Shock-induced formation mechanism of seifertite in shergottites
The Martian meteorites Shergotty, Zagami and Dhofar 378 have been re-investigated in order to elucidate the shock-induced formation of seifertite. The occurrence of orthorhombic seifertite (α-PbO2 structured SiO2) has been confirmed for the mesostasis of Shergotty and Zagami by transmission electron microscopy with lattice parameters of a = 4.05(1) Å, b = 5.05(1) Å and c = 4.45(1) Å. Seifertite crystals are interpreted as shock-induced transformation products occurring together with maskelynite of both plagioclase and alkali-feldspar composition in a largely preserved eutectic crystallisation texture. Shock-induced microstructures in accessory minerals demonstrate that these regions cannot have been completely re-molten. No further features indicating shock-pressures above ~30 GPa are detected. Hence, seifertite must have been formed below its stability field by a fast solid-state process. Significantly higher shock-pressures of Dhofar 378 indicate an inhibition of a potential seifertite crystallisation by resulting high post-shock temperatures. Crystallographic considerations reveal that a direct formation of seifertite from a high-pressure derivate of cristobalite is possible without breaking any silicon-oxygen bonds. Important implications arise from the existence of such a non-equilibrium pathway. Inferring shock-pressures from metastably formed phases appears implausible, and the transition pressure could be even below 30 GPa. Furthermore, the transformation product is determined by the precursor phase. Epitaxial intergrowth with other silica high-pressure polymorphs should be induced by certain features of the precursor, for example, planar defects, or heterogeneous strain conditions. Due to symmetrical considerations, seifertite will get amorphous during a potential back-transformation, which provides an explanation for the formation of numerous amorphous lamellae.
KeywordsSilica High-pressure Polymorphism Metastability Shergottites Transmission electron microscopy
The author thanks Falko Langenhorst, Friedrich-Schiller-Universität Jena, Germany, for providing samples of Shergotty and Zagami and together with Ahmed El Goresy, Bayerisches Geoinstitut, Universität Bayreuth, Germany, for several discussions. Hubert Schulze, Bayerisches Geosinstitut, Universität Bayreuth, Germany, is kindly acknowledged for the precise preparation of thin sections. The manuscript benefits from suggestions of Roman Scala and two further anonymous reviewers. The work was supported by Deutsche Forschungsgemeinschaft (grants BL 917/1-1).
- Aramovich CJ, Herd CDK, Papike JJ (2002) Symplectites derived from metastable phases in Martian basaltic meteorites. Am Mineral 87:1351–1359Google Scholar
- Chopin C, Henry C, Michard A (1991) Geology and petrology of the coesite-bearing terrain, Dora-Maira Massif, Western Alps. Eur J Mineral 3:263–291Google Scholar
- Dera P, Prewitt CT, Boctor NZ, Hemley RJ (2002) Characterization of a high-pressure phase of silica from the Martian meteorite Shergotty. Am Mineral 87:1018–1023Google Scholar
- Downs RT, Palmer DC (1994) The pressure behavior of alpha-cristobalite. Am Mineral 79:9–14Google Scholar
- Dubrovinsky LS, Dubrovinskaia NA, Saxena SK, Tutti F, Rekhi S, Le Bihan T, Shen GY, Hu J (2001) Pressure-induced transformations of cristobalite. Chem PhysLet 333:264–270Google Scholar
- Gatehouse BM, Grey IE, Lovering JF, Wark DA (1977) Structural studies on tranquillityite and related synthetic phases. Proceedings of the 8th lunar science conference, pp 1831–1838Google Scholar
- Hemley RJ, Prewitt CT, Kingma KJ (1996) High pressure behaviour of silica, in: silica: physical behavior, geochemistry and materials applications. Rev Mineral 29:41–81Google Scholar
- Hyde BG, Andersson S (1989) Inorganic crystal structures. Whiley, New York, pp 69–72Google Scholar
- Ikeda Y, Kimura M, Takeda H, Shimoda G, Kita NT, Morishita Y, Suzuki A, Jagoutz E, Dreibus G (2006) Petrology of a new basaltic shergottite: dhofar 378. Antarct Meteorite Res 19:20–44Google Scholar
- Lakshtanov DL, Sinogeikin SV, Litasov KD, Prakapenka VB, Hellwig H, Wang JY, Sanches-Valle C, Perrillat JP, Chen B, Somayazulu M, Li J, Ohtani E, Bass JD (2007) The post-stishovite phase transition in hydrous alumina-bearing SiO2 in the lower mantle of the earth. Proc Nat Acad Sci 104:13588–13590CrossRefGoogle Scholar
- Lovering JF, Wark DA, Reid AF, Ware NG, Keil K, Prinz M, Bunch TE, El Goresy A, Ramdohr P, Brown GM, Peckett A, Phillips R, Cameron EN, Douglas JAV, Plant AG (1971) Tranquillityite: a new silicate mineral from Apollo 11 and Apollo 12 basaltic rocks. Proceedings of the Second Lunar Science Conference, vol 1, pp 39–45Google Scholar
- Palmer DC, Finger LW (1994) Pressure-induced phase-transition in cristobalite—an X-ray-powder diffraction study to 4.4 GPa. Am Mineral 79:1–8Google Scholar