship, especially at the more reducing conditions. Likewise, the A H for the reduction reaction is calculated to be about 8 k c a l / m o l from the results of fig. 2; this A H is significantly lower than the 20-30 k c a l / m o l estimated on the basis of other silicate melts [16]. These abnormahties have been ascribed to the interference of iron metal absorption into the Pt containers under the reducing conditions of low oxygen fugacity and high temperature. Another possible rationale for the deviations is that the analyses by redox titrations may measure other reduced ion concentrations such as Ti(III), Cr(II), or Eu(II) or other oxidized ion concentrations such as Mn(III). Nevertheless, the results provide an estimate for the T - f o 2 regimes of these tektites.

354

H . D . Schretber et a l /

The redox state o f iron m tektttes

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Fig. 1. The iron redox eqmhbna an remelted mdochmltes with respect to imposed oxygen fugaclty at 1350 o C. R = [Fe 2+ ]/[Fe 3+ ] or [Fe°]/[Fe 2+ ]. Symbols represent the measured Fe(III)-Fe(II) equdlbnum for I-1 (c]rcles), 1-2 (squares), 1-3 (triangles), and 1-4 (diamonds). The straight hne of slope four Is drawn through the points for reference. The Fe(II)-Fe(0) stabihty field, as estimated from Schrelber et al [12,13], is outhned by dashed hnes

A n a l y s e s of the v i r g i n tektites glasses p r o v i d e d the f o l l o w i n g r e s u l t s : I - I , [Fe(II)] = (3.59 -t- 0.05)wt%, [EFe] = (3.45 + 0.10)wt% I-2, [Fe(II)] = (3.48 + 0.03)wt%, [EFe] = (3.58 + 0.10)wt%. I-3, [Fe(II)] = (3.27 + 0.02)wt%, [EFe] = (3.25 + 0.04)wt%. 1-4, [Fe(II)] = (3.48 + 0.04)wt%, [EFe] = (3.39 + 0.07)wt%. Clearly, essentially all the i r o n i n the tektites c a n b e c o n s i d e r e d to b e F e ( I I ) o r the f e r r o u s ion. H o w e v e r , figs. 1 a n d 2 i n d i c a t e t h a t at all r e a s o n a b l e m e l t t e m p e r a t u r e s or o x y g e n fugacities, either m e a s u r e a b l e a m o u n t s o f F e ( I I I ) o r Fe(0) s h o u l d b e i n e q u i l i b r i u m w i t h the F e ( I I ) i n the melt.

H D Schretber et al / The redox state of tron m tektites

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5.2. P a r t B

Before any thermal or atmospheric dependence of the iron redox eqmlibrium could be obtained by spectrophotometric monitoring of the Fe(II) absorption at 1000 nm, the linearity of the relationship with Fe(II) content in the glass had to be tested. Therefore, all samples were analyzed for Fe(II) content by redox titration as well as for Fe(II) spectral absorption. The range of synthesis conditions assured a wide range of Fe(II) concentrations in the glasses. Fig. 3 illustrates the results of this calibration. Evidently, the 1000 nm absorption peak saturates at about 1 wt% Fe(II) and is rendered unsatisfactory for quantitative work at higher Fe(II) concentrations. Since the absorption of Fe(II) at 1000 nm in the tektite samples fell in this region of < 1 wt% Fe(II), this spectrophotometric procedure was not suitable for the determination of the redox state of iron in tektites. This procedure had been used previously for the analyses of Fe(II) in glasses, but the content of Fe(II) was limited to less than 1 wt% [12]. 5.3. P a r t C

The results of this analytical procedure are summarized in fig. 4 which shows the thermal and atmospheric dependence of the Fe(III)-Fe(II) and

356

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H D Schretber et al // The redox state oftron m tektttes

357

Fe(II)-Fe(0) equilibria (as in fig. 1) for the tektite composition. Although there is much more experimental scatter of points than desired, the stability regions for the iron redox equilibria are clearly defined. Analyses of the virgin tektite glasses yielded (3.60 + 0.05)wt% Fe(II) and (3.62 + 0.05)wt% total Fe. Redox titrations on these same samples indicated the glasses possessed (3.63_ 0.06)wt% Fe(II). Once again, the tektites possessed essentially all their dissolved Iron as Fe(II), whereas the re-synthesized samples for this composition always show an F e ( I I I ) - F e ( I I ) or Fe(II)-Fe(0) equilibrium present in the glass. The analytical procedure for determination of iron redox states in this part is believed to be superior to the other chemical methods for these tektite samples. Ferrous iron and total iron were measured on the same aliquot of sample by the same experimental technique in order to rmnimaze the error in ferric iron concentrations by difference. Interferences should also be minimlzed. Nevertheless, the results of part A are in basic agreement with those of part C.

6. Discussion 6.1. The redox state o f tron m t e k t t t e s

The glass matrix of the tektites analyzed in this study contained essentially all its iron as ferrous iron. Within experimental error, no ferric iron was detected in the glasses by chemical procedures. As an upper limit to the amount of ferric iron that might be present and escaped detection, no more than 1 to 2% of all dissolved iron as Fe(III) is plausible. This is in agreement with the previously discussed magnetic and Mossbauer studies that estabhshed an upper limit of 0.01 or 0.05 for the ferric/ferrous ratio m tekUtes. Unfortunately, it is difficult to analyze directly for ferric iron in glasses by chemical means. Fe(III) concentrations are obtained by the differences between the experimental total iron content and the measured Fe(II) concentration; or potentially very small numbers are calculated from the difference of two relatively large numbers. Nevertheless, the results indicate with confidence that there must be very little, if any, Fe(III) in the tektite glasses. Perhaps any trace quantity of analyzed Fe(III) in the glass can be attributed to contamination or weathered surfaces. 6.2. Iron r e d o x equtlibrta m t e k t t t e m e l t s

Tektites remelted under a variety of controlled, known conditions estabhshed the position of the iron redox equilibria with respect to melt temperature and imposed oxygen fugacity. Under relatively oxidizing conditions (low temperature and high oxygen fugacity) the F e ( I I I ) - F e ( I I ) redox equilibrium m the melt is operational, while Fe(II)-Fe(0) redox equilibrium was defined under relatively reducing conditions. The T - f o 2 regions for the establishment

358

H D Schrezber et al / The redox state o f Iron m tektttes

of these equilibria in tektite melts are in agreement with those defined by Schrelber et al. [12] in basaltic compositions and by Schreiber et al. [14] in borosilicate melts. The definition of the Fe(III)-Fe(II)-Fe(0) equdlbria with respect to temperature and oxygen fugacity in tektite and other glass-forming melts is such that there is no T - f o 2 region where only Fe(II) exists in the melt.

6.3. Iron redox equihbrla and the formatton of tektttes Figs. 1 and 3 illustrate the difficulty in synthesizing a glass from a tektite composition containing only Fe(II). If one assumes that a trace of Fe(III) is present in tektites, the exact oxygen fugacity approximately intermediate to the Fe(III)-Fe(II) and Fe(II)-Fe(0) equilibria must be attained in the formation of all tektites. It is unhkely that these redox conditions could be achieved uniformly on a large scale such as tektite melts. An alternative way to obtain Fe(II) in a glass at the expense of Fe(III) or Fe(0) in an equilibrium situation is to add reducing agents or oxidizing agents respectively. However, the only other multivalent elements in sufficient concentration in tektites to affect the redox state of iron are manganese and titanium. The presence of Mn(III) or Ti(III) m the melt would produce more Fe(III) or Fe(0) in the melt, instead of more ferrous iron [12,17]. The evidence seems to indicate that tektites contain essentially no ferric iron. However, previously cited studies have reported that perhaps some tektites contain residual iron metal as N i - F e spherules. This is in agreement with the observation that Fe(III) and Fe(0) probably do not coexist in glass-forming melts [12]. Although other scenarios may be plausible to explain the redox state of iron in tektites, the following seems to fit the results of this study best. The source melt of the tektites initially contained iron in the Fe(II)-Fe(0) equilibrium. Of course, this implies a relatively reducing set of conditions, most likely low oxygen fugacitaes. Thus the dissolved iron in the melt is essentially all ferrous iron, but finely dispersed molten metal is present in equilibrium. If sufficient time is allowed for the metal to coagulate, definite phase separation can occur whereby a metal rich layer is in equilibrium with the silicate melt. Because of density differences, the metal rich layer separates from the melt immediately before the melt quenches to the glass. The glass thus freezes the redox state of iron contained therein as Fe(II), so that the tektites possesses little, if any, ferric iron. Residual metallic spherules would be expected in some tektites, as observed. O'Keefe [4] also presents addiuonal evidence that tektites formed in equilibrium with molten iron: the magnetic inclusions with nickel-free iron cores and the nickel content of the metallic spherules. With the choice of a lunar volcanic or a terrestrial impact origin for tektites [18], the described scenario seems to agree better with the lunar origin of tektites in order to achieve the needed reducing conditions. Although it makes the terrestrial origin unhkely, it does not preclude this possibility. Terrestrial materials contain iron as Fe(III) and Fe(II), and it is not inconceivable that a

H D Schretber et a l /

The redox state o f tron m tektttes

359

m e l t c o n t a i n i n g b o t h F e ( I I I ) a n d F e ( I I ) c a n s e p a r a t e t h e F e ( I I I ) l e a v i n g the t e k t i t e m e l t w i t h o n l y F e ( I I ) . O n e w o u l d h a v e to i m p o s e t o t a l r e d u c t i o n of all i r o n in the m e l t to the F e ( I I ) - F e ( 0 ) in the i m p a c t , w h i c h w o u l d a p p e a r to be u n l i k e l y f r o m a k i n e t i c p o i n t o f v i e w as s u c h r e d o x r e a c t i o n s r e v o l v e the d i f f u s i o n o f o x y g e n i n t o o r o u t o f the m e l t [16]. In c o n c l u s i o n , the e v i d e n c e f r o m the r e d o x s t a t e of w o n in t e k t i t e s f a v o r s the l u n a r o r i g i n b u t d o e s n o t t o t a l l y e x c l u d e the terrestrial a l t e r n a t i v e .

7. Conclusions T h e r e d o x state o f i r o n in t e k t i t e s ts e s s e n t i a l l y all F e ( I I ) . H o w e v e r , the t e k t i t e m e l t m a y h a v e f o r m e d in e q u i l i b r i u m w i t h i r o n m e t a l , b u t t h e m e t a l w a s s e p a r a t e d f r o m t h e m e l t i m m e d i a t e l y b e f o r e t h e t e k t i t e m e l t was q u e n c h e d to the glass. F o r m e l t s o f t h e t e k t i t e b a s e c o m p o s i t i o n , F e ( I I I ) o r F e ( 0 ) a l w a y s e x i s t e d m e q u i l i b r i u m w i t h F e ( I I ) o v e r the r a n g e o f m e l t t e m p e r a t u r e s a n d i m p o s e d o x y g e n fugacities. T h i s r e s e a r c h was s u p p o r t e d b y the N a t i o n a l A e r o n a u t i c s a n d S p a c e A d m i n i s t r a t i o n ( N A S A ) u n d e r c o n t r a c t no. N S G - 7 3 5 5 . T h e a u t h o r s t h a n k L a u r e l C o m s t o c k a n d T h o m a s M a n l e y for d o i n g s o m e e x p l o r a t o r y w o r k in t h e initial p h a s e o f this research. I n a d d i t i o n , the a u t h o r s are g r a t e f u l to C h a r l o t t e W. S c h r e i b e r for h e l p in the p r e p a r a t i o n o f this m a n u s c r i p t .

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