Magnetic properties of natural goethite-III. Magnetic behaviour and properties of minerals originating from goethite dehydration during thermal demagnetization

Abstract

The magnetic behaviour during stepwise demagnetization of a set of artificial samples, containing well-defined natural goethite from five localities (chemical composition, crystallite size and grain size are known) is reported. Differential Scanning Calorimetry measurements indicated that the goethites converted to haematite between 260" and 360°C. In this temperature range a small decrease in initial susceptibility and an additional remanence decay leading toward an extra bending point in the thermal decay curve are observed. Because titanomagnetite or pyrrhotite are absent in the samples, this extra bending point is tentatively interpreted as being due to recrystallization of the minor haematite already present in the original goethite concentrates, facilitated by water made available through the goethite/haematite conversion. At much higher temperatures, at some 600 "C, a self-reversal of the remanence is observed in most specimens.The haematite derived from the goethites between 260" and 360 "C appeared to be prone to further alteration at successive higher demagnetization temperatures. Chemical alterations in the magnetic mineral suite, starting at temperatures from 380 "C upward-depending on the grain size of the original goethite-were inferred from an increase in initial susceptibility. The susceptibility behaviour showed an appreciable variation, due to the creation of varying trace amounts of magnetite, which appeared to be already present after the 400°C step. After the final 685°C demagnetization step the magnetic mineralogy was usually dominated by magnetite. The observed variation is thought to be the result of local within-specimen differences in availability of the amount and possibly also the composition of the vapour phase. The vapour phase is due to chemical reactions between the matrix constituents: waterglass and calcite, leading to a C 0 2 / H 2 0 vapour phase. Reducing capacity, necessary for the magnetite formation, is envisaged to be created by decomposition of trace amounts of organic matter at relatively low temperatures, up to some 400°C. At higher temperatures (over 550°C) decomposition of traces of ferrous iron bearing clayminerals presumably donates the ferrous iron.Differences in the magnetite/haematite ratio as well as in the properties of both minerals were assessed in some detail with more or less routine rockmagnetic methods: acquisition of the isothermal remanent magnetization at room temperature (in fields up to 11.2MAm-' or 14T), AF demagnetization and low-temperature cycling of the saturation remanence. This was done after the 400 "C step to study the magnetic properties of the newly formed haematite and after the 685°C step to evaluate the properties of the magnetite and more evolved haematite.After the 400°C step the haematite appeared to be very fine-crystalline and magnetically extremely hard. Its saturation remanence is considerably lower than that of well-crystalline haematite. After the 685 "C step its hardness has decreased and its saturation remanence slightly incre...

Preliminary Rock Magnetic Study of Archaeomagnetic Samples from Bulgarian Prehistoric Sites.

“…On the other hand, the domain state of the ferrimagnetic grains determines, to a great extent, the values of the hysteresis parameters. For non-interacting SD grains the ratio J,.9/J, should be 0.5, but this value is rarely achieved because of magnetostatic interactions among grains (Davis and Evans, 1976;Dunlop, 1981). Thus as a result of interactions J,.9/J9 and H112 may be lowered, while Hc,, and Hcr' may become higher (Dankers, 1981).…”

“…Since goethite is unstable upon heating, its transformation will affect NRM and TRM behaviour during the Thellier experiments. Unblocking temperatures are typically found to be between 70-120'C, but its dehydration occurs in the temperature interval 250-400'C (Dekkers, 1990). As the remanent magnetization carried by goethite is weak, its concentration must be considerable to have a significant influence on the total NRM (considered as a TRM).…”

Rock-magnetic analyses as a tool to investigate archaeological fired sediments: a case study of Mirador cave (Sierra de Atapuerca, Spain)

“…However, the contribution of this component in clays is in absolute numbers lower than in the ashes, rubefactions and facies TF, which are magnetically enhanced by low-coercive material. According to this, a slightly harder and recrystallized magnetic variety of haematite might have been generated by annealing as was shown by Dunlop (1971) and Dekkers (1990). Finally, a third component is fitted with percentages between 5 and 10 per cent and a very low coercivity (3-5 mT).…”

“…• C) are the dehydroxylation of lepidocrocite (Gehring & Hofmeister 1994;Gendler et al 2005), goethite (Gehring & Heller 1989;Dekkers 1990) and the oxidation of siderite (Hus 1990;Hirt & Gehring 1991). Heating does not only involve magnetic phase transformations, but also changes in the magnetic grain sizes of the assemblage (Jordanova et al 2001).…”

Low‐temperature magnetic properties of lepidocrocite

“…Although there have been numerous studies on the magnetic properties of goethite [Bocquet and Kennedy, 1992;Dekkers, 1989aDekkers, , 1989bDekkers, , 1990Hedley, 1971], less is known about the other common soil iron hydroxides (e.g., ferrihydrite and lepidocrocite). Zergenyi et al [2000] examined the magnetic properties of a highly crystalline ferrihydrite.…”