Creator:
Date Created:May 2, 1959
Place Created:
Keywords:Spinel-Olivine inversion,phase changes
Context:article reprinted from "Nature"
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(Reprinted from Nature, Vol. 183, p. 1257 only, May 2, 1959)
The Spinel-Olivine Inversion in Mg2Ge04*
During the past few years there has been active interest in the problem of alternative explanations other than layers of different composition in the Earth to explain seismic data. The most important of these arises from an early suggestion by Bernal1 regarding the possible inversion of the orthorhombic mineral olivine to a dense form with the spinel structure. An actual example of this inversion from the spinel to the olivine had been reported by Goldschmidt* for the compound Mg2Ge04. However, there have always been some doubts regarding this observation, since it was mentioned only in a footnote and could not be repeated.
With the revived interest, there have apparently been several attempts3 to synthesize the spinel form of Mg2Ge04. But the general conclusion was that this form probably does not exist. However, in 1954, Roy and Roy1 reported the reproducible synthesis and reversible inversion to olivine of a spinel form of MgaGe04. This work has been generally missed, since the paper describing it was concerned with the morphology of synthetic serpentines. In 1955, a quantitative equilibrium study of the whole problem was started with the view of determining the pressure-temperature conditions for the inversion of natural olivines into spinel structures. The problem was tackled in stages by determining a series of t-x sections of the Mg2Si04-MgaGe04 system ; the study of t-x sections with 10 and 20 mol. per cent Fe4* replacing Mg2+, and finally a p-x section of the MgaGe04-MgaSi04 system at 542° C. up to the experimental limit of 65,000 atm. Many of these results were presented orally at the 1956 annual meeting of the Geological Society of America5. While the results of this extensive experimental study will be published later elsewhere, it is considered worth while to summarize here the new data on the spinel-olivine inversion obtained from some four hundred runs in hydrostatic or uniaxial pressure devices.
The work utilized high-pressure water as a catalyst, and under such conditions there is no problem with reproducible synthesis of the spinel phase. The inversion temperature for the Mg2Ge04 (spinel) MgaGe04 (olivine) equilibrium (obtained by extrapolation from runs as low as 1,000 lb./sq. in.) is 810° C. at atmospheric pressure. The AF of inversion
•Contribution No. 58-28, College of Mineral Industries, The Pennsylvania State University. University Park,Pennsylvania, U.S.A .
from X-ray data is 3 -5 cm.s/mol.; AH (calculated from the pressure-dependence slope) is 3,690 cal./mol. The entropy of inversion is thus a reasonable 3 -4 e.u. The inversion temperature is raised by 0 025 deg. C./ bar for the first 5,500 bars. X-ray intensities and infra-red absorption spectra clearly show that Mg2Ge04 is an inverse spinel. It may be of interest to record that the inversion in Mg2Si04 as determined by extrapolation of experimental points up to 60 mol. per cent Mg2Si04 in the p-x section is set at 100 ± 15 Idlobars at 542° C. Further, the change in lattice spacings of the spinel solid solutions of Mg2Ge04-Mg2Si04 shows tliat the spinel form of Mg2SiG4 will have a cell edge of 8-22 A. Therefore, the AF of this transition is 2 0 cm.3/mol. The dependence of the Mg2SiOi transition upon pressure is beyond direct experimental study at present, but from a consideration of various sections in the p-t-x volume constructed with the data for the system Mg2Ge04-Mg2Si04, one would expect that it will be only 0 013 deg. C./bar.
Frank Dachiixr Rustum Roy
Department of Geophysics and Geochemistry, College of Mineral Industries, Pennsylvania State University. Jan. 28.
> Bernal, J. U., Geophys. Dis. Roy. Astro. Soc., No. 748, 267 (1936).
*	Goldscluuidt, V. M., Nacht. Uesell. Wissensch. Gdttingen, Math. Phys.
Kl., Fachgr. IV, Bd. 1, 184. 190 (1931). ' Drey, H. C., "The Planets", 69 (Yale University Press, New Haven. 1952). Romeijn, 1'. C., Phillips Res. Rep., 8, 321 (1953). Bertaut, E. F., Durlf-Varambon, A., and Pauthenet, E., "ProprieWs Cristallographiques et Magnt'-tique de Quelques Nouvelles Series de Spinelles Jlistes", Third Int. Cong. Crvst.. Paris (1954). Ring wood, A. E., Amur. J. Sci., 254, No. 11, 707 (1956). 'Hoy, D. M., and Roy, R., .Inter. Min., 39, 957 (1954).
•	D.iehille, F., and Roy, R., Hull. (Jeol. Sar. Amer.. 67. 1682 (1956)
(Abstract).
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