High-coercivity magnetic minerals in archaeological baked clay and bricks

The thorough understanding of magnetic mineralogy is a prerequisite o f 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 and >600 degrees C. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 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. Stoichiometic 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 and 620 degrees C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140 and 240 degrees 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.

Automated summary

The study conducted detailed rock magnetic analysis on archaeological ceramics from Bulgaria and Russia, revealing the presence of magnetically soft and hard minerals with differing unblocking temperatures. A unique high coercivity, stable, low-unblocking temperature phase was identified in some samples, while others contained a combination of high- and low-unblocking temperature, magnetically hard phases.