Barite is an occasional voluminous component of the non-magnetic heavy mineral fraction of altered volcanic tuffs and sandstones, and can swamp out the target zircon population. Unfortunately barite is remarkably immune to dissolution in strong mineral acids. To quantitatively remove this barite, we have modified the method described by Breit et al. (Chemical Geology, 52:333-336, 1985) for barite dissolution via anion replacement in the presence of high concentrations of aqueous sodium carbonate, followed by dissolution of the product barium carbonate in dilute HCl. Our modifications include: a) treatment of the heavy mineral separate without any grinding; b) scaling up of the volumes of reagent, maintaining a ~10:1 ratio of sodium carbonate:barite in an aqueous solution of approximately 50 ml; c) prolonging the reaction time to at least 24 hours; d) applying sequential treatments to remove large amounts of coarse barite. As many as four 24 hour treatments have been required to quantitatively dissolve several grams of coarse bladed barite, with no ill-effects on the zircon crystals, and your patience is rewarded with a barite-free zircon separate!
Alternatively, if you’re equipped to handle hydrofluoric acid dissolution, you can more quickly and quantitatively remove barite by doing a bulk chemical abrasion step in concentrated HF at 180°C for 12 hours on the ANNEALED heavy mineral separate. Make sure the separated has been annealed in a 900°C muffle furnace for 48 hours before attempting this chemical abrasion, otherwise the U-Pb systematics of the zircons will be compromised. After the chemical abrasion step, pipette the HF solution away, add a few milliliters of 6M HCl and flux on a hotplate for an hour, after which you should have a clear solution with just zircon in the bottom of the beaker. Pipette the HCl away, rinse a couple of times with water and enjoy a barite-free zircon separate.