IRON-OXIDES IN A SOIL DEVELOPED FROM BASALT

A dusky red Oxisol forming on a tholeiitic basalt is found to contain varying proportion of aluminous hematite (Hm) and titanoaluminous magh emite (Mh) in the different size fractions. Maghemite is the main iron oxide in the sand and silt fractions whereas Hm is dominant in the cl ay fraction, together with gibbsite (Gb), kaolinite (Ka), rutile (Rt) land probably anatase, An) and Mh. Maghemite is also the major oxide m ineral in the magnetic separates of soil fractions (sand, about 65% of the relative Mossbauer spectral area; silt, 60%). Hematite (sand, 30% ; silt, 15%) and ilmenite (Im) (sand, 5%; silt, 16%) are also signific antly present in the magnetic extract. Accessory minerals are Rt and A n. No magnetite (Mt) was detected in any soil fraction. Sand- and silt -size Mh have similar nature (a(o) = 0.8319 +/- 0.0005 nm; about 8 mol % of Al substitution; saturation magnetization of 49 J T-1 kg(-1)), an d certainly a common origin. Lattice parameters of clay-Mh are more di fficult to deduce, as magnetic separation was ineffective in removing nonmagnetic phases. Al content in Hm varies from 14 mol% (clay and sil t) to 20 mol% (sand). The proposed cation distribution on the spinel s ites of the sand-size Mh is: [Fe<INF>0.92</INF>Al<INF>0.08</INF>] INF> Ti<INF>0.18</INF>square<INF>0.39</INF>}O<INF>4 </INF>(square = vacancy , [ ] = tetrahedral sites and {} = octahedral sites), with a correspon ding molar mass of 208.8 g mol(-1). The predicted magnetization based on this formula is a sigma congruent to 68 J T-1 kg(-1), assuming coll inear spin arrangement. The large discrepancy with the experimentally determined magnetization is discussed.