Washington University home page back to Department home page




Magnetic Separation

Magnetic separation takes advantage of differences in the magnetic properties of minerals. Minerals fall into one of three magnetic properties: ferromagnetic, paramagnetic and diamagnetic. Ferromagnetic minerals are themselves magnetic (i.e., magnetite and pyrrhotite) and can be easily separated from other minerals with a magnet since they will stick to the poles of the magnet. These minerals can be separated by wrapping the poles of a magnet in paper, passing the magnet over the mineral mixture. The ferromagnetic minerals will stick to the magnet and may be easily separated by removing the paper covering the magnet. Paramagnetic and diamagnetic minerals are not magnetic, but they differ in how they interact with a magnetic field. Paramagnetic minerals are weakly attracted into a magnetic field and diamagnetic minerals are weakly repelled by a magnetic field. Thus, if a mixture of paramagnetic and diamagnetic minerals is passed through a magnetic field, they will be pulled into the field (paramagnetic) or repelled from the field (diamagnetic) and may be separated. Furthermore, paramagnetic minerals with different degrees of paramagnetism can be separated from one another in the same way. The device used to separate minerals based on their magnetic properties is called a Frantz Isodynamic Magnetic Separator. The magnetic separator consists of a large electromagnet through which mineral mixtures can be passed on a metal trough which is divided near its exit end. Varying the strength of the magnetic field and/or slope of the separation trough is used to separate minerals.

All forms of mineral separation suffer from one difficulty. It is impossible to completely eliminate impurities. Depending on what the impurities are, that may or may not be a major problem. For example, if you were separating hornblende from a granite for Zr analysis, potential contamination by zircon inclusions in the hornblende might be a major problem. A typical hornblende crystal might have a Zr content of 50 ppm. A zircon crystal (ZrSiO4) has approximately 500,000 ppm Zr. Thus, if the hornblende separate contained only 0.01% Zr, the hornblende would contribute 4999.5 units of Zr and the zircon impurity would contribute 5000 units of Zr. The resulting concentration you would measure would be 100 ppm, which is twice the correct result. This is a major problem that cannot be eliminated when mineral separations are involved in the analysis.