4R.W.Linton,A.Loh,D.F.Natusch,C.A.Evans and P.Williams.Science,Vol,191,1976,p852
5J.C.Hower,R.F.Rathbone,G.Peterson and A.S.Trimble.Petrology,minerlogy &chemistry of magnetically separated sized fly ash.In:1997 International Ash UtilizationSymposium,October 20-22,1997,Lexington,Kentuchy,1997,pp738-744