Amoraite, Ca12Al6(OH)36(CO3)2(SO3)·15H2O, a new mineral, the first natural sulfite-bearing layered double hydroxide

Amoraite is a new mineral in the hydrocalumite group − a Ca-Al subdivision of the hydrotalcite supergroup. It was discovered in pyrometamorphic rocks of the Hatrurim Basin, near the Israeli coast of the Dead Sea, and was named for the Amora, a Pleistocene lake that situated at the basin currently occupied by the Dead Sea. Amoraite occurs in the cavities of altered larnite-jasmundite-brownmillerite marble, in association with katoite and other hydrocalumite-group minerals: kuzelite, hydrocalumite, mariakrite Ca4Al2(OH)12(H2O)4(Fe2S4), and mampsisite Ca4Al2(OH)12(CO3)·5H2O. The mineral forms thin colorless platelets up to 0.2 mm in size flattened on {010}, with perfect cleavage in this plane. Mohs’ hardness ∼3; the calculated density is 2.100 g·cm-3. Optically biaxial (−), 2Vmeas ∼ 15 °, α =1.528(3), β = γ = 1.545(2). Chemical composition (electron microprobe, wt.%, H2O and CO2 derived from the structural data): CaO 39.65, Al2O3 17.67, SO2 3.59, CO2 4.55, H2O 34.96, total 100.42. The empirical formula calculated on the basis of (Ca+Al) = 18 per formula unit is Ca12.08Al5.92(OH)36.46(CO3)1.77(SO3)0.96·14.92H2O, corresponding to the ideal one Ca12Al6(OH)36(CO3)2(SO3)·15H2O. Triclinic, space group P1¯, a =15.2630(2), b = 16.0664(2), c = 23.0448(3) Å, α = 100.259(1), β = 100.936(1), γ = 94.299(1) °, V = 5425.05(12) Å3, Z = 4. From a crystal chemical point of view, amoraite belongs to the Ca-Al branch of layered double hydroxides (LDH). In cement industry, these Ca-Al LDH are known as the AFm phases (from early “alumina, ferric oxide, monosubstituted”). Amoraite is the first sulfite-bearing LDH with determined crystal structure (R1 = 0.058 for 15362 unique observed reflections). Its unit cell contains 295 independent structural sites with fully ordered arrangement of (CO3) and (SO3) groups. Strongest lines of powder X-ray diffraction pattern [d in Å(I)(hkl)]: 7.89(100)(020), 3.933(41)(040), 3.815(39)(3¯30, 330), 2.882(29)(5¯04 + mult), 2.526(26)(4¯44 + mult), 2.441(25)(6¯20 + mult), 2.323(19)(1¯38, 1¯58). The mineral is a product of low-temperature aqueous alteration of larnite marble. It is a natural analogue of a sulfite-containing AFm phase, a common constituent of cements produced with the addition of desulfurization ashes. It is noteworthy that interlayer d-spacing of amoraite, 7.89 Å, nearly coincident with that of a more common AFm phase, hydrocalumite that significantly complicates the task of powder XRD discrimination of these phases in cementitious compositions.