Enricofrancoite, KNaCaSi 4 O 10 , a new Ca–K–Na silicate from Somma–Vesuvius volcano, southern Italy

Abstract

Enricofrancoite (IMA2023–002), ideally KNaCaSi 4 O 10 , 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 colourless Si-glass. Single crystals of enricofrancoite are transparent colourless or light blue with a vitreous lustre. Mohs hardness 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 (−), α = 1.542(5), β = 1.567(5),γ = 1.575(5); 2V (meas) = 60(2)° and 2V calc = 58°. The mean chemical composition (wt.%, electron-microprobe data) is: SiO 2 64.81, Al 2 O 3 0.03, TiO 2 0.08, FeO 0.07, MgO 1.71, CaO 10.64, CuO 2.22, Na 2 O 8.56, K 2 O 11.41, total 99.94. The empirical formula based on 10 O apfu is: K 0.90 Na 1.03 (Ca 0.71 Mg 0.16 Cu 0.10 ) Σ0.97 Si 4.02 O 10 . 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 absorption bands are: 424, 470, 492, 530, 600, 630, 690, 750, 788, 970, 1040 and 1160 cm –1 . The eight strongest lines of the powder X-ray diffraction pattern are [ d , Å ( I , %) hkl ]: 6.75 (42) 01 $\bar{1}$ , 3.65 (20) 11 $\bar{2}$ , 3.370 (100) 02 $\bar{2}$ , 3.210 (52) 102, 3.051 (18) 111, 3.033 (25) 2 $\bar{1}\bar{2}$ , 2.834 (22) 02 $\bar{3}$ and 2.411 (72) 03 $\bar{2}$ . Enricofrancoite is triclinic, space group P $\bar{1}$ , 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)° and V = 472.74(5) Å 3 . The crystal structure has been refined from single-crystal X-ray diffraction data to R 1 = 0.035 on the basis of 2078 independent reflections with F o > 4σ( F o ). Enricofrancoite is an H 2 O-free analogue of calcinaksite with 5-coordinated Ca 2+ at the M site.