Sekaninaite
| Sekaninaite | |
|---|---|
| General | |
| Category | Cyclosilicate |
|
Formula (repeating unit) |
(Fe+2,Mg)2Al4Si5O18 |
| Strunz classification | 9.CJ.10 |
| Dana classification | 61.02.01.02 Cordierite group |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Cccm |
| Unit cell | a = 17.18 Å, b = 9.82 Å c = 9.29 Å; Z = 4 |
| Identification | |
| Color | Blue to blue-violet |
| Crystal habit | As poorly developed crystals |
| Twinning | Commonly twinned on {110} and {310} |
| Cleavage | {100}, imperfect; parting on {001} |
| Mohs scale hardness | 7 - 7.5 |
| Luster | Vitreous |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.76 - 2.77 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.561 nβ = 1.572 nγ = 1.576 |
| Birefringence | δ = 0.015 |
| 2V angle | Measured: 66°, Calculated: 60° |
| References | |
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, Ireland sekaninaite occurs in bauxitic clay within the contact aureole of a diabase intrusive plug.
The chemical formula of sekaninaite is: (Fe2+, Mg2+)2Al4Si5O18*nH2O. Grapes 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, 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, 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]*nH2O-sekaninaite (Fe,Mg)2[Al4Si4O18]*nH2O. 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, 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 SiO¬4 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. Thus, the framework can be described as a semi-layered structure formed of layers of tetrahedra linked into rings by sharing vertices and octahedra and tetrahedra sharing edges, alternating along the c axis. The distortion of the orthorhombic unit cell is determined by the chemical composition rather than the degree of ordering in the tetrahedral framework (Yakubovich, 2003). The temperature at which the liquidous phases crystallize in a sequence: mullite + tridymite, followed by sekaninaite and finally fayalite + clinoferrosilite (Grapes, 2010). Similar trends are observed for amphiboles, clinopyroxenes, olivines’, and others. The increase in the Fe mole fraction of minerals was not related with iron input, but was caused by its redistribution during contact metamorphism (Korchak, 2010).
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