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Ferroåkermanite; Ca2FeSi2O7 – a new mineral from the reduced kirschsteinite-bearing paralava; Hatrurim Complex; Israel
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DOI：10.1180/mgm.2025.10120.png)
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Abstract
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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 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/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 P21m 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.
Keywords:
ferroåkermanite
new mineral
melilite-group minerals
paralava
crystal structure
Raman spectroscopy
Hatrurim Complex
Israel
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