sekaninaite

Sekaninaite ((Fe+2,Mg)2Al4Si5O18) is a silicate mineral, the iron-rich analogue of cordierite. It was first described in 1968 for an occurrence in Dolní Bory, Vysočina Region, Moravia, Czech Republic, and is now known also from Ireland, Japan, and Sweden. It was named after a Czech mineralogist, Josef Sekanina (1901–1986). In Brockley on Rathlin Island, Ireland sekaninaite occurs in bauxitic clay within the contact aureole of a diabase intrusive plug. Structure and composition The chemical formula of sekaninaite is: (Fe^2+, Mg^2+)2[Al4Si5O18]*\mathit{n}H2O. Grapes et al. (2010) calculated the percentage weights of the sample from Dolni Bory, This compound exists in nature in the form of two polymorphs: one having a disordered hexagonal structure and the other arranged in an ordered orthorhombic structure. As an aluminosilicate, the repeated and ordered structure is based on polymerization of one or the other's tetrahedral framework of Si, Al tetrahedra (Yakubovich et al., 2003). Nearly all analyses show excess of Al and deficiency in Si with respect to tetrahedral components. The overall substitution of alkalis causes excess in cations found in (K2O, Na2O, CaO), implying that sekaninaite is essentially anhydrous (Grapes et al., 2010). The atomic structures of cordierites are interpreted as a continuous series of structures that vary based on the content of octahedrally coordinated Mg and Fe cations. The varying content of atoms in the octahedral M position has an effect on the orthorhombic unit cell's parameters. The wide range of isomorphism of Mg and Fe(4-96%) suggest the existence of a continuous isomorphic series cordierite (Mg,Fe)2[Al4Si4O18]*\mathit{n}H2O-sekaninaite (Fe,Mg)2[Al4Si4O18]*\mathit{n}H2O. It is shown via crystallographic data that a shift in the iron content leads to a corresponding variance in a and b unit cell parameters (Yakubovich et al., 2003). As an aluminosilicate/cyclosilicate, the octahedral M-O distances consist of 5 independent tetrahedra form a 3-dimensional anionic framework of ordered and distributed Al3+ and Si4+ cations. One independent AlO4 and two SiO4 vortex-sharing tetrahedra share oxygen atoms to form six-member rings along the c axis of the unit cell. Mg, Fe octahedra share edges with SiO4 to form rings from alternating octahedra and tetrahedra.