A chondrodite–humite-spinel marble from Sørfinnset,near Glomfjord, northern Norway

Abstract

SummaryA chemical and modal analysis of the marble, together with chemical analyses of the coexisting spinel and the chondrodite-humite mixture are given. The spinel, a 8·101 ± 0·001 Å, n 1·731 ± 0·003, is much richer in iron (Niggli's mg 0·90), than the humite mixture (mg 0·99), which agrees with the only previous analysis of a coexisting spinel and humite mineral. It was not possible to separate the two humite minerals from each other and analyse them, but the analysis of the mixture shows less iron than any previously analysed chondrodite or humite; the composition is very near to that of humite although the mixture contains at least 50 % chondrodite. It is pointed out that there are serious discrepancies between the chemical analyses of some of the humite minerals and their accepted structural formulae. The possibility that some humite minerals contain irregular numbers of olivine and Mg(OH,F)2-TiO2 layers would fit the available chemical analyses of the humite minerals very well but more structural work is needed as the present scanty X-ray results do not confirm this view. It is suggested that Ti substitution in the humite minerals is primarily controlled by the availability of F in the rock.