Ferroåkermanite, Ca₂FeSi₂O₇-a new mineral from the reduced kirschsteinite-bearing paralava, Hatrurim Complex, Israel

Ferroåkermanite, Ca2FeSi2O7-a new member of the melilite group, has been found in coarse-grained kirschsteinite-bearing paralava in the Hatrurim Basin outcrop between the Zohar and Halamish Wadies of the Hatrurim Complex in Israel. Ferroåkermanite rarely forms single subhedral light-yellow crystals up to 30-50 m in size with a prismatic habit. The most common are irregular grains, aggregates and intergrowths with gehlenite or ferroåkermanite crystals with perovskite inclusions. The mineral is transparent, exhibits vitreous lustre and has a distinct cleavage on (001). It is non-fluorescent, brittle and has a conchoidal fracture, a Mohs hardness of ∼4.5-5 and a calculated density of 3.20 g/cm3. Ferroåkermanite is uniaxial (-), ω = 1.652(3) and ϵ = 1.643(3) (λ = 589 nm), and exhibits a visible pleochroism from light-yellow (ω) to intense yellow (ϵ). The empirical formula of ferroåkermanite calculated on the basis of 7 O is (Ca1.77Na0.18Sr0.02Ba0.02K0.02)Σ2.01(Fe2+0.68Al0.28Mg0.04)Σ1.00(Si1.93Al0.07)Σ2.00O7. The chemical data obtained confirm the presence of ferroåkermanite-gehlenite solid solution (Fe2+ + Si4+ ↔ 2Al3+) in the studied rock, which was verified by Raman spectroscopy investigation. The crystal structure of the new mineral was refined to R = 0.0617 in the space group P 21m with the following unit-cell parameters a = 7.7813(7) Å, c = 5.0114(5) Å, V = 303.43(6) Å3, Z = 2. Ferroåkermanite has a melilite-Type structure with layers consisting of (Si2O7)6- disilicate units and (Fe2+O4)6- tetrahedra intercalated by layers formed of eightfold-coordinated Ca atoms. Moreover, the T1 site in the holotype specimen shows a mixed occupancy refined to 0.63(3) Fe2+ and 0.37(3) Al. The presence of rock-forming minerals such as gehlenite or rankinite suggests that the paralava analysed formed under high-Temperature conditions, confirming that the new mineral ferroåkermanite is indeed a high-Temperature phase. Furthermore, the presence of Fe2+-bearing phases, such as kirschsteinite, ferroåkermanite, chromite, ulvöspinel and bennesherite indicates the reduced conditions.