[1] Greigite (Fe 3 S 4 ) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low-field magnetic susceptibility is reported here.The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low-temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17-200 nm for cubic crystals and 17-500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo-single-domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.

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[1] Magnetic hysteresis measurements of sediments have resulted in widespread reporting of "pseudo-single-domain"-like magnetic properties. In contrast, the ideal single domain (SD) properties that would be expected to be responsible for high quality paleomagnetic records are rare. Determining whether SD particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment. We apply a range of such techniques and find that SD particles are much more common than has been reported in the literature and that magnetite magnetofossils (the inorganic remains of magnetotactic bacteria) are widely preserved at depth in a range of sediment types, including biogenic pelagic carbonates, lacustrine and marine clays, and possibly even in glaci-marine sediments. Thus, instead of being rarely preserved in the geological record, we find that magnetofossils are widespread. This observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments. Magnetofossils also have grain sizes that are substantially smaller than the 1-15 mm size range for which there is reasonable empirical support for relative paleointensity studies. The different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies. This issue needs much closer attention. Despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years, much remains to be understood about how they become magnetized.Citation: Roberts, A. P., L. Chang, D. Heslop, F. Florindo, and J. C. Larrasoaña (2012), Searching for single domain magnetite in the "pseudo-single-domain" sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations,