The grain size dependence of various mineral (rock) magnetic parameters has been determined, using a series of essentially pure, fine-grained (single domain, SD) and ultrafine-grained (superparamagnetic, SP) magnetites. The parameters measured include low-field susceptibility, frequency-dependent susceptibility, saturation remanence (SIRM), anhysteretic susceptibility (XARM), and coercivity of remanence ((B0)CR). The magnetites were produced in experiments designed to simulate possible pedogenic and biogenic pathways of magnetite formation. Their mean grain sizes range from 0.012 um to 0.06 um, and hence span the SP/SD boundary. Isothermal magnetic measurements were performed on two separate subsets of differing packing densities. The response of the magnetic parameters is modified by interaction effects, but they display continuous variation across the entire grain size range, confirming their value for rapid magnetic granulometry. Within the fine and ultrafine end of the magnetite grain size spectrum, susceptibility, frequency dependent susceptibility and XARM are notably responsive to grain size change. In terms of magnetic response (and also possibly of grain size, shape and absence of cation substitution), these synhtetic magnetites represent close analogues of those found in some soils and sediments.

/pdf/magnetic-properties-of-some-synthetic-sub-micron-magnetites-1woz75adew.pdf

Magnetic properties of some synthetic sub-micron magnetites

The magnetic properties of certain soils indicate the widespread presence of ultrafine-grained magnetite, even where there is no detrital input of magnetite. This suggests that in situ formation of magnetite can occur under soil-forming conditions. Schwertmann and Taylor have stated, however, that magnetite has not been found as a newly formed mineral in soils. Here we report the recovery and identification of pure, ultrafine-grained magnetite from some UK soils that have no apparent external sources of magnetite. We consider this magnetite to be of inorganic, in situ origin. The identification of magnetite formation during soil development has significance for studies of iron in soils but also has wider environmental implications. Soil-derived magnetite may contribute to the natural remanent magnetism of sediments, and act as a specific indicator of erosional events.

Formation of ultrafine-grained magnetite in soils

http://web.gps.caltech.edu/~jkirschvink/pdfs/magnetofossils.pdf

Elongated prismatic magnetite crystals in ALH84001 carbonate globules: Potential Martian magnetofossils

SUMMARY
A range of low-temperature and isothermal magnetic measurements are used to identify the secondary ferrimagnetic mineral (SFM) grain sizes in 10 representative soil samples from Wales. A comparison of percentage frequency-dependent susceptibility (χFD percentage) and low-temperature remanence measurements shows that they are sensitive to different ranges of superparamagnetic (SP) grains. The relative distributions of SP grains and stable single domain (SSD) grains are similar in nine of the samples. Typical distributions for soils dominated by SFMs are ˜˜20-30 per cent SSD and 70–80 per cent SP. Multidomain (MD) grains were not detected in the samples studied. There is evidence that some soils contain significant numbers of ultrafine SP grains <0.010 μm that are not detected by low-temperature remanence measurements at 20 K and which will have the effect of depressing values of low-field susceptibility (χLF) and χFD percentage. A mixing model suggests that χFD percentage may be used semi-quantitatively to estimate the proportion of SP grains in a sample. The positively skewed grain-size distributions strongly suggest a mechanism of SFM formation that is driven by processes at the < 10-8 m scale, thus supporting weathering and fermentation as controlling processes, rather than the degradation of SSD bacterial magnetosomes and primary minerals.

/pdf/secondary-ferrimagnetic-minerals-in-welsh-soils-a-comparison-22z9knn8wn.pdf

Secondary ferrimagnetic minerals in Welsh soils: a comparison of mineral magnetic detection methods and implications for mineral formation

Magnetite (Fe_3O_4) is one of the most stable carriers of natural remanent magnetization (NRM) in sedimentary rocks, and paleomagnetic studies of magnetite-bearing sediments, such as deep-sea cores and pelagic limestones, have provided a detailed calibration between the biostratigraphic
and magnetic polarity time scales. Despite this important role, there is as yet a very poor understanding of how ultrafine-grained (< 0.1 µm) magnetite is formed, transported, and preserved in marine environments.
A major conceptual advance in our understanding of these
processes is the recent discovery that biogenic magnetite, formed by magnetotactic bacteria and/or other magnetite-precipitating organisms, is responsible for much of the stable magnetic remanence in many marine
sediments and sedimentary rocks. Since these magnetite particles are of biogenic origin, they are termed properly magnetofossils (Kirschvink & Chang 1984).

G.E. Morgan

Papers

Transmission electron microscope and rock magnetic investigations of remanence carriers in a Precambrian metadolerite