The Magnetic Properties of Naturally Occurring Goethite

Strangway, D. W.; Honea, Russell M.; McMahon, B. E.; Larson, Edward J.

doi:10.1111/j.1365-246x.1968.tb00191.x

Cited by 72 publications

(31 citation statements)

References 17 publications

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“…A fall in Xi, during the goethite/haematite conversion reaction was also observed by Strangway et al (1968). These authors also detected that some goethites showed hardly any decrease in Xi, during this reaction (like the MKB goethite); this different behaviour of Xi, could be related to the spontaneous magnetization of the created haematite.…”

Section: Initial Susceptibility Behaviourmentioning

confidence: 71%

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

Dekkers

1990

Geophysical Journal International

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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...

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Section: Initial Susceptibility Behaviourmentioning

confidence: 71%

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

Dekkers

1990

Geophysical Journal International

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“…A likely candidate for the third component of the mineralogy is goethite. The maximum unblocking temperature of goethite ranges from 60°C to 120°C (Strangway et al, 1968;Dekkers, 1988Dekkers, , 1989), but goethite dehydrates during heating at temperatures as low as 50°C (Goss, 1987), leading to complete transformation into hematite at about 250°C (Wolska and Schwertmann, 1989). We interpret the thermal unblocking of the hard and intermediate components at temperatures up to 240°C to be due to unblocking of goethite, accompanied by goethite dehydration, hematite formation, and partial unblocking of hematite (see Roberts et al, 1995).…”

Section: Wasp-waisted Hysteresis Loopsmentioning

confidence: 89%

Mineral Magnetic Studies of Archaeological Samples: Implications for Sample Selection for Paleointensity Determinations.

Cui¹,

Verosub²,

Roberts³

et al. 1997

J. geomagn. geoelec

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Samples with a significant fraction of multidomain magnetic grains, hard secondary components and thermally unstable magnetic phases have been shown to be unreliable for paleointensity studies. However, mineral magnetic screening is rarely performed before paleointensity determinations are made even through non-ideal magnetic properties are the main reasons for rejecting data after the work has been completed. We have conducted a detailed mineral magnetic investigation of 23 archaeological samples from Bulgaria which yielded both satisfactory and unsatisfactory paleointensity results. Our study demonstrates how the non-ideal magnetic properties lead to unacceptable paleointensity results. We have used our findings to develop a simple and practical sample selection procedure which requires only two specimens from each sample and which can be done with conventional paleomagnetic equipment. We suggest that any sample which fails to pass this screening should not be subjected to time-consuming Thellier-Thellier experiments.

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“…It may occur in trace amounts in virtually all sediments. Goethite may in principle carry a thermoremanent magnetization (TRM) as was first established by Strangway et al (1967Strangway et al ( , 1968, but most goethite remanences in natural rocks are considered to have been acquired below its NCel point by growth of goethite through its blocking volume, which makes it a chemical remanent magnetization (CRM) (e.g. O'Reilly 1984).…”

Section: Introductionmentioning

confidence: 98%

Magnetic Properties of Natural Goethite-I. Grain-Size Dependence of Some Low- and High-Field Related Rockmagnetic Parameters Measured At Room Temperature

Dekkers

1989

Geophysical Journal International

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S U M M A R YFive goethite concentrates, separated from natural samples of different supergene geological origin, are sieved in 12 well-defined grain-size fractions ranging from 250 pm down to <5 pm. The goethites show differences in their isomorphous substitution and in their contents of dispersed non-Fe elements. The crystallite size ranges from 240 to 480& which is considerably smaller than the grain size of even the smallest fraction. Nevertheless grain-size dependent trends are present in most observed rockmagnetic parameters. The behaviour in a grain-size dependent framework is reported for the initial susceptibility (Xi"), the high field susceptibility (xhf), the saturation magnetization (J,), the saturation remanence (Jrs; acquired in fields of 16 X lo6 Am-' or 20 T), the coercive force (H,), ferromagnetic coercive force (H, remanent coercive force (Her) and the remanent acquisition coercive force (Her,). X,,, H, and H,, decrease with grain size for each sample. A tendency to decrease with grain size is observed for Jr,. Xi, shows a weak maximum in the intermediate grain-size range. H,,,,,. is approximately constant in the coarse grain-size fractions and falls in the same grain-size range as Xi, rises. No grain-size dependence of J, is observed. Her. does not show a consistent grain-size dependence: in three samples it is grain-size independent and the other two it decreases with grain sue. There is a great variety between the samples in the absolute figures of J, (30-450 X Am2 kg-I), J,, (15-115 X lop3 Am2 kg-I), H, (2-20 x lo4 Am-'), H,,,,,. (2-70 X lo4 Am-'), H,, (40-320 X lo4 Am-') and H,,, (100-550 Xlo4 Am-'). Xi, shows smaller differences (0.50-1.50 X m3 kg-I) and Xh, is approximately equal for all samples (varying from 0.40 x lop6 m3 kg-' down to 0.20 x lop6 m3 kg-' with decreasing grain size). Sample differences and some ratios of these parameters are discussed in terms of different goethite chemical composition, crystallite size and the presence of intergrowths. The magnetic properties of mirocrystalline goethite are best understood by taking into account crystallite size and magnetic interaction amongst crystallites.

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The Magnetic Properties of Naturally Occurring Goethite

Cited by 72 publications

References 17 publications

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

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

Mineral Magnetic Studies of Archaeological Samples: Implications for Sample Selection for Paleointensity Determinations.

Magnetic Properties of Natural Goethite-I. Grain-Size Dependence of Some Low- and High-Field Related Rockmagnetic Parameters Measured At Room Temperature

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