The electromagnetic radiation from semiconductor minerals in orebody

In complex ore deposits composing semiconductor minerals, electromagnetic radiation in the radio frequency (30 kHz ~ 3 MHz) is induced by propagation of elastic waves [1]. Semiconductor minerals are divided into n- or p-type. When each p-type and n-type is joined, the resulting junction (p-n junction) has the rectifying property. Many natural orebodies show this property, but it has not been evaluated qualitatively. A lot of p-n junctions exist as which connect in parallel and in series in the orebody [2]. They can be regarded as a single p-n junction at large scale. Hence elucidating the electric property of micro p-n junction is required to understand the semiconductor properties of orebody. To discuss the electromagnetic emission from semiconductor minerals in the orebody associated with tectonic process, we measure the electric property of the semiconductor pyrite. Composition and electric properties of natural semiconductor minerals are heterogeneous due to the presence of impurities and defects. Therefore, it is needed to clarify the properties at each microscopic region. In this research, we apply electroetching method and SEM analysis to acquire composition characteristics and use an indentation probe to reveal microscopic electric properties. Sample of pyrite is from Waga-Sennin mine, Akita prefecture, Japan. The area of cross section is 1.4 cm2 with thickness of 0.38 mm2. In the electrolytic etching, the surface of samples showed etching figures and zonal structures with widths of about 10 -100 μm. According to the SEM analysis, Pb inclusions were observed to be precipitated parallel to crystallographic planes. The heterogeneous change in electric properties of each area was observed to be as etching figure. Thermal probing method clarified that the regions of n-p type differences were also coincidence well to etching figure patterns. P-type regions showed a higher solubility than n-type regions. At p-n junction regions, rectifying property was observed. The reverse and forward breakdown voltage was estimated to be 1.6 V and 0.3 V respectively. Rectifying effect was appeared up to an input frequency reaching 100 kHz. However its effect decreased over the 100 kHz and disappeared above 900kHz. The contact potential between p-type and n-type semiconductors is given by the forward breakdown voltage at the p-n junction. If we apply the measured forward breakdown voltage of 0.3 V to the contact potential, the surface bound charge density is estimated to be 2.7×10-4 C/m2. This value is consistent with order of the values estimated previous rock fracture experiments [3] and enough to cause corona discharge of air. Otherwise, without separation, charge in the p-n junction is redistributed and cause discharge when the external voltage reaches a reverse breakdown voltage. This process also could be a source of electromagnetic radiation in the orebody. Reference [1] Sobolev, G. A. et al., 1980, Dokl. Akad. Nauk SSSR, 252, 1353-1355. [2] Demin, V. M. et al., 2004. Phys. Sol. Earth., 40, 262-266. [3] Takeuchi,A. and Nagahama, H., 2002, Phys. Earth Planet. Inter., 130, 285-291.