The state of iron in natural zeolites: A M6ssbauer study Rolando Roque-Malherbe, Carlos Diaz-Aguila, and Edilso Reguera-Ruiz National Center for Scientific Research, Havana, Cuba Juan Fundora-Lliteras, Lazaro L6pez-Colado, and Manuel Hernaindez-V~lez Physical Faculty, Higher Pedagogical Institute of Havana, Ciudad Libertad, Havana, Cuba The state of iron in natural zeolites is an important feature for the practical applications of these materials. A M6ssbauer study, complemented with other methods, of iron-containing zeolitic rocks from several countries (Mexico, Cuba, Czechoslovakia, and the USSR) was developed. It has been determined that iron is located as high spin Fe 3+ in framework tetrahedral sites, in extraframework octahedral sites as free Fe(H20) 3+, and as high spin Fe 2+ in octahedral coordination in extraframework sites or in another aluminosilicate associated with the zeolite. Iron is also located in magnetite contained in the zeolite rocks. Keywords: Iron; natural zeolite; M6ssbauer spectroscopy; tetrahedral site; octahedral site; magnetite INTRODUCTION M6ssbauer spectroscopyis a powerful tool in the elucidation of the state of iron in solid materials. In zeolites, this method has been applied during the last 2 decades in the study of the ion exchange of ferrous and ferric ions in several zeolites ~-7 and in adsorption studies. 8-13 Other authors have applied the method for the study of iron clusters in zeolites H-16 and for the elucidation of the state of iron included in the framework of ferrisilicate, FAPO-5, etc. 17-21 More- over, iron in aluminosilicates has been thoroughly investigated by M6ssbauer spectroscopy, 22-25 but stn- dies about the state of iron in natural zeolites are not often found. 26 Natural zeolites are raw material for several pro- cesses, 27-29 and the knowledge of the state of iron is of real importance in applications where brightness is relevant or for use in agriculture or for zeolite synthesis. The present paper attempts a closer approach to the state of iron in natural zeolite rocks. Well- characterized samples of zeolite rocks from several countries (Mexico, Cuba, Czechoslovakia,and the USSR) containing different zeolite phases (clinoptilo- lite, mordenite, and erionite) were studied by M6ss- bauer spectroscopy combined with other methods to provide information about the location and chemical state of iron. Address reprint requests to Dr. Roque-Malherbe at the National Center for Scientific Research, P.O. Box 6990, Havana, Cuba. Received 17 July 1989 accepted 5 February 1990 EXPERIMENTAL Natural zeolite rocks were obtained from deposits of Aguas Prietas, Sonora, Mexico (label AD); Castillas, Havana, Cuba (HC); Tasajeras, Villa Clara, Cuba (CMT); Camaguey, Cuba (C1, C2, C4, C5, C6, C7); Palmarito, Santiago de Cuba, Cuba (MP); Nizny Hra- bobec, Czechoslovakia(CZ); and Dzerbi, Georgia, USSR (GR). The correspondingelemental chemical analyses are reported in Table 1 and phase analysis in Table 2.3°'31 Characterization methods are described elsewhere. 39 Samples CI,C2, C4, C5, C6, C7, MP, CMT, and HC were refluxed five times during 4 h with 3 M solution of NaCI, KCI, and NH4C! (liquid/solid ratio = 2). The presence of iron in the solution after each exchange was tested by atomic absorption with a Pye Unican SP- 1900 spectrophotometer. Sample HC was thermally treated fi)r 10 h at 400, Table 1 Elemental composition of natural zeolites in oxide weight percent Sample SiO2 AI203 Fe203 CaO MgO Na20 K20 H20 C1 62.3 13.1 1.7 2.8 1.2 0.9 1.4 15.1 C2 64.3 12.9 3.6 5.0 1.3 1.5 1.3 10.5 C4 65.3 10.2 2.1 2.7 0.5 1.7 1.2 14.0 C5 66.2 11.4 3.8 4.2 0.7 1.3 1.0 12.8 C6 67.5 11.0 1.9 4.9 0.5 1.9 0.7 11.9 C7 64.2 13.1 3.0 4.2 1.4 1.5 0.7 11.5 MP 66.9 11.6 2.7 4.4 0.8 1.8 0.8 12.1 CMT 66.6 12.5 2.0 2.7 0.7 1.7 0.8 12.9 HC 66.8 13.1 1.3 3.2 1.2 0.6 1.9 12.1 AD 59.6 14.2 2.3 2.2 1.5 2.4 3.3 13.8 CZ 69.8 14.2 1.2 3.0 1.5 2.4 3.3 13.9 GR 62.4 i2.0 2.9 4.1 1.8 2.0 1.2 14.1 © 1990 Butterworth-Heinemann ZEOLITES, 1990, Vol 10, September~October 685