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Density Separation

Density separation is based on the fact that different minerals have different densities. Thus, if a mixture of minerals with different densities can be placed in a liquid with an intermediate density, the grains with densities less than that of the liquid will float and grains with densities greater than the liquid will sink. Typical mineral densities range from about 2.2 g/cc to as much as 8 g/cc, but are generally between 2.5 and 3.5 g/cc for silicate minerals. Suitable liquids for density separation include bromoform (density = 2.84 g/cc) and diiodomethane (density = 3.31 g/cc). High density liquids with a range of densities can also be prepared by dissolving sodium tungstate powder in water. Because of the use of high density liquids, density separation is often referred to as heavy liquids separation. Heavy liquids separations are generally done in separatory funnels. The procedure is very simple. The sample is placed in the separatory funnel and the heavy liquid is added. The funnel is then left for some time to permit light minerals to float and heavy minerals to sink. When the minerals have been separated, the separatory funnel is opened and the heavier minerals are transferred onto a piece of weighing paper in a funnel (to allow the heavy liquid to drain away). The mineral separate is then washed and examined optically for purity. The densities of bromoform and diiodomethane can be adjusted by adding acetone (density about 0.7 g/cc) and the densities of sodium tungstate solutions can be varied by adding water or more sodium tungstate. Thus, minerals with only slightly different densities can be separated by adjusting the density of the heavy liquid until it lies between those of the minerals. The negatives of density separation are based on two factors. First, bromoform and diiodomethane are halogenated organic liquids and, as such, present significant health hazards. Sodium tungstate solutions are safer, but are quite expensive. Second, when density differences or mineral grain sizes are small, heavy liquids separation can take many hours. These latter effects can be reduced if the separation is done in a centrifuge.

Shape Separation

Shape separation is based on the fact that minerals with different shapes may roll (equant grains) or slide (platy grains) and that grains that slide will have different coefficients of friction, when placed on a sloped surface. A good example of shape separation is the separation of micas (flat platy shapes) from other minerals with more equant shapes. This type of separation can be done simply by taping a piece of paper to piece of wood or sheet metal. The mineral mixture is then placed at one end of the sheet, the sheet is slightly tilted, so that the mixture is at the high end, and the sheet is vibrated. The equant grains will then roll or slide rapidly off of the sheet, while the flat micas move very little. This simple sort of shape separation can generate >95% pure mica separates in a few seconds. Shape separation can be combined with gravity separation by using a separation surface with a series of grooves cut at an angle to the tilt. The mineral mixture is placed on the high side of the surface and water is flowed over it to move the grains while the surface is vibrated. Heavier grains will collect in the grooves while lighter grains are washed down the surface. This type of shape/density separator is known as a Wifley table and is used extensively to separate zircons for U-Pb dating.