Unusually iron-rich julgoldite-(Fe3+) from the Karadag volcanic massif (Crimea Peninsula)
Kasatkin A. V., Zubkova N. V., Chukanov N. V., Ksenofontov D. A., Škoda R., Tishchenko A. I., Voronin M. V., Britvin S. N., Pekov I. V.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 6. P. 96-112
Julgoldite-(Fe3+), a rare mineral of the pumpellyite group, has been found at the Karadag volcanic massif (Crimea peninsula) as black radial aggregates up to 0.5 cm in diameter with. It occurs in cracks of a volcanic rock and is associated with quartz. Its chemical composition [wt. %, electron microprobe analysis (EMPA), Fe3+ : Fe2+ ratio calculated from Mössbauer data] is: CaO 20.40, MgO 0.35, MnO 0.10, FeO 3.62, Al2O3 0.10, Fe2O3 37.91, SiO2 32.86, H2O (calc.) 5.68, total 101.02. Parameters of monoclinic unit cell refined from powder X-ray diffraction (XRD) data are: a = 8.8885(7), b = 6.0791(4), c = 19.3679(14) Å, β = 97.499(5)°, V = 1037.58(13) Å3; space group A2/m. The combination of the results of EMPA and XRD together with Mössbauer, IR and Raman spectroscopic data makes it possible to write the crystal chemical formula of Karadag julgoldite-(Fe3+) with such possible variations in the distribution of Fe3+ and Fe2+ between the X and Y structural sites: Ca2.00X(Fe3+0.77-0.62Fe2+0.13-0.28Mg0.05Mn0.01) Σ 0.96Y(Fe3+1.84-1.99Fe2+0.15-0.00Al0.01)Σ2.00[Si2O6O10 (OH,O)] [SiO4]O11(OH,O)O7(OH,H2O)O5(OH). The Karadag mineral is unusually rich in iron and almost does not contain a pumpellyite component. In addition, unlike many other minerals of the pumpellyite group, where all the hydrogen belongs to OH groups, julgoldite-(Fe3+) from Karadag contains molecular water, albeit in a small amount.
Keywords: julgoldite-(Fe3+), pumpellyite group, crystal structure, Mössbauer spectrum, IR spectrum, Raman spectrum, Karadag volcanic massif, Crimea peninsula