Geoscientist’s Toolkit: Heavy Liquid Separation

Heavy liquid separation.  Mixed dense (red) and light (purple) minerals are poured into a liquid of intermediate density and stirred.  After they come to equilibrium, the dense mineral(s) will sink, and the light mineral(s) will float.  Image credit: Bill Mitchell (CC-BY).
Heavy liquid separation. Mixed dense (red) and light (purple) minerals are poured into a liquid of intermediate density and stirred. After they come to equilibrium, the dense mineral(s) will sink, and the light mineral(s) will float. Image credit: Bill Mitchell (CC-BY).

When purifying a mineral from whole rock, one of the most useful separations is by density. Water, being less dense than most rock, is not especially useful for this. However, lithium metatungstate (LMT, mixed with water) and sodium polytungstate (SPT, also mixed with water) can create denser—albeit more viscous—liquids, with densities approaching 2.9–3.1 g/cm3. These denser liquids are enough to separate feldspar and quartz (<2.7 g/cm3) from zircon, titanite (sphene), and barite (densities >3.5 g/cm3).

Separations are fairly straightforward. A crushed, sieved rock sample is poured into a separatory funnel filled 1/2–2/3 full with the heavy liquid. The slurry is stirred vigorously with a stirring rod, and allowed to settle (it may take a couple hours if the grain size is fine and the liquid viscous). After it settles, the dense minerals should have sunk to the bottom, while the light minerals will float. A filter funnel is then placed under the separatory funnel. When the stopcock is opened, the dense minerals and some of the heavy liquid will pour out the bottom. The stopcock is then closed when the heavy separate has passed through. A second filter funnel is then used to capture the light fraction. With good filtering, the heavy liquid can be reused. The separates can be washed with distilled water and dried.

Heavy liquid separation is often used in combination with magnetic separation to purify minerals for analysis. Depending on the difference in densities being separated, a liquid may need to be fairly precisely calibrated with larger samples of the desired minerals. Sanidine (~2.55 g/cm3) and quartz (~2.65 g/cm3) need a well-calibrated liquid to achieve good separation, while either (or both) of them from zircon can be done with any LMT solution >2.7 g/cm3.

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