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

Khesinite, Ca₄Mg₂Fe³⁺₁₀O₄ (Fe³⁺₁₀Si₂)O₃₆, is a new member of the rhönite 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 (rarelywollastonite),flamite, kalsilite, cuspidine and members of the solid-solution series: Schorlomite-andradite, gehlenite-ackermanite-"Fe³⁺-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: Ca₄ (Fe³⁺_8.528Mg_1.635Ca_0.898Ti⁴⁺_0.336Ni²⁺_0.217Mn²⁺_0.155Cr³⁺_0.132Fe²⁺_0.098)∑₁₂ [(Fe³⁺_6.827Al_2.506Si_2.667)S₁₂O₄0]. 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 Mohs' hardness of 6; microhardness VHN50 is 943 kgmm^-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 [P1 a = 10.5363(1), b = 10.9242(2), c = 9.0612(1) Å, a = 106.340(1)°, b = 95.765(1)°, g = 124.373(1)°, V= 780.54(2)Å3] was refined fromX-ray single-crystal data to R1 = 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 onmagnesioferrite in associationwith pseudowollastonite and flamite at temperature not lower than 1200 °C.