Bentonite Mineral Separations

Overview Rare crystals of sanidine and/or zircon are separated from a clay-rich matrix based on their density and magnetic properties Aqueous separations Crushing and sieving (as needed) Magnetic separation Heavy Liquids Separation Hand picking SEM verification

Crushing and sieving As needed, dry the sample in the oven, prior to crushing. Most bentonites require milling, and some crushing prior to milling. Following crushing, sieve to 125 – 355 um and begin work on this size fraction, unless larger crystals are observed Do not discard any material.

Aqueous separations Clay minerals often adhere to the minerals of interest and must be removed. Washing at the sink is sufficient for some samples while the additional agitation of the gold table is required for others. For very clay-rich samples, washing in sodium hexametaphosphate is needed to disaggregate and remove the clay minerals.

Magnetic separations Sanidine and zircon possess a low magnetic susceptibility. Highly magnetic material is first separated with a hand magnet The Frantz separator concentrates the low susceptibility fraction

Heavy liquids separations Density: Sanidine: 2.55 – 2.62 g/cm 3 Zircon: 4.6 – 4.7 g/cm 3 Zircon is separated using undiluted Methylene Iodide (MI; density 3.32) Sanidine is separated by two passes in diluted MI at densities 2.55 and 2.62 g/cm 3

Picking Picking under reflected light is typically sufficient For especially small zircon, picking under cross-polarized transmitted light can be helpful.

SEM verification To ensure separation of sanidine from plagioclase, feldspar grains are mounted on carbon tape and analyzed using EDS.