Abstract
The kinetics of cation disordering in a natural ordered (P2/n) omphacite have been followed at P=18 and 30 kb, T= 750–1,260° C, for times of between 1.5 min and 16 days in a piston-cylinder apparatus. Time-temperature-transformation (TTT) analysis of the experimental data, using the presence or absence of the 11¯1 reflection in single crystal X-ray precession photographs to indicate the extent of reaction, yields an equilibrium order/disorder temperature (T ord) of 865±10° C, an activation enthalpy (1 bar) of 71±6 kcal mole−1 and an activation volume of 9±4 cm3 mole−1 (plus and minus figures represent the precision of a best fit between experimental data and TTT theory rather than absolute errors). The activation volume is consistent with a vacancy mechanism of cation diffusion. H2O, added in the form of oxalic acid, appears to speed the process up slightly. The overall transformation mechanism is continuous, involving neither the nucleation of a disordered phase nor a change in antiphase-domain distribution. This is consistent with both first- and non-first-order character for the C2/c⇋P2/n transformation, though a range of ordered states below T ord is indicated by the weakening of h+k=odd reflections. A simple extrapolation of the disordering rates to geological conditions leads to the first estimate of how long disordered omphacites would take to order in nature, ranging from less than one year at T≈800° C to more than 107 years at T<350° C.
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References
Besancon JR (1980) Rate of cation disordering in orthopyroxenes. Trans Am Geophys Union 17: (abstr) 407–408
Boyd FR, England JL (1960) Apparatus for phase equilibrium measurements at pressures up to 50 kilobars and temperatures up to 1,750° C. J Geophys Res 65:741–748
Boyd FR, England JL (1963) Effect of pressure on the melting point of diopside, CaMgSi2O6 and albite, NaAlSi3O8 in the range up to 50 kilobars. J Geophys Res 68:311–323
Buening DK, Buseck PR (1973) Fe-Mg lattice diffusion in olivine. J Geophys Res 78:6852–6862
Carpenter MA (1979) Omphacites from Greece, Turkey and Guatemala: composition limits of cation ordering. Am Mineral 64:102–108
Carpenter MA (1980a) Kinetics of disordering reactions in omphacite at high pressures and temperatures. Geol Soc Am Abstr Progr 7:398
Carpenter MA (1980b) Mechanisms of exsolution in sodic pyroxenes. Contrib Mineral Petrol 71:289–300
Carpenter MA (1980c) Composition and cation order variations in a sector-zoned blueschist pyroxene. Am Mineral 65:313–320
Carpenter MA (1981) Omphacite microstructures as time-temperature indicators of blueschist- and eclogite-facies metamorphism. Contrib Mineral Petrol 78:441–451
Carpenter MA, Okay A (1978) Topotactic replacement of augite by omphacite in a blueschist rock from north-west Turkey. Mineral Mag 42:435–438
Carpenter MA, Smith DC (1981) Solid solution and cation ordering limits in high temperature sodic pyroxenes from the Nybö eclogite pod, Norway. Mineral Mag 44:37–44
Clark JR, Appleman DE, Papike JJ (1969) Crystal-chemical characterisation of clinopyroxenes based on eight new structure refinements. Mineral Soc Am Spec Pap 2:31–50
Clark JR, Papike JJ (1968) Crystal-chemical characterisation of omphacites. Am Mineral 53:840–868
Cook HE (1969) The kinetics of clustering and short range order in stable solid solutions. J Phys Chem Solids 30:2427–2437
Curtis L, Gittins J, Kocman V, Rucklidge JC, Hawthorne FC, Ferguson RB (1975) Two crystal structure refinements of a P2/n titanian ferro-omphacite. Can Mineral 13:62–67
Donnay G, Wyart J, Sabatier G (1959) Structural mechanism of thermal and compositional transformations in silicates. Z Kristallogr 112:161–168
Fleet ME, Herzberg CT, Bancroft GM, Aldridge LP (1978) Omphacite studies, I: The P2/n→C2/c transformation. Am Mineral 63:1100–1106
Hilliard JE (1970) Spinodal decomposition. In: HI Aaronson (ed) Phase transformations, pp 497–560. Metals Park, Ohio: Am Soc for Metals
Holland TJB (1979) High water activities in the generation of high pressure kyanite eclogites of the Tauern window, Austria. J Geol 87:1–28
Johannes W, Bell PM, Mao HK, Boettcher AL, Chipman DW, Hays JF, Newton RC, Seifert F (1971) An interlaboratory piston-cylinder calibration using the albite-breakdown reaction. Contrib Mineral Petrol 32:24–38
Matsumoto T, Tokonami M, Morimoto N (1975) The crystal structure of omphacite. Am Mineral 60:634–641
McCallister RH (1978) The coarsening kinetics associated with exsolution in an iron free clinopyroxene. Contrib Mineral Petrol 65:327–331
McConnell JDC (1975) Microstructures of minerals as petrogenetic indicators. Ann Rev Earth Planet Sci 3:129–155
Misener DJ (1974) Cationic diffusion in olivine to 1,400° C and 35 kbar. In: AW Hoffman, B Giletti, HS Yoder Jr, RA Yund (eds) pp 117–129. Carnegie Inst Washington
Råheim A, Green DH (1975) P, T paths of natural eclogites during metamorphism — a record of subduction. Lithos 8:317–328
Rossi G, Tazzoli V, Ungaretti L (1978) Crystal-chemical studies on sodic pyroxenes. Proc XI IMA meeting, Novosibirsk 1:29
Shewmon PG (1963) Diffusion in solids. 203 p. New York, McGraw Hill
Slimming EH (1976) Electron optical studies of the feldspars. Ph D Thesis, Cambridge University
Walker D, Longhi J, Hays JF (1972) Experimental petrology and origin of Fra Mauro rocks and soil. Proc 3rd Lunar Sci Conf 1:797–817
Wood BJ, Holland TJB, Newton RC, Kleppa OJ (1980) Thermochemistry of jadeite-diopside pyroxenes. Geochem Cosmochim Acta 44:1363–1371
Yamauchi H, de Fontaine D (1974) Kinetics of order-disorder. In: H Warlimont (ed) Order-disorder transformations in alloys, pp 148–178. Berlin Heidelberg New York: Springer Verlag
Yokoyama K, Banno S, Matsumoto T (1976) Compositional range of P2/n omphacite from the eclogitic rocks of central Shikoku, Japan. Mineral Mag 40:773–779
Yund RA, Tullis J (1980) The effect of water, pressure and strain on Al/Si order-disorder kinetics in feldspars. Contrib Mineral Petrol 72:297–302
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Carpenter, M.A. Time-temperature-transformation (TTT) analysis of cation disordering in omphacite. Contr. Mineral. and Petrol. 78, 433–440 (1982). https://doi.org/10.1007/BF00375205
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DOI: https://doi.org/10.1007/BF00375205