Ferroåkermanite, Ca ₂ FeSi ₂ O ₇ – a new mineral from the reduced kirschsteinite-bearing paralava, Hatrurim Complex, Israel

Abstract Ferroåkermanite, Ca 2 FeSi 2 O 7 – 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 μm 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/cm 3 . 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 (Ca 1.77 Na 0.18 Sr 0.02 Ba 0.02 K 0.02 ) Σ2.01 (Fe 2+ 0.68 Al 0.28 Mg 0.04 ) Σ1.00 (Si 1.93 Al 0.07 ) Σ2.00 O 7 . The chemical data obtained confirm the presence of ferroåkermanite–gehlenite solid solution (Fe 2+ + Si 4+ ↔ 2Al 3+ ) 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 $\overline4$ 2 1 m with the following unit-cell parameters a = 7.7813(7) Å, c = 5.0114(5) Å, V = 303.43(6) Å 3 , Z = 2. Ferroåkermanite has a melilite-type structure with layers consisting of (Si 2 O 7 ) 6– disilicate units and (Fe 2+ O 4 ) 6– tetrahedra intercalated by layers formed of eightfold-coordinated Ca atoms. Moreover, the T 1 site in the holotype specimen shows a mixed occupancy refined to 0.63(3) Fe 2+ 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 Fe 2+ -bearing phases, such as kirschsteinite, ferroåkermanite, chromite, ulvöspinel and bennesherite indicates the reduced conditions.