Unmixing magnetic assemblages and the magnetic behavior of bimodal mixtures

Stable single-domain (SSD) grains were mixed separately with superparamagnetic, pseudosingle-domain, and multidomain (MD) magnetite/maghemite particles in order to test the linearity of various magnetic parameters as a function of mixing ratio. Hysteresis loops, isothermal remanent magnetization acquisition curves, DC demagnetization curves, and low-temperature thermal demagnetization curves were measured on the mixtures. The experiments demonstrate that magnetization parameters are linearly dependent on the mixing ratio, while more complex parameters, e.g., coercivities, do not behave linearly as a function of mixing ratio. Armed with linearity, we apply a mathematical technique which, given a database of type curves, uses singular value decomposition to solve for the various concentrations of the magnetic phases in the mixture and a Monte Carlo simulation to determine the error in the inversion. We then test the technique on numerical mixtures, on the physical mixtures, and on a small set of natural samples from Lake Pepin, Minnesota. Finally, the magnetic behavior of the mixture of MD and SSD grains is considered, and two more mixture strains of MD and SSD grains (numerically produced) are considered to facilitate this discussion.