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Composition and morphology of hyalophanes from Busovača, Bosnia and Herzegovina

Hyalophane is a barium-rich silicate belonging to the feldspar group, and an intermediate member of the series orthoclase-celsian-albite with general formula (K, Ba, Na) [Al (Al, Si) Si2O8]. Few world localities with hyalophane as the main constituent have been described, one of them being Zagrlski creek near Busovača in Bosnia and Herzegovina. Hyalophane crystals, along with quartz, albite and carbonate crystals, are primary constituents of alpine veins cutting fillites. Crystal habit of hyalophane changes during the crystallization in order: adular type (Carlsbad twins – early crystals) ; orthoclase type (Mannebach- Bavenno twins) ; adular type (single crystals) ; special type of habit dominated by vicinal faces (late crystals). Thus, hyalophane crystals from this locality can be divided in four groups according to their morpho-genetic characteristics: 1. – Carlsbad twins – early crystals 2. – Mannebach- Bavenno twins 3. – single crystals 4. – late single crystals which grew parallel on the early crystals – two variances are noticed: with and without channels. Microprobe analysis of studied hyalophanes yield following chemical composition: SiO2 = 48, 01-51, 48 ; Al2O3 = 21, 57-22, 74 ; FeO =0, 00-0, 08 ; BaO = 17, 38-19, 62 ; CaO = 0, 00-0, 04 ; Na2O = 1, 06-1, 59 ; K2O = 6, 62-8, 01 ; TiO2 = 0, 00-0, 08, which is in good agreement with literature. Data from this investigation as well as data from literature, show dual trend in Al/Si ratio. One trend (which most crystals show) is the usual one, in agreement with substitution of alkaline metal for alkaline earth metal and Si for Al. The other trend corresponds to relative depletion in Al (or relative enrichment in Si) which causes depletion in other cations in order to maintain the neutrality, which is observed in minority of analyzed crystals. Infra-red spectra of analyzed crystals show usual absorption bands. But, in all analyzed crystals absorption band at approximately 3130 cm-1 appears which corresponds to absorption of NH4+ ion. NH4+ ion was already determined in hyalophane from Busovača, but its’ position and role in hyalophane crystal structure was not determine. In order to determine mentioned parameters, polarized oriented spectra have been taken on differently oriented plates of hyalophanes from Zagrlski creek. It was determined that the position of ammonium ion is constant, that ion is oriented, but band intensities are different for differently oriented plates. This means that ammonium ion is bounded in crystal structure in the cation position rather than it is a constituent of a fluid inclusion. Quantitative IR measurements yielded 0, 08-0, 17 % of NH4+. Perceived contents of NH4+ ion are meaningful from several points of view: 1. – values are relatively high for natural feldspar minerals 2. – it is determined with high certainty that NH4+ ion is accommodated in crystal structure and is a constituent as any other cation 3. – from genetical point of view, it is worth noting that NH4+ ion implies highly basic conditions during crystallization of alpine veins.

hyalophane; morphology; chemical composition

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