Jagoite revisited: crystal structure, mineral composition and paragenesis

The rare Pb silicate jagoite, known only from the Långban and Pajsberg Mn–Fe oxide deposits in Värmland, Sweden, is associated with a more diverse mineral assemblage than originally described: alamosite, barysilite, hyttsjöite, margarosanite, melanotekite, nasonite and yangite and other, not fully characterised Pb silicates. Primary melanotekite and barysilite, formed as skarn (together with hematite, quartz, clinopyroxene and andradite) during regional metamorphism, are prone to alteration, with Cl⁻, SiO₂, Ca2⁺ and H₂O acting as modifying agents. In the process, newly formed Pb silicates exhibit increasing Si content, reflecting a higher degree of SiO₄ polymerisation at high pH and decreasing temperatures. A refinement of the crystal structure of jagoite from X-ray diffraction data, to R1 = 1.2% [space group P $\bar 6$ 2c, a = 8.53926(5) Å and c = 33.3399(2) Å], confirms previous work, and provides significantly improved structural parameters. New data were also obtained with Mössbauer spectroscopy, laser-Raman micro-spectroscopy, electron-microprobe and laser-ablation inductively coupled plasma mass spectrometry analyses. The results indicate that jagoite accommodates minor elements, notably Al at an octahedrally coordinated Fe-dominated site and Mn3⁺, Zn and Mg at four-coordinated mixed Fe–Si sites, and small amounts of Ca+Na replacing Pb. Jagoite is also enriched in Be, Sb, Bi and Br, but those elements have a limited role in its crystal chemistry. Mössbauer measurements show that Fe3⁺ is distributed over three different crystallographic sites, two 4-coordinated and one 6-coordinated, and that jagoite remains paramagnetic down to 77 K. The ideal chemical formula for jagoite should be written Pb11Fe5Si12O41Cl3 for Z = 2.