Monazite alteration mechanisms and depletion measurements in kaolins

Two widely different kaolin deposits of Greece were studied in order to compare the impact of the alteration mechanism to the presence of thorium and rare earth elements (REE) in the deposits and to find a measure for their migration during the alteration processes. Halloysite and kaolinite coexist in gneissic weathering profiles, developed under a temperate climate in the Leucogia area, NE Greece. The kaolin occurrences of Kos island, Aegean sea, Greece, are products of hydrothermal alteration of rocks of rhyolitic composition. Monazite is found as an accessory mineral in both areas and it is the only mineral that contains lanthanum, cerium and neodymium and in addition thorium, in the kaolins of Leucogia area. To resolve the genetic relationship between monazite and its alteration products, the mineralogy of the primary rocks and kaolins and their microtextures were examined, and bulk rock chemical analyses were performed in order to correlate major and trace elements behaviour with the alteration mechanisms. In the gneissic weathering profiles of the Leucogia area, monazite (Ce) is partially altered to a ThSiO4 or Th-OH silicate microcrystalline phase with very low concentrations of light rare earth element (LREE). The relict monazite crystals are depleted in thorium. The depletion and/or the enrichment of the various elements in the bulk sample appear to be related to mineral breakdown rather than to mild leaching of primary minerals by meteoric water. The only element that is apparently immobile is thorium and this element can be used as a measure for the depletion in this environment. Monazite (Ce), in the hydrothermally altered rhyolitic rocks of Kos island, is altered to P-bearing crandallite-goyazite. During this alteration process, LREE are depleted with respect to the fresh monazite crystals. Phosphorus, which was found to be the only element with an immobile behaviour, can be used as a reference to evaluate the amount of REE leached during monazite alteration in this hydrothermal environment. (C) 2003 Elsevier B.V. All rights reserved.