Enricofrancoite (IMA 2023-002), ideally KNaCaSi4O10, is a new litidionite-group member found as the product of high-temperature alteration of hosting silicates with the enrichment by Cu-bearing fluids at the rock-fumaroles interface related to the 1872 eruption of Somma-Vesuvius volcano, southern Italy. It occurs as euhedral and platy crystals or crusts together with litidionite, tridymite, wollastonite and Al- and Fe-bearing diopside, kamenevite, perovskite, rutile, Ti-rich magnetite and colorless Si-glass. Single crystals of enricofrancoite are transparent colorless or light blue with a vitreous lustre. Mohs hardness is 5.5. Dmeas is 2.63(3) g/cm3 and Dcalc is 2.63 g/cm3. The mineral is optically biaxial (-), α = 1.542(5), β = 1.567(5),γ = 1.575(5); 2V(meas) = 60(2)° and 2Vcalc = 58°. The mean chemical composition (wt.%, electron-microprobe data) is: SiO2 64.81, Al2O3 0.03, TiO2 0.08, FeO 0.07, MgO 1.71, CaO 10.64, CuO 2.22, Na2O 8.56, K2O 11.41, total 99.94. The empirical formula based on 10 O apfu is: K0.90Na1.03(Ca0.71Mg0.16Cu0.10)Σ=0.97Si4.02O10. The Raman spectrum contains bands at 133, 248, 265, 290, 335, 400, 438, 510, 600, 690, 1120 cm-1 and the wavenumbers of the IR absorption bands are: 424, 470, 492, 530, 600, 630, 690, 750, 788, 970, 1040, 1160 cm-1. The eight strongest lines of the powder X-ray diffraction pattern are (I-d(A)-hkl): 42-6.75-01-1, 20-3.65-11-2, 100-3.370-02-2, 52-3.210-102, 18-3.051-111, 25-3.033-2-1-2, 22-2.834-02-3, 72-2.411-03-2. Enricofrancoite is triclinic, space group P-1, unit-cell parameters refined from the single-crystal data are a = 7.0155(4) Å, b = 8.0721(4) Å, c = 10.0275(4) Å, α = 104.420(4)°, β = 99.764(4)°, γ = 115.126(5),° V = 472.74(5) Å3. The crystal structure has been refined from single-crystal X-ray diffraction data to R1 = 0.035 on the basis of 2078 independent reflections with Fo > 4σ(Fo). Enricofrancoite is an H2O-free analogue of calcinaksite with 5-coordinated Ca2+ at the M site. © 2024 Cambridge University Press. All rights reserved.
