I think these grains are impurities. It may be due to the low quality of chemicals that you used for the analysis or flask (for the SEM instrument).

Regards,

K

Rakibul Hassan

This is called 'piggy-backing' and the large number of small particles (with huge surface area) adhere through van der Waals and solid-solid diffusion to the fewer larger particles. See this webinar (registration needed):

Adhesion and cohesion

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W180725AdhesionCohesion

Best way to figure this out is to so do elemental mapping using EDS attachment to SEM.