Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Luboš Vrtiška, Jiří Sejkora, Jaromír Tvrdý, Jan Filip, J Loun, Radana Vrtišková

Julgoldite-(Fe²⁺) and Fe-rich prehnite from pectolite veins at Košťálov quarry, Czech Republic: Mössbauer, WDS and PXRD study

Journal of Geosciences, volume 70 (2025), issue 1, 43 - 51

DOI: http://doi.org/10.3190/jgeosci.403

Julgoldite-(Fe²⁺) and Fe-rich prehnite were found in pectolite veins filling fissures in basaltic andesite at the Košt’álov quarry near Semily (Liberec region, Czech Republic). Empirical formula of julgoldite-(Fe²⁺) based on EPMA and Mössbauer spectroscopy considering so-called intervalence electron hopping in the structure may be written as Ca_2.01(Fe²⁺_0.56Fe^2.5+_0.25Mg_0.14)_Σ0.95(Fe³⁺_1.19Fe^2.5+_0.25Al_0.57)_Σ2.01Si₃O_9.94(OH)_4.06. Refined unit-cell parameters for monoclinic space group A2/m are a = 8.927(3) Å, b = 6.080(2) Å, c = 19.428(7) Å, β = 97.59(4)°, V = 1045.2(8) ų. Prehnite with the empirical formula Ca_2.01(Al_0.53Fe_0.45)_Σ0.98(AlSi₃O₁₀)(OH)_1.93F_0.02 is located approximately in the middle of the prehnite-ferriprehnite series. Mössbauer spectroscopy indicated that all Fe is present in the trivalent form. The increased Fe content is also reflected in this unit-cell: a = 18.617(5) Å, b = 5.502(2) Å, c = 4.672(2) Å, V = 478.5(2) ų. Pectolite is the main mineral in the mineral assemblage. Its unit-cell parameters refined from powder X-ray diffraction are a = 7.9848(14) Å, b = 7.0391(19) Å, c = 7.0243(11) Å, α = 90.53(2)°, β = 95.19(1)°, γ = 102.48(2)° and V 383.7(1) ų. Chemical composition of pectolite corresponds to ideal stoichiometry and give the empirical formula Na_1.00(Ca_2.02Mn_0.01Fe_0.01)_Σ2.04Si₃O₈(OH_1.04)F_0.02. Other minor minerals of the association are hematite, pyrite and the youngest calcite. The source of mineralization of the circulating fluids can be found in alteration of rock components (feldspars, olivine, glass); however, external input from the surrounding calcareous sediments cannot be excluded.