Khesinite, Ca4Mg2Fe3+ 10O4[(Fe3+ 10Si2)O36], a new rhönite-group (sapphirine supergroup) mineral from the Negev Desert, Israel– natural analogue of the SFCA phase

Abstract

Khesinite, Ca 4 Mg 2 Fe 3+ 10 O 4 (Fe 3+ 10 Si 2 )O 36 , is a new member of the rhonite group of the sapphirine supergroup. Khesinite was discovered in thin veins of paralavas within fine-grained gehlenite rocks (hornfels) of the Hatrurim Complex in the Negev Desert, Israel. Paralavas are composed of rankinite, pseudowollastonite (rarely wollastonite), flamite, kalsilite, cuspidine and members of the solid-solution series: schorlomite–andradite, gehlenite–ackermanite–“Fe 3+ -gehlenite”, magnesioferrite–spinel and fluorapatite–fluorellestadite. Accessory and rare minerals are represented by baryte, walstromite, fresnoite, vorlanite, barioferrite, hematite, perovskite, gurimite, zadovite, aradite and hexacelsian. Electron-microprobe analysis of the holotype khesinite gives the following empirical formula for 40 oxygens and 28 cations: Ca 4 (Fe 3+ 8.528 Mg 1.635 Ca 0.898 Ti 4+ 0.336 Ni 2+ 0.217 Mn 2+ 0.155 Cr 3+ 0.132 Fe 2+ 0.098 ) Σ12 [(Fe 3+ 6.827 Al 2.506 Si 2.667 ) Σ12 O 40 ]. Khesinite is black to dark brown. It has semi-metallic lustre and does not show fluorescence. Cleavage and parting are not observed, fracture is irregular. Khesinite has a Mohs9 hardness of 6; microhardness VHN 50 is 943 kg mm −2 . The calculated density is 4.097 g cm −3 . In reflected light khesinite is grey with weak internal brown reflections. Reflectance data for the COM (Commission of Ore Mineralogy, IMA) wavelengths vary from ~13.4% (470 nm) to ~11.8% (700 nm). The crystal structure of khesinite [ P 1 ¯ a = 10 . 5363 ( 1 ) , b = 10.9242(2), c = 9.0612(1) A, α = 106.340(1)°, β = 95.765(1)°, γ = 124.373(1)°, V = 780.54(2) A 3 ] was refined from X-ray single-crystal data to R 1 = 0.046. The khesinite structure is close to that of the synthetic compounds SFCA and SFCAM. Khesinite crystallized in paralava from melt, sometimes forming isolated crystals, but more commonly reaction rims on magnesioferrite in association with pseudowollastonite and flamite at temperature not lower than 1200 °C.