Measurements were performed with a Beckman Coulter Laser diffraction particle size analyzer LS 13 320 at MARUM laboratories (University of Bremen). Samples were pretreated with a dispersing agent (tetrasodium pyrophosphate) while boiling for few minutes to speed up the process. A standard procedure was followed to prepare the sample for analysis to remove the carbonate, organic carbon and opal by using hydrogen peroxide, hydrochloric acid and sodium hydroxide.