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/cm3

• Zircon: 4.6 – 4.7 g/cm3

• 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/cm3

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.