MINERAL MAGNETIC-PROPERTIES OF MIXTURES OF ENVIRONMENTAL AND SYNTHETIC MATERIALS - LINEAR ADDITIVITY AND INTERACTION EFFECTS

Mineral magnetic properties have been used recently to classify and to attempt to quantify the sources of sediments through environmental sy stems. Linear modelling techniques could be used with a wide range of concentration-dependent magnetic measurements to quantify the sources of sediments. To investigate wider application of linear modelling tec hniques using mineral magnetic properties, research has been conducted using laboratory mixtures of up to six source materials, including bo th natural environmental materials and synthetic compounds. While six sources may seem ambitious, this figure was used as an absolute upper limit rather than giving a real prospect of mathematically unmixing si x sources. It has been found that even with the most magnetically diff erentiable materials, large errors are encountered when modelling the sources of the mixtures. This paper investigates the causes of 'non-ad ditivity' of certain magnetic measurements and the failure of the line ar modelling of the sources of the mixtures. Possible reasons for this failure include source homogeneity, calibration and linearity of equi pment, magnetic viscosity of materials and/or the changing physical ch aracteristics of the source materials once mixed together (interaction effects). In testing linear additivity, low-frequency susceptibility is the most reliable mineral magnetic measurement, while remanence mea surements suffer from a systematic error in the expected results. Resu lts have shown that in the best controlled conditions where the source s are identified and are artificially mixed together, the results of l inear modelling are quite poor and at best four sources can be 'unmixe d' with reasonable success. It is suggested that interaction within th e mixtures, especially when containing highly ferrimagnetic burnt envi ronmental materials, causes some of the non-additivity phenomena.