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A certified crystalline content is important. Quartz particles are usually covered in a layer of amorphous silica about 0.Q3 pm thick (Nagelschmidt, 1956; Edmonds et aI., 1977; O'Connor and Chang, 1986). Finer particles show a relative decrease in diffracted intensity per unit mass as the amorphous layer becomes a larger fraction of the particle volume. The effect is generally fairly small in practice, but a certified crystalline content of the standard allows a correction to be made.