Tululite, Ca14(Fe3+,Al)(Al,Zn,Fe3+,Si,P,Mn,Mg)15O36: a new Ca zincate-aluminate from combustion metamorphic marbles, central Jordan

Abstract

Tululite (Ca14(Fe3+,Al)(Al,Zn,Fe3+,Si,P,Mn,Mg)15O36 (the hypothetical end-member formula Ca14{Fe3+O6}[SiO4][Zn5Al9]O26) (IMA2014-065) is a new natural Ca zincate-aluminate, identified in medium-temperature (800–850 °C) combustion metamorphic (CM) spurrite-fluorellestadite marbles from central Jordan. The type locality (Tulul Al Hammam area) is situated in the northern part of the Siwaqa complex, the largest area of the “Mottled Zone” Formation in the Dead Sea region. The marbles originated from bitumen-rich chalky marine sediments of the Maastrichtian-Paleogene Muwaqqar Chalk Marl Formation, which have low clay content (and, consequently, low Al) and high Zn, Cd, and U enrichments. The bulk CM rocks derived from the low-Al protolith have unusually high (Zn + Cd)/Al ratios (~0.2) and, as a result, a mineralogy with negligibly small percentages of Ca aluminates having low Ca:Al molar ratios (minerals of mayenite supergroup, Ca:Al = 6:7) common to most of calcareous CM rocks in the Mottled Zone. Instead, the mineral assemblage of the Zn-rich marbles contains tululite, with high Ca:Al = 2.55 molar ratios and Zn substituting for a large portion of Al (Zn:Al = 1.1). Tululite occurs in thin clusters as irregular grains with indented outlines (20–100 μm in size), having typical open-work textures associated with rock-forming calcite, fluorellestadite, spurrite, and accessory Zn-rich periclase, lime-monteponite solid solutions, calcium uranates, and zincite. Marbles also bear brownmillerite, dorrite, fluormayenite, high-fluorine Ca aluminate, and lakargiite. Secondary phases are brucite, gel-like calcium silicate hydrates and calcium silicate aluminate hydrates, including Zn- and U-bearing and Cd-rich compounds, Si-bearing hydrated compounds after calcium uranates, and basic Cd chlorides. The empirical formula of the holotype tululite (a mean of 32 analyses) is (Ca13.29Cd0.75)Σ14.04(Al5.46Zn5.20Fe3+ 2.23Si0.95Mn3+ 1.01Mg0.78P0.41)Σ16.04O36. Tululite is cubic, space group F23; a = 14.9346(4) A; V = 3331.07(15) A3, Z = 4. The strongest lines of the X-ray powder-diffraction pattern [d, A – (I obs )] are: 2.874(57), 2.640 (100), 2.524(42), 2.278(41), 1.760(54), 1.725(25), 1.524(33), 1.500(33). The crystal structure was solved from single-crystal X-ray diffraction data and refined to wR2 = 0.0672 on the basis of 913 unique reflections with I 0 > 2σ(I). Tululite belongs to a group of compounds with the general formula Ca14 MT 15O35+x (0 ≤ x ≤ 1), and is a new structure type. The tetrahedral framework of tululite structure is formed by T7O13 secondary building units (SBU), which consist of four corner-linked tetrahedra sharing a common oxygen atom and three tetrahedra sharing two O atoms with the neighbor SBU. Ca2+ cations occupy three positions; two of them also contain a minor amount of Cd2+. The Ca sites surround an island (Fe3+,Al)O6 octahedron and a (Si,P)O4 tetrahedron in the centers of framework cages at the junction of eight SBUs. The (Fe3+,Al)O6 octahedron is coordinated by fourteen Ca positions into a 6-capped cube, whereas the (Si,P)O4 tetrahedron is coordinated by six Ca positions into a regular octahedron. The structural formula of tululite is Ca14{Fe3+O6}M1[(Si,P)O4]T1[(Al,Zn)7O13]2 T2-T4. The mineral is yellow with greenish tint, transparent with vitreous luster, non-fluorescent under ultraviolet light, and showing neither parting nor cleavage; Mohs hardness is 6.5. The density calculated on the basis of the empirical formula is 3.826 g/cm3. Its Raman spectrum shows strong bands at 522, 550 and 636 cm−1 and weak bands at 199, 260, 295, 456, and 754 cm−1.