High-coercivity magnetic minerals in archaeological baked clay and bricks

Abstract

Summary The thorough understanding of magnetic mineralogy is a prerequisite of any successful palaeomagnetic or archaeomagnetic study. Magnetic minerals in archaeological ceramics and baked clay may be inherited from the parent material, or, more frequently, formed during the firing process. The resulting magnetic mineralogy may be complex, including ferrimagnetic phases not commonly encountered in rocks. Towards this end, we carried out a detailed rock magnetic study on a representative collection of archaeological ceramics (baked clay from combustion structures and bricks) from Bulgaria and Russia. Experiments included measurement of isothermal remanence acquisition and demagnetization as a function of temperature between 20° C and > 600° C. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 K and 300 K have been carried out. All studied samples contain a magnetically soft mineral identified as maghemite probably substituted by Ti, Mn and/or Al. Stoichiometric magnetite has never been observed, as evidenced by the absence of the Verwey phase transition. In addition, one or two magnetically hard mineral phases have been detected, differing sharply in their respective unblocking temperatures. One of these unblocking between 540° C and 620° C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140° C and 240° C, and its magnetic properties correspond to an enigmatic High Coercivity, Stable, Low-unblocking Temperature (HCSLT) phase reported earlier. In a few samples high- and low unblocking temperature, magnetically hard phases appear to coexist, in the others the HCSLT phase is the only magnetically hard mineral present.

High spatial resolution magnetic mapping using ultra-high sensitivity scanning SQUID microscopy on a speleothem from the Kingdom of Tonga, southern Pacific

“…The magnetic property of ε-Fe 2 O 3 corresponds to a high coercive field and low unblocking temperature (e.g., Tuček et al 2010;López-Sánchez et al 2017;Kosterov et al 2020b). Furthermore, the occurrence of ε-Fe 2 O 3 is related to the anthropogenic fire (e.g., López-Sánchez et al 2017;Kosterov et al 2020a) or the vegetation change (McClean et al 2001). This is consistent with the environmental magnetic implication we discussed below.…”

Applying thermal demagnetization to archaeological materials: A tool for detecting burnt clay and estimating its firing temperature

“…The mineralogical assemblages in the clays depend on the local geology. Yet, it is common to all iron-bearing clay minerals that when they are heated to temperatures starting from ~150C and up to ~700C, they are transformed into stable ferrimagnetic minerals such as magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ) and hematite (α-Fe 2 O 3 )-in a stochiometric form or with substitution of Ti, Mn, or Al [30][31][32]. The effect of the heating is twofold.…”

Diversity of Iron Oxides: Mechanisms of Formation, Physical Properties and Applications

“…Structures with β-Fe 2 O 3 , ε-Fe 2 O 3 , ζ-Fe 2 O 3 , δ-Fe 2 O 3 or Fe 4 O 5 metastable phases (Table 6) are much less described compared to the main stable compounds of iron oxides. ε-Fe 2 O 3 is the only one seriously applicable in biomedicine [130,131], electronics [132,133] or geosciences [134,135] and can be obtained both synthetically [37,136,137] or by extraction from various archeological objects [138][139][140]. There have also been some attempts to use β-Fe 2 O 3 NPs in biomedicine [63], sensors and lithium-ion batteries [20,64].…”