Authigenic Greigite Research Articles

On the detailed magnetostratigraphy of greigite — (Smythite) mineralization, Sokolov Brown-Coal Basin, bohemia

In the Middle Miocene claystones containing fossil micro-organic matter and overlying the brown-coal seams in the “Jiři” quarry of the Sokolov Brown-Coal Basin, a strongly magnetic layer was found and subjected to detailed palaeomagnetic investigations. The principal carrier of magnetism in this bed (called Kocian's bed) is the ferrimagnetic mineralization of greigite or greigitesmythite showing pronounced metastable properties. This mineralization must be treated with caution during laboratory tests aimed at deriving palaeomagnetic directions. In Kocian's bed, about two metres thick, two zones of palaeomagnetic field transition were identified. The high degree of demagnetization achieved by using thermal stepwise procedures and a MAVACS apparatus enabled the origin of the self-reversal of remanence to be defined after heating to 360°C. The process of chemo-remanent magnetization of the authigenic greigite-(smythite) mineralization fossilizing the palaeomagnetic field was relatively fast; the transition of the palaeomagnetic field is recorded in layers not exceeding a thickness of 2 × 10−2 m.

A stable chemical remanence in Holocene sediments

The ferrimagnetic iron sulphide, greigite, has been extracted from freshwater sediments from Loch Lomond and Llyn Geirionydd and identified by X‐ray diffraction and thermomagnetic analyses. The authigenic greigite is probably the dominant carrier of a stable natural remanent magnetization in the freshwater sediments underlying marine sediments in Loch Lomond and in the Lateglacial interstadial sediments from Llyn Geirionydd. Mineral magnetic analysis shows that greigite tends to have a higher SIRM/X ratio (approximately 70 kAm−1) than many natural magnetite assemblages. This magnetic characteristic coupled with greigite's ability to oxidize easily and to change to a less magnetic mineral upon exposure to air may be used to indicate its presence in downcore magnetic profiles. Alternating field demagnetization cannot be used to distinguish chemical from detrital remanences in Loch Lomond or Llyn Geirionydd on account of the similar magnetic stabilities of greigite and magnetite. The time lag between the acquisition of the primary detrital remanence and the secondary chemical remanence remains unknown and creates difficulties in understanding the record of paleomagnetic secular variation contained in these lake sediments.