Modraite, Ca19Fe2+Al4(Al7Fe2+)(SiO4)10(Si2O7)4O(OH)9, a new vesuvianite-group mineral from the Modra skarn, Malé Karpaty Mountains, Slovakia

Abstract Modraite, ideally Ca19Fe2+Al4(Al7Fe2+)(SiO4)10(Si2O7)4O(OH)9, is a new member of the vesuvianite group. It was found at Modra, Harmónia skarn rock in the Malé Karpaty Mountains, Pezinok District, Slovakia. Modraite formed as a high-temperature and low-pressure metamorphic mineral on the contact zone between Devonian limestones and Carboniferous granitic rocks, in association with diopside-hedenbergite, rarely grossular, calcite, titanite, and clinozoisite-epidote. The new mineral forms dark brown microscopically transparent euhedral to subhedral prismatic crystals or radiating aggregates (up to 5 cm long) in diopside-hedenbergite groundmass. Modraite crystals are characterized by dominant {100} and minor {110} prism faces; the mineral is brittle, has an irregular (uneven) fracture without cleavage or parting. The Mohs’ hardness is 6−7. The measured and calculated densities are 3.35(2) and 3.42 g/cm3, respectively. The new mineral is microscopically transparent to translucent, optically uniaxial, negative, ω = 1.7221(2), ε = 1.7151(3). The mean chemical composition of modraite (wt%, electron microprobe, REE and Li by LA-ICP-MS, Fe2O3 and FeO by Mössbauer spectroscopy, H2O calculated from stoichiometry) is: SiO2 36.42, TiO2 1.79, Al2O3 16.37, V2O3 0.03, Cr2O3 0.03, La2O3 0.03, Ce2O3 0.08, Pr2O3 0.01, Nd2O3 0.04, Sm2O3 0.01, Gd2O3 0.01, Fe2O3 1.32, FeO 3.65, MnO 0.23, ZnO 0.05, MgO 0.87, CaO 34.92, BaO 0.26, Li2O 0.03, Na2O 0.09, H2O 1.80, F 1.49, Cl 0.16, O=F -0.63, O=Cl -0.04, total 99.02. The empirical formula based on 50 cations (excluding H) per formula unit is: X1–X4(Ca18.60Na0.09Ba0.05Ce0.02La0.01Nd0.01)Σ18.78Y1(Fe2+0.81Fe3+0.13Al0.09)Σ1.03Y2Al4Y3(Al5.50Fe2+0.71Ti4+0.67Mg0.65Fe3+0.36Mn2+0.10Li+0.06Zn2+0.02 V3+0.01Cr3+0.01)Σ8.09T1(□4) T2□ (Z1–Z2Si1.01O4)10 (Z3Si2O7)4O10–O11(OH5.97F2.34O1.55Cl0.14)Σ10.00. Mössbauer spectroscopy indicates a dominant proportion of Fe2+ (76 %) over Fe3+ (24 %), with the dominancy of Fe2+ over Fe3+ at both the Y1 and Y3 sites. The crystal structure was refined to R1 = 0.0215 for 3970 unique reflections. The mineral is tetragonal, space group P4/nnc, single-crystal X-ray diffraction data of a modraite are: a = 15.559(2) Å, c = 11.804(2) Å, V = 2857.4(9) Å3, Z = 2. The seven strongest lines of the powder X-ray diffraction pattern are [d(Å)−I(%)−hkl]: 3.235−19−402, 2.949−29−004, 2.749−100−432, 2.593−59−522, 2.458−37−620, 1.625−16−526, 1.621−30−922. Modraite is the Y1Fe2+-dominant member, ideally with 7 Al3+ and 1 Fe2+ cations at the Y3 site, and a strong O10-H10…O10 hydrogen bond. Modraite is named after the type locality, Modra, western Slovakia.