Testing the effectiveness of mechanical magnetic extraction in riverine and lacustrine sediments

Abstract

Rock magnetic studies often require analysing extracted magnetic grains so that the mineralogy, granulometry, and concentration of magnetic minerals could be determined accurately. Magnetic extraction is often cumbersome, tedious, and subjective as this process is carried out manually by placing strong magnet inside sediment slurries. Moreover, the magnetic mineral content in sediment is generally much less than 1% of its weight. In this study, the effectiveness of mechanical magnetic extraction using magnetic stirrer is tested for riverine and lacustrine sediments. The samples are sediments from Citarum River in West Java and from Brantas River in East Java as well as surface sediment from Lake Towuti in South Sulawesi. The effectiveness tests were carried out by comparing the magnetic properties of bulk samples with that of extracted magnetic grains. Additional tests were also carried out in the form of SEM (scanning electron microscopy) and XRD (X-Ray diffraction) analyses. Apart from the expected higher values of mass-specific magnetic susceptibility, the extracted samples also tend to have larger magnetic grains than that of bulk samples. The predominant magnetic mineral in bulk and extracted samples is magnetite, although other type of magnetic minerals might be present in extracted samples. The presence of SP (superparamagnetic) grains in extracted samples is also negligible unlike that in bulk samples. SEM and XRD analyses support these findings. Magnetite grains could easily be identified in extracted samples.