Transition in chemical and mineralogical composition around unweathered relicts of granodiorite in the kaolin deposit on Bornholm, Denmark

Abstract

In the kaolin deposit near Rønne on the island of Bornholm in the Baltic Sea a number of well-rounded, unweathered, boulder-like relicts of the granodioritic parent rock are found, embedded in kaolin. Samples have been taken of one relict at 2 cm interval from the fresh material outwards into the kaolin. The distribution pattern of the major elements in these samples has been studied in order to gain an impression of the leaching conditions at the time of formation, in the immediate vicinity of the relict. This distribution shows on a minor scale, what previously has been demonstrated in bulk for the whole deposit that leaching effects have not been uniform for the different elements, leading to a relative enrichment of calcium in the transition zone close to the unweathered parent rock. TEM and SEM observations revealed that halloysite and smectite are the dominant clay minerals of the transition zone, where kaolinite occurs only sporadically. These minerals apparently become unstable when alkalis and alkali earths are progres­sively leached, while kaolinite emerges as the pincipal clay mineral, and amphibole and plagioclase no longer remain in the residual mineral assemblage.