Magnetite-like nanocrystals formed by laser-driven shocks in siderite
Introduction
Siderite (FeCO3) is a common carbonate mineral in iron-bearing sediments and soils. It is present on Earth in a large variety of environments, which can be targets of impacts. This mineral has also been identified in meteorites (e.g. [1]). Tiny magnetite crystals were found in the ALH84001 meteorite, a Martian orthopyroxenite, in carbonate globules consisting of MgCO3–FeCO3 solid solutions with variable Ca and Mn contents. It was initially suggested that the morphologies and grain sizes of these grains were typical of magnetite produced by magnetotactic bacteria living on Earth, suggesting a biological origin [2], [3], [4], [5]. A second hypothesis is that the magnetite from ALH84001 was formed by shock-induced decomposition of siderite [6], [7], [8], [9], [10], [11], [12], [13], [14]. The purpose of the present study is to investigate whether ultra short shocks on iron carbonates can generate nanometer-sized magnetite crystals.
Section snippets
Materials and methods
The siderite samples were collected from the Saint Georges d'Hurtière iron mine, Savoie, France. This natural siderite is Mn- and Mg-rich and contains a small amount of hematite due to partial oxidation of the natural rock. In a previous paper [15], we studied the mineralogical transformations of Mn-siderite heated in air. In the present study, we performed laser-driven shock experiments in order to document the transformations of siderite under dynamic pressure conditions. The laser-driven
Characterization of the starting material
In order to detect subtle shock effects, it is necessary to have a good knowledge of the accessory phases in the starting material. The samples used for the laser-driven shock experiments consisted of well-crystallized, millimeter-sized grains of Mn-rich siderite Fe0.79(2)Mn0.12(2)Mg0.07(2)Ca0.02(1)CO3 partially oxidized into Mn-hematite. A detailed analysis of the initial material has been given in a previous study [15]. Raman spectra of the siderite powder were recorded at ambient
Shock-recovered samples
The shock-recovered samples have a color slightly darker than the initial chips and show craters of a few millimeters in diameter, corresponding to the irradiated areas (see Table 1).
Discussion
Raman spectra and TEM observations revealed the coexistence of three different phases in the centre of the impact craters, two types of iron oxides (hematite and a spinel phase) associated with the iron carbonate. Hematite was present in the initial material and comes from the oxidation of siderite, which is a metastable phase under ambient conditions of temperature, pressure and oxygen fugacity. The interesting feature of the present results is the appearance of a magnetite-type phase in the
Conclusion
We have shown that natural Mn-bearing siderite decomposes to Mn-magnetite-like phase by high-pressure shock-induced decarbonation within durations as short as a few nanoseconds. Magnetite, stabilized by the presence of Mn in its crystal lattice, which prevents it from further oxidation, is able to record the magnetic field present during its growth. The existence of stable remanence linked to such transformations may have profound implications for paleomagnetic studies of carbonate-bearing
Acknowledgements
The authors are pleased to acknowledge Dominique Gasquet from ENSG, Nancy, for having provided us with siderite and Gilles Montagnac for his assistance in the Raman experiments. Two helpful and constructive reviews helped to improve the manuscript. R van der Hilst is thanked for his careful edition of the paper.
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