The origin of Ba- and REE-bearing aluminium-phosphate-sulphate minerals from the Lohrheim kaolinitic clay deposit (Rheinisches Schiefergebirge, Germany)

Two types of aluminium-phosphate-sulfate minerals are described for the first time from the Lohrheim kaolinitic clay deposit which was laid down during the Tertiary on Devonian rocks of the Rheinisches Schiefergebirge: type A is dominant in gorceixite BaAl3(PO4)2(OH)5 · H2O, type B is abundant in florencite-(Ce) CeAl3(PO4)2(OH)6 and goyazite SrAl3(PO4)2(OH)5· H2O. Both types resulted from weathering of pyritiferous parent material, which caused the low pH of the meteoric water. Their cations Ba and REE, however, were derived from different sources. The Ba in gorceixite was provided by supergene alteration of barite-bearing Eifelian bituminous shales and volcanics which locally underlie the kaolinitic saprolite, whereas the REE are likely to have been derived from the Devonian keratophyres which form the direct substrate of the loam of the weathering zone. saprolite profile an overall decrease of the REE content from bottom to top as well as HREE/LREE changes may be observed; they are a function of pH. During the incipient stages, HREE were preferentially removed, while during later stages a moderate rise in the pH led to a marked depletion in LREE relative to HREE. Chemical analyses of different size fractions of kaolin demonstrate that goyazite-florencite (type B) is LREE selective, whereas gorceixite (type A) is HREE selective.