Chromian spinels and magnetite of serpentinites, steatitic rocks, tremolite asbestos and chloritites from Bragança massif, northeastern Portugal

Abstract

At Bragança region, northeastern Portugal, accessory chromian spinels and magnetite occur in serpentinites and steatitic rocks and rarely in tremolite asbestos and chloritites. The chromian spinels have a large range in composition, which is related to serpentinisation and other subsequent alteration processes such as steatisation. Four varieties of zoned chromian spinels were distinguished: 1- zoned crystals with a picotite core and a ferrian chromite rim in serpentinite from Donai; 2- zoned crystals with an aluminian chromite core and a ferrian chromite rim in steatitic rock from Donai; 3- zoned aluminian chromite crystals showing a rim richer in Fe²⁺ and poorer in Mg than the core, found in Donai tremolite asbestos; 4- zoned chromian magnetite crystals showing a rim richer in Fe³⁺ and poorer in Cr than the core, in serpentinites and steatitic rocks from Sete Fontes and Soeira/Pena Maquieira. Unzoned aluminian chromite crystals were found in Donai chloritite. Magnetite from the Donai serpentinite and steatitic rocks from Sete Fontes and Soeira/Pena Maquieira has a composition close to the ideal formula.