Sekaninaite

sekaninaite

fayalite

cordierite

titanomagnetite

Images

Formula: Fe²⁺₂Al₄Si₅O₁₈
Cyclosilicate (ring silicate), paramorph of ferroindialite, forms a series with cordierite
Crystal System: Orthorhombic
Specific gravity: 2.76 to 2.77 measured, 2.78 calculated
Hardness: 7 to 7½
Streak: White
Colour: Greyish blue to bluish violet
Common impurities: Ti,Mn,Ca,Na,K,H₂O
Environments

Pegmatites
Coal-seam fires

Localities

At Arthurs Seat, Mornington Peninsula Shire, Victoria, Australia, sekaninaite occurs in a newly discovered Devonian (419.62 to 358.86 million years ago) rhyolite that shows well developed sector twinning in response to structural ordering during cooling. Reverse zoning is attributed to the reheating of the host magmas due to interaction, such as mixing, with a higher-temperature magma prior to eruption. The ranges of cordierite–sekaninaite compositions in all volcanic rocks bear no systematic relationship to the bulk compositions of their host rocks. It appears that sekaninaite might be stable in silicic volcanic magmas over a wide range of melt compositions, pressures and temperatures but is favoured for low-Mg# bulk compositions at low pressure and low temperature (MM.89.4.463-475).

At the type locality, Pegmatite vein No. 4, Dolní Bory, Bory, Žďár nad Sázavou District, Vysočina Region, Czech Republic, sekaninaite occurs in the albite zone of the pegmatite in granulite and gneiss, associated with albite and quartz. The sekaninaite occurs as poorly developed vitreous crystals up to 70 cm in size, usually twinned, simulating hexagonal symmetry (AM 62.395).
Sekaninaite from Dolní Bory - Image

At Brockley, Rathlin Island, County Antrim, Ulster, Northern Ireland, UK, sekaninaite occurs in bauxitic lithomarge intensely altered by a dolerite plug (HOM)

At the Mystic Creek Coal Basin, Denali Borough, Alaska, USA, the coal is Miocene (23.03 to 5.33 million years ago) and the host rock is a silty sandstone consisting mainly of quartz, feldspar, and minor hematite and clay. A coal-seam fire fused and melted the country rock producing a metasediment-clinker and paralava. Sekaninaite, plagioclase and fayalite are the main minerals that formed along with titanium-bearing magnetite, mullite, augite, and an unidentified aluminium - iron - titanium oxide mineral.
The paralava is an andesite with rhyolitic residual glass. Oxidation and fusion of the sediment was the first phase of pyrometamorphism, where the sediment becomes brown-red and sekaninaite begins to form. The metasediment melts forming vesicles in a black glass; sekaninaite formation is well underway. The melt separates from the host and coalesces to form the paralava. As the paralava cools, fayalite and sekaninaite precipitate, accompanied by plagioclase, quartz, titanium-bearing magnetite, and an aluminium - iron - titanium oxide. Proximity to the surface allowed quenching of the remaining liquid to rhyolitic glass. In all models, sekaninaite precipitation is the most important mineral leading to the rhyolitic glass (AM 108.1794-1804).

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