Characteristic low‐temperature magnetic properties of aluminous goethite [α‐(Fe, Al)OOH] explained

Abstract

[1] Goethite (a-FeOOH) is an antiferromagnetic iron oxyhydroxide that forms as a weathering product of iron-bearing minerals. We systematically investigated the low-and room temperature properties of well-defined aluminous goethites [a-(Fe, Al)OOH] with varying grain size and Al content. A marked decrease in the Néel temperature with increasing Al content for goethite lowers the blocking temperature distribution, which produces an increase in the remanent magnetization on cooling. The zero-field-cooled (ZFC) and field-cooled (FC) curves are irreversible. This is due to an additional partial thermal remanent magnetization (pTRM), which is acquired at low temperatures during the FC process because the initial 300 K remanence is far from being saturated due to the extremely high saturation field of goethite. This pTRM can be thermally demagnetized when reheating a sample back to 300 K. Finally, a sharp decrease in the bulk coercivity at 11-13 mol % Al is mostly caused by a broad coercivity distribution due to nonuniformity of Al substitution, which becomes more significant with increasing Al content. The positive correlation between pTRM acquisition during the FC process and the bulk coercivity strongly indicates that the ZFC/FC behavior is controlled by the bulk coercivity, which is, in turn, determined by Al substitution. This explanation of the characteristic low-temperature magnetic properties of Al goethite provides important constraints for identifying natural Al goethite bearing samples and quantitatively estimating the contributions of this mineral to the bulk magnetic properties of such samples.

Iron oxide minerals in dust-source sediments from the Bodélé Depression, Chad: Implications for radiative properties and Fe bioavailability of dust plumes from the Sahara

“…However, since goethite is extremely hard to magnetize at room temperature in fields below 2.0 T (Maher et al, 2004;Rochette et al, 2005) it may contribute a negligible amount to HIRM or the S 300 -ratio value even though it may be the abundant phase. Furthermore, depending on particle sizes and cation impurity, the magnetic ordering temperature for goethite may be below room-temperature and thus be invisible to remanence measurements at room-temperature [Liu et al, 2006]. Low-temperature remanent magnetization and ac susceptibility () were measured using a Quantum Design Magnetic Properties Measurement System.…”

“…These additional steps are designed to minimize the contribution from strongly magnetic minerals like magnetite and maghemite, which can dominate the FC, ZFC, and RTSIRM magnetizations, and target weakly magnetic, antiferromagnetic minerals with high coercivities like goethite and hematite. The Carter-Guyodo method makes use of the low coercivity of magnetite and maghemite compared to hematite and goethite plus the difference in LT magnetic behavior between hematite and goethite Fillion, 1989: Maher et al, 2004;Liu et al, 2006]. Our experimental protocol is described in Reynolds et al [2014a, b].…”

“…The remanence ratio, R LT =IRM(10K)/IRM (300K), was calculated from the remanence cooling segment of the Carter-Guyodo method after alternating-field demagnetization at 300 K, where IRM (10 K) and IRM (300 K) are remanence values measured at 10 K and 300 K, respectively (Liu et al, 2006). In pure minerals, goethite can have R LT ratios greater than 2.0 (Guyodo et al, 2003;Liu et al, 2006), whereas magnetite and hematite have R LT ratios near 1.0 or less.…”

“…In addition, no magnetic signatures for goethite compositions with sufficiently high cation substitution (e.g., Al) to lower their Néel temperature below 300 K were observed on any of the FC and ZFC curves. Such cation loading would likely have produced a small drop in remanence between 200 and 300 K. For example, the observations indicate that if Al substitution is present in goethite it must be less than approximately 14 mole % Al [e.g., Liu et al, 2006].…”

“…We also determined the type of ferric oxide mineral, whether primarily hematite (-Fe 2 O 3 ) or goethite (-FeOOH), which are the two most common forms in desert soils and surfaces (hereafter collectively referred to as iron oxides or ferric oxides), and which have different optical properties. Reflectance spectroscopy can be used to identify different ferric oxide minerals (e.g., Clark et al, 2007), as can certain magnetic methods (Maher et al, 2004;Guyodo et al, 2006;Carter-Stiglitz et al, 2006;Liu et al, 2006;Maher, 2011). Mössbauer spectroscopy was also employed to identify with high specificity various iron phases, including amounts of hematite and goethite as well as the fraction that occur as nanoparticles, in a sample.…”