Diatomite contains from 70 -97% SiO2. If you are interested in quartz, then you cannot isolate it from the mixture.. He has a certain structure inherent only to him. If you are interested in SiO2, then the oxide can be separated by zone zone melting. Pre-disperse the powder, place it in a quartz ampoule and do zone melting. Powder with impurities will have a lower melting point than diatomite. Read about zone melting or refining in the literature.