particle size of 10 ppm emulsion of pyrite is being measured by Malvern. but the result is not accurate because the refractive index of pyrite is unknown.
Iron pyrites is a solid so I can't understand how you can make an emulsion with it. Please explain. Anyway iron pyrites is basically opaque with an RI of 2.94. 10 ppm is not sufficient concentration to measure, almost certainly.