5 Mössbauer Studies of Tektites,
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Pyroxenes, and Olivines 1
J. G. MARZOLF, J. T. D E H N ,
2
and J. F. S A L M O N
3
Woodstock College, Woodstock, M d .
Mössbauer spectra have been measured for various tektites, as well as for both natural and synthetic iron-bearing silicate minerals. These results are reported and compared with other similar studies available in the literature. The ratios of the intensities of the appropriate Mössbauer lines have been used to determine the ferric-ferrous ratios where possible. The spectra of the ferrosilite-enstatite series of pyroxenes show four lines which are interpreted as two quadrupole split doublets, and the ratio of the intensities of these lines indicates the degree of ordering in filling the available metal ion sites. Similar studies on the fayalite-forsterite series of olivines are also reported.
' T ' e k t i t e s are natural glasses found in various strewn fields around the earth, hundreds or thousands of kilometers in diameter. Their spe cific gravities range from 2.3 to 2.5, and the specimens so far discovered vary from a fraction of a gram to about 3000 grams. Those found i n the Far East seem to form one strewn field and are designated indochinites, javanites, philippinites, australites, etc., according to the locality in which they are found. A field in Czechoslovakia contains tektites called moldavites, while in Texas a large number of tektites called bediasites occur. Different samples have been found in Georgia and Massachusetts and on the Ivory Coast in Africa. These objects range in age up to 30 million years. Their physical and chemical properties indicate that they were formed under violent 1
Present address: LeMoyne College, Syracuse, Ν. Y. * Present address: Materials Research Laboratory, Pennsylvania State University, Uni versity Park, Pa. * Present address: Loyola College, Baltimore, Md.
61 Herber; The Mössbauer Effect and Its Application in Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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T H E MÔSSBAUER E F F E C T A N D ITS APPLICATION I N CHEMISTRY
conditions at high temperatures ( ~ 2 0 0 0 ° C . ) in a reducing atmosphere, probably i n a collision involving a comet or a meteorite (since some tektites contain iron spherules) either on earth or on the moon. More distant origins are excluded because of the amount of radioactive Α 1 still present i n tektites. The variety of "button," "bowl," "teardrop," "peanut," etc., forms in which tektites are found suggests a three-step process for at least some of them: (1) spin shaping after impact ejection in the molten state, (2) aerodynamic sculpturing in flight, followed by (3) long term weathering and fragmentation. Some physical factors that distinguish them from other natural glasses are the presence of flow structures and strain birefringence, the presence of siliceous glass inclusions of lower refractive index than the surrounding glass, and the absence of microlites. The index of refraction of tektites ranges from about 1.48 to 1.52 and varies inversely with their silica content and directly with their iron content ( 1 - 5 % ) and magnetic susceptibility (2.3 Χ 10 to 8.4 χ 10" e.m.u./gram). The ratio of the real part of their permittivity to their permeability varies from 42 χ 10~ to 52 X 10~ sq. mho. Their chemical composition—silica ( 7 0 - 8 0 % ) , alumina ( ~ 1 0 % ) / F e O and F e 0 ( 1 - 5 % total), M g O , C a O , N a 0 , K 0 , T i 0 (0.5-2.5% each), and smaller amounts of other elements—is roughly the same as that of other natural glasses yet sufficiently different to justify separate classification. For example, tektites have a higher alumina content, a higher ( F e O + M g O ) / ( N a 0 + K 0 ) ratio, an unusually low water content ( 6, the F s case already referred to, was chosen. Morimotos (22) x-ray coordinates for clinoenstatite were used as representative of this end of the series, while the coordinates of hortonolite (Mg .47,Feo.53)2Si0 determined from x-ray studies by Gibbs and co-workers (14) were taken as representative of the olivine series. 2
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0
5 3
4
Preliminary calculations involving only the six nearest-neighbor oxygen ions yielded the observed temperature-dependent quadrupole splittings but were unable to distinguish the M i and M pyroxene sites without using a covalency factor a ~ 1 for the site with the larger quadrupole splitting and a « 0.75 for the site with the smaller quadru pole splitting. This was true both for the orthopyroxene and the clinopyroxene calculations. (For the clinoenstatite case note that Morimoto reverses the more common designation of M i and M sites used by Ghose. ) In any case these preliminary calculations failed to yield a satisfactory independent check on the observation that the Mossbauer iron-favoring site of smaller quadrupole splitting is indeed the M site of Ghose's x-ray analysis. 2
2
2
2
2
The necessity of assigning M as the more covalent of the two sites (a ~ 0.75) is consistent with its slightly smaller quadrupole splitting (9, 10). The fact that M is the more distorted octahedron and yet corre sponds to the smaller quadrupole splitting is not particularly surprising since less than one-fifth of the E F G arises from the direct contribution of the lattice, the rest arising from the sixth d-electron i n F e . A differ ence i n the Lamb-Môssbauer fraction between the two sites is a possible complicating factor which is difficult to estimate. The olivine calculation also was able to yield the observed doublet splitting by taking a ~ 1 for both sites. More elaborate calculations on pyroxenes and olivines are i n progress. 2
2
2
2+
2
Summary The Mossbauer spectra have been measured for various tektites, is well as for both natural and synthetic iron-bearing silicate minerals. The tektite spectra show a pair of broad lines, unequal i n both width and height, which may be interpreted as a quadrupole split doublet owing to F e i n a glasslike environment. The mineral spectra show narrower lines, consistent with a crystal structure, and the ratios of the intensities of the appropriate Mossbauer lines have been used to determine the ferric-ferrous ratios. The values obtained this way are less precise than (but i n generally good agreement with) those obtained by the methods of wet chemistry. The spectra of the ferrosilite-enstatite series of pyrox2+
Herber; The Mössbauer Effect and Its Application in Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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T H E MÖSSBAUER E F F E C T A N D ITS APPLICATION I N CHEMISTRY
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enes show four lines which are interpreted as two quadrupole split doublets arising from two different electric field gradients and indicating that there are two unequivalent sets of sites for the iron. The ratio of the intensities of these two doublets indicates a high degree of ordering, and the preferred site for iron has the smaller electric field gradient. Similar studies on the fayalite-forsterite series of olivines indicate that at room temperature all iron nuclei see the same electric field gradient and electron density. Acknowledgment The authors gratefully acknowledge the financial support of the National Aeronautics and Space Administration through research grant NsG-670, which has made this study possible. W e are also grateful for many helpful discussions with John A . O'Keefe and Louis Walter, Goddard Space Flight Center ( N A S A ) , who supplied us with generous computer and laboratory assistance through the extensive facilities of Goddard. Finally, we thank Donald Lindsley and George Fisher, Car negie Geophysical Laboratory of Washington, George Switzer, John S. White, Jr., and Roy S. Clarke, Jr., U . S. National Museum, and Donald L . Bailey, Pittsburgh Plate Glass Co., for having supplied us with a number of the samples used in these experiments. Literature Cited (1) Afanas'ev, A. M . , Kagan, Yu, Soviet Phys. JETP (English Transl.) 18, 1139 (1964). (2) Bancroft, G. M . , Burns, R. G., International Mineralogical Society meet ing, Cambridge, England, 1966. (3) Bancroft, G. M . , Maddock, A. G., Burns, R. G., Strens, R. G. J., Nature 212, 913 (1966). (4) Belyustin, Α. Α., Ostanevich, Yu. M . , Pisarevskiĭ, A. M., Tomilov, S. B., Bai-shi, U., Cher, L., Soviet Phys. Solid State 7, 1163 (1965). (5) Blume, M . , Phys. Rev. Letters 14, 96 (1965). (6) Bradford, E., Marshall, W., Proc. Phys. Soc. (London) 87, 731 (1966). (7) de Coster, M . , Pollak, H . , Amelinckx, S., Phys. Status Solidi 3, 283 (1963). (8) Deer, W . Α., Howie, R. Α., Zussman, J., "Rock-Forming Minerals," Vols. 1 and 2, Wiley, New York, 1962. (9) Epstein, L. M., J. Chem. Phys. 36, 2731 (1962). (10) Ibid., 40, 435 (1964). (11) Evans, B. J., Ghose, S., Hafner, S., J. Geol. (to be published). (12) Ghose, S., Amer. Mineral. 47, 388 (1962). (13) Ghose, S., Z. Kristallogr. 122, 81 (1965). (14) Gibbs, G. V . , Moore, P. B., Smith, J. V . , Ann. Meeting, Geol. Soc. Am., New York, Ν. Y., Nov. 17-20, p. 66A, 1963. (15) Gol'danskiĭ, V. I., Makarov, E . F., Khrapov, V . V., Phys. Letters 3, 344 (1963). (16) Hutchings, M . T., Solid State Phys. 16, 227 (1964). (17) Ingalls, R., Phys. Rev. 133, A787 (1964).
Herber; The Mössbauer Effect and Its Application in Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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5.
MARZOLF E T AL.
Tektites, Pyroxenes, and Olivines
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(18) Kurkjian, C. R., Buchanan, D . Ν. E . , Phys. Chem. Glasses 5 (3), 63 (1964). (19) Kurkjian, C. R., Sigety, Ε. Α., VIIth International Congress on Glass, Brussels, 1965, Paper 39, 1966. (20) Lindsley, D . H . , Davis, B. T. C., MacGregor, I. D., Science 144, 73 (1964). (21) Mattern, P. L . , Ph.D. thesis, Cornell University, Ithaca, Ν. Y., 1965. (22) Morimoto, Ν., Z. Kristallogr. 114, 120 (1960). (23) O'Keefe, J. Α., "Tektites," University of Chicago Press, Chicago, 1963. (24) Pollak, H . , de Coster, M . , Amelinckx, S., Phys. Status Solidi 2, 653 (1962). (25) Roothaan, C. C. J., J. Chem. Phys. 19, 1445 (1951). (26) Rose, M. E., "Elementary Theory of Angular Momentum," p. 60, Wiley, New York, 1957. (27) Spijkerman, J., et al., private communication. (28) Sprenkel-Segel, E . L . , Hanna, S. S., Geochim. Cosmochim. Acta 28, 1913 (1964). (29) Walker, L . R., Wertheim, G. K., Jaccarino, V., Phys. Rev. Letters 6, 98 (1961). (30) Watson, R. E., Solid State and Molecular Theory Group, M.I.T., Tech. Rept. 12 (1959). RECEIVED
January 10, 1967.
Herber; The Mössbauer Effect and Its Application in Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
