Thermally activated mineralogical transformations in archaeological hearths: inversion from maghemite γFe2O4 phase to haematite αFe2O4 form

Abstract

AbstractA series of laboratory experiments were conducted in an effort to understand why magnetic field gradient survey techniques failed to detect hearths at a prehistoric archaeological site in southern California. The study used various methods of environmental magnetism to examine the effects of exposing soil samples to a temperature of 650°C over a period 26 h. Results of the study indicate that the failure was associated with a reduction in soil magnetic susceptibility to below background levels within hearth soils. This reduction was due to high‐temperature transformation of iron oxides from a highly magnetic form to a relatively non‐magnetic form. The reduction in susceptibility is thought to have proceeded via the oxidation of primary (lithogenic) magnetite, Fe3O4 to maghemite, γ Fe2O4 followed by the inversion of maghemite to haematite, αFe2O4. The study suggests that in some instances high temperature inversion can reduce the proportion of ferrimagnetic minerals within a hearth to below initial concentrations (resulting in a negative magnetic field gradient anomaly, the opposite of what is normally expected). A field experiment was also conducted to determine what soil temperatures might be achieved 2 cm beneath a hearth. The experiment recorded temperatures ranging from approximately 400°C to 650°C, with an average temperature of about 470°C. Soil colour changes and magnetic susceptibility enhancement observed at the conclusion of the field experiment indicate that these temperatures were sufficient to activate some mineralogical changes, possibly including inversion to haematite. The implications of high temperature inversion to archaeological prospection are discussed, as is a potential archaeological application. Copyright © 2006 John Wiley & Sons, Ltd.