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.
Original languageEnglish
Pages (from-to)277-287
Number of pages11
JournalMineralogical Magazine
Volume88
Issue number3
Early online date26 Feb 2024
DOIs
StatePublished - 26 Jun 2024

    Research areas

  • crystal structure, enricofrancoite, Italy, litidionite group, new mineral, Somma-Vesuvius, Absorption spectroscopy, Alumina, Aluminum oxide, Copper oxides, Feldspar, Magnesia, Magnetite, Perovskite, Potassium compounds, Silica, Silicon, Single crystals, Sodium compounds, Ternary alloys, Titanium dioxide, Volcanoes, Crystals structures, Cu bearings, Enricofrancoite, Group members, Highest temperature, Litidionite group, New mineral, Southern Italy, Crystal structure

ID: 117487485