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

Vladimír ÁEK^1,*, Roman SKÁLA¹, Marta CHLUPÁOVÁ² and Zdenk DVOÁK³

¹ Czech Geological Survey, P.O. BOX 85, Klárov 3, CZ-118 21 Praha 1, Czech Republic
² Boháova 866/4, CZ-149 00 Praha 4, Czech Republic
³ Doly Bílina, a.s., Bílina, Czech Republic

^* Corresponding author, e-mail: zacek{at}cgu.cz

Si-undersaturated Ca-Fe-rich buchite produced by fossil burning of a coal seam occurs in porcelanite deposit at elénky in the North-Bohemian Brown Coal Basin, Czech Republic. Buchite is highly inhomogeneous, dense ( = 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 Ca₂Fe₂³⁺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)_0.5Ti_0.5 AlSiO₆ (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, åkermanite 1–49, ferroåkermanite 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⁺_-1 Ca_-1, and suggest a hypothetical end-member Ca (SiTi)₂ O₅. Calcium ferrite associated with srebrodolskite, ideally CaFe₄³⁺O₇, shows extensive compositional variability: Ca_0.6–0.8Fe³⁺_2.7–3.6Al_0.1–0.6Mg_0.2–0.6Si_0.1–0.5O₇. First natural occurrence of barium hexaferrite, (Ba_0.6–1.1Ca_0.1–0.2)(Fe³⁺_10.0–11.7Al_0.3–0.9Ti_0.1–0.6Mg_0.1–0.4Mn_0.0–0.2) O₁₉, ideally BaFe³⁺₁₂O₁₉, 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.

Key-words: esseneite, melilite, srebrodolskite, ferrites, pyrometamorphism, Czech Republic.

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