Original paper

Ca-Fe3+-rich, Si-undersaturated buchite from Zelenky, North-Bohemian Brown Coal Basin, Czech Republic

Žáček, Vladimír; Skála, Roman; Chlupáčová, Marta; Dvořák,

European Journal of Mineralogy Volume 17 Number 4 (2005), p. 623 - 634

published: Jul 25, 2005

DOI: 10.1127/0935-1221/2005/0017-0623

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Abstract

Si-undersaturated Ca-Fe-rich buchite produced by fossil burning of a coal seam occurs in porcelanite deposit at Želénky in the North-Bohemian Brown Coal Basin, Czech Republic. Buchite is highly inhomogeneous, dense (ρ = 2.9–3.1 g/cm3), strongly magnetic rock (10-2–10-1 [SI]) composed of melilite, clinopyroxene (esseneite), spinel-group minerals, hematite, Ca2SiO4, calcite, aragonite, anorthite, anhydrite, perovskite, barite, two calcium ferrites – orthorhombic Ca2 Fe23+O5 (srebrodolskite) and CaFe43+O7, barium hexaferrite – BaFe123+O19, along with other several poorly identified phases. Clinopyroxene is rich in esseneite component (19–63 mol. %) and in a hypothetical end-member CaFe3+Fe3+SiO6 (up to 25 mol. %) due to Fe3+ substitution in tetrahedral sites (up to 0.26 apfu), and also in a hypothetical component Ca(Fe2+Mg)0.5Ti0.5AlSiO6 (2–36 mol. %) due to incorporation of Ti (0.012–0.18 apfu) in M1 site. Melilite shows a wide compositional variability (mol. %): gehlenite 21–93, ferrigehlenite 0–34, Ã¥kermanite 1–49, ferroÃ¥kermanite 0–25, soda-melilite 0–15. Spinel-group minerals show solid solutions of Mf 12–80, Mt 1–62, Sp 2–68, Hc 0–21 (mol. %). Srebrodolskite, orthorhombic Ca2(Fe3+Al)2O5 is widely substituted by Si and Ti. There are negative correlations among Fe + Al vs. Si + Ti (R2=0.998), and Ca (or R2+) vs. total vacancy (R2=0.94), which are consistent with an exchange vector 2R4+□2R3+-1Ca-1, and suggest a hypothetical end-member Ca□(SiTi)2O5. Calcium ferrite associated with srebrodolskite, ideally CaFe43+O7, shows extensive compositional variability: Ca0.6-0.8Fe3+2.7-3.6Al0.1-0.6Mg0.2-0.6Si0.1-0.5O7. First natural occurrence of barium hexaferrite, (Ba0.6-1.1Ca0.1-0.2)(Fe3+10.0-11.7Al0.3-0.9Ti0.1-0.6Mg0.1-0.4Mn0.0-0.2) O19, ideally BaFe3+12O 19, is reported. Variations of magnetic susceptibility with temperature were also investigated. Buchite originated by fusion of Tertiary clay and residual ash with overlying Quaternary sediments (loess loam with carbonate horizon) during fossil combustion of Miocene coal seam.

Keywords

esseneite • melilite • srebrodolskite • ferrites • pyrometamorphism • czech republic