I found that 20 Mule Team technical grade borax comes in granular or powder form. The specs for the granular say insoluble impurities are <400 ppm. The specs for the powder grade, which is what you get at the supermarket for about 50 cents a pound, show no insoluble particles. The soluble impurities are guaranteed <700 ppm chloride, <600 ppm sulphate and less than 30 ppm iron.