Geochemical behaviour of boron during the metamorphism of carbonate rocks

Studies were made of the polymorphic form and the boron content of illites from variously metamorphosed carbonate rocks. The per cent of the 2 M polymorph in the illite from a given sample is used as an index of metamorphism. Some samples were reanalyzed for boron, after destruction of illite. The results show that increasing metamorphism causes (1) no significant change in the mean boron content (referred to illite) of insoluble residues and clay fractions, (2) an increase in the variance of individual values about the mean boron content of illite from a group of similarly metamorphosed samples, and the release of boron from the illite structure during the 1 Md-2 M transformation, with the concomitant generation, in situ, of magnesian tourmaline (dravite). The redistribution of boron appears to take place progressively with increasing metamorphism. Processes of polymorphic transformation and dravite formation are completed at greenschist facies metamorphic conditions.