Enricofrancoite, KNaCaSi4O10, a new Ca-K-Na silicate from Somma-Vesuvius volcano, southern Italy

Enricofrancoite (IMA2023-002), ideally KNaCaSi4O10, is a newlitidionite-group member found as the product of high-temperaturealteration of hosting silicates with the enrichment by Cu-bearing fluidsat the rock-fumaroles interface related to the 1872 eruption ofSomma-Vesuvius volcano, southern Italy. It occurs as euhedral and platycrystals or crusts together with litidionite, tridymite, wollastoniteand Al- and Fe-bearing diopside, kamenevite, perovskite, rutile, Ti-richmagnetite and colourless Si-glass. Single crystals of enricofrancoiteare transparent colourless or light blue with a vitreous lustre. Mohshardness is 5.5. D-meas is 2.63(3) g/cm(3) and D-calc is 2.63 g/cm(3).The mineral is optically biaxial (-), alpha = 1.542(5), beta =1.567(5),gamma = 1.575(5); 2V((meas)) = 60(2)degrees and 2V(calc) = 58degrees. 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 basedon 10 O apfu is: K0.90Na1.03(Ca0.71Mg0.16Cu0.10)(Sigma 0.97)Si4.02O10.The Raman spectrum contains bands at 133, 248, 265, 290, 335, 400, 438,510, 600, 690 and 1120 cm(-1) and the wavenumbers of the IR absorptionbands are: 424, 470, 492, 530, 600, 630, 690, 750, 788, 970, 1040 and1160 cm(-1). The eight strongest lines of the powder X-ray diffractionpattern are {[}d, Acirc; (I, %) hkl]: 6.75 (42) 01 (1) over bar, 3.65(20) 11 (2) over bar, 3.370 (100) 02 (2) over bar, 3.210 (52) 102, 3.051(18) 111, 3.033 (25) 2 (1) over bar(2) over bar, 2.834 (22) 02 (3) overbar and 2.411 (72) 03 (2) over bar. Enricofrancoite is triclinic, spacegroup P (1) over bar}, unit-cell parameters refined from thesingle-crystal data are a = 7.0155(4) Acirc;, b = 8.0721(4) Acirc;, c =10.0275(4) Acirc;, alpha = 104.420(4)(degrees), beta = 99.764(4)degrees,gamma = 115.126(5)(degrees) and V = 472.74(5) Acirc;(3). The crystalstructure has been refined from single-crystal X-ray diffraction data toR-1 = 0.035 on the basis of 2078 independent reflections with F-o > 4sigma(F-o). Enricofrancoite is an H2O-free analogue of calcinaksite with5-coordinated Ca2+ at the M site.