Ba2+ substitution in heulandite-clinoptilolite series from Hončova Hůrka hill (Czechia): the emergence of a new mineral species

Abstract

Hončova hůrka Hill represents an Early Cretaceous strongly altered picritic effusive body. In the low-temperature hydrothermal mineralisation of both amygdales and rock fissures, a calcite-dominated mineral association including zeolites (harmotome, ferrierite-Mg, and heulandite-clinoptilolite series), chlorite, baryte, various clay minerals, quartz, and some accessories are known. Up to this moment, the only published analysis of the heulandite-clinoptilolite series mineral was a single wet analysis proving the existence of a Ca-dominant extra-framework cation. Two hundred twenty-three new energy-dispersive (EDS) microanalyses of 11 samples revealed a much more complicated picture. Two hundred spots are classified as clinoptilolites, with a continuous transition from more common barium-rich clinoptilolite-Ca to the Ba-dominant member (4 spot analyses). Twenty-three analyses correspond to the heulandite subgroup, with a continuous transition from barium-rich heulandite-Ca to the Ba-dominant member (5 spot analyses). The electron microscope backscattered electron images revealed the lamellar structure of the crystal aggregates of the heulandite-clinoptilolite series minerals and strong zoning, with the cores richest in barium and the outer parts enriched in calcium. We believe that originally the mineral aggregates were dominantly barian, while at the end of the hydrothermal solution precipitation resulting in carbonate crystallisation (calcite, minor dolomite), there was an extra-framework cation exchange of Ba²⁺ for Ca²⁺. The excess of barium at that moment stimulated baryte deposition in minute cracks of the heulandite-clinoptilolite series minerals. Four microanalyses revealing the presence of yet undescribed and unapproved clinoptilolite-Ba are not sufficient for a new phase description at this time but clearly indicate its presence in nature.