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.
The study of impact glasses from the Zhamanshin meteorite crater has a long history extending back to early 1970ies. Most attention is paid to unique impact-related rocks known as zhamanshinites (bombs) and irghizites (lapilli). But, as first shown by V.L. Masaitis, other, more common types of impactites, suevites and massive melts (tagamites), are also present in Zhamanshin. We study the distribution in the crater, structure and composition of these particular rocks using powder X-ray diffraction and scanning electron microscopy. It is shown that all impact glasses from the Zhamanshin crater are genetically related and derive from the impact event, while the observed petrographic differences between them reflect the conditions of their formation. Individual varieties of studied impactites (suevites and tagamites) are spatially separated and, as a rule, do not intersect. This points to the absence of global mixing of the target material as a result of the impact event, as well as to the complex nature of the impactor.
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