Anisotropy of susceptibility in rocks which are magnetically nonlinear even in low fields

0490845 - GLÚ 2019 RIV GB eng J - Journal Article
Hrouda, F. - Chadima, Martin - Ježek, J.
Anisotropy of susceptibility in rocks which are magnetically nonlinear even in low fields.
Geophysical Journal International. Roč. 213, č. 3 (2018), s. 1792-1803. ISSN 0956-540X. E-ISSN 1365-246X
R&D Projects: GA AV ČR IAAX00130801
Institutional support: RVO:67985831
Keywords : electrical properties * magnetic properties * magnetic fabrics and anisotropy * magnetic mineralogy and petrology * rock and mineral magnetism
OECD category: Geology
Impact factor: 2.777, year: 2018

Theory of the low-field anisotropy of magnetic susceptibility (AMS) assumes a linear relationship between magnetization and magnetizing field, resulting in field-independent susceptibility. This is valid for diamagnetic and paramagnetic minerals by definition and also for pure magnetite, while in titanomagnetite, pyrrhotite and hematite the susceptibility may be clearly field-dependent even in low fields used in common AMS meter. Consequently, the use of the linear AMS theory is fully legitimate in the former minerals, but in principle incorrect in the latter ones. Automated measurement of susceptibility in 320 directions in variable low-fields ranging from 5 to 700 A m^-1 was applied to more than 100 specimens of various pyrrhotite-bearing and titanomagnetite-bearing rocks. Data analysis showed that the anisotropic susceptibility remains well represented by an ellipsoid in the entire low-field span even though the ellipsoid increases its volume and eccentricity. The principal directions do not change their orientations with low-field in most specimens. Expressions for susceptibility as a function of field were found in the form of diagonal tensor whose elements are polynomials of low order. In a large proportion of samples, the susceptibility expressions can be further simplified to have one common skeleton polynomial.
Permanent Link: http://hdl.handle.net/11104/0284982