and when {HEAD ~ the amount} of {TAIL ~ presence} of the silicon component exposed on the surface, expressed in terms of silicon, with respect to the magnetite particles is designated as B, the relationship C/B≧15 is satisfied 
of
and when { HEAD ~ the amount } of { TAIL ~ presence } of the silicon component exposed on the surface , expressed in terms of silicon , with respect to the magnetite particles is designated as B , the relationship C / B≧15 is satisfied
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH