Sekaninaite
Sekaninaite ((Fe+2,Mg)2Al4Si5O18) is a silicate mineral, the iron-rich analogue of cordierite.
| Sekaninaite | |
|---|---|
Sekaninaite from Dolní Bory, Czech Republic | |
| General | |
| Category | Cyclosilicate |
| Formula (repeating unit) | (Fe+2,Mg)2Al4Si5O18 |
| IMA symbol | Skn |
| 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 | |
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.
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