Mossbauer, magnetic, X-ray fluorescence and transmission electron microscopy study of natural magnetic materials from speleothems: haematite and the Morin transition

Detrital magnetic materials within cave stalagmitic formations, e.g., haema tite or magnetite, carry remanence whose vector is of value in dating. Magn etometry measurements on a particular haematite-bearing sample reveal that remanence was substantially restored and/or conserved on rewarming after co oling below the Morin transition temperature. Mossbauer measurements indica te the presence of two types of haematite, distinguished primarily by parti cle size. The majority is small in size, partially exhibiting superparamagn etism, and does not undergo a Morin transition above liquid nitrogen temper ature. Superparamagnetic goethite is the second major component. Mine haema tite samples of surface location with different color and mineralogical com position have also been studied. Possible relations between the mineralogic al composition of the mine samples and detrital stalagmitic magnetic materi al, the modifications and the origin of this mineralization are discussed. Special attention is paid to the "irreversible" Morin transition in large e nough (> 20 nm) haematite particles and the possible loss of natural remane nt magnetization and hence of palaeomagnetic records.