Osumilite

osumilite

tridymite

enstatite

cordierite

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Formula: KFe2(Al5Si10)O30
Cyclosilicate (ring silicate), osumilite group, titanium-bearing mineral
Crystal System: Hexagonal
Specific gravity: 2.58 to 2.68 measured, 2.71 calculated
Hardness: 5 to 6
Colour: Blue
Solubility:
Common impurities: Ti,Mn,Ba,K
Environments

Volcanic igneous environments
Metamorphic environments

Osumilite occurs in acid volcanic rocks such as rhyolite and dacite, as anhedral phenocrysts in the groundmass and as cavity-lining crystals associated with tridymite and quartz. It also occurs in contact metamorphic xenoliths in volcanic rocks, in fused hornfels and in high grade contact metamorphic aureoles. The conditions of crystallisation range from volcanic up to granulite facies metamorphic. Osumilite is not stable in hydrous, low-temperature conditions (Dana). Associated minerals include tridymite, cristobalite, quartz, oligoclase, K-feldspar, fayalite, enstatite-ferrosilite, biotite, magnetite and zircon (HOM).

Localities

At the Nain Complex, Labrador, Newfoundland, Canada, pink osumilite occurs with quartz, enstatite-ferrosilite, orthoclase, cordierite, plagioclase, graphite and pyrrhotite in granulite in the contact aureole of the complex. It occurs as a discrete phase as well as with quartz and enstatite-ferrosilite in symplectite. Associated rock types in this contact aureole include plagioclase-enstatite-ferrosilite-quartz granulite and cordierite-garnet-enstatite-ferrosilite-quartz-feldspar granulites. The osumilite is commonly separated from polycrystalline cordierite knots by a zone of plagioclase. There are places, however, where the cordierite and osumilite are in direct contact. The polycrystalline cordierite knots enclose minor amounts of a brown hercynitic spinel. Locally, the osumilite occurs in spectacular intergrowths with quartz and enstatite-ferrosilite (AM 61.20-37).

At the Fosso Ricomero, Vetralla, Viterbo Province, Lazio, Italy, osumilite occurs as rare, transparent, flattened, hexagonal light-blue prisms in vugs of a volcanic ejectum from inside the basal trachytic flow of the Vico Volcanic Complex. The osumilite is intimately associated with a vanadium-rich mineral of the pseudobrookite group and hematite (AM 74.1278-1284).

At the type locality, Sakkabira, Kagoshima Prefecture, Japan, osumilite occurs in biotite-bearing enstatite-ferrosilite rhyolite/dacite that contains andesine, quartz, enstatite-ferrosilite, biotite and magnetite as phenocrysts scattered in the groundmass. osumilite is present in the brittle porous part of the rhyolite/dacite and not in the hard compact portion. Euhedral crystals occur in association with tridymite and quartz in drusy cavities in the brittle porous part, whereas anhedral crystals occur in the groundmass. The groundmass consists mainly of tridymite, oligoclase and K-feldspar, with small amounts of glass, quartz, enstatite-ferrosilite, fayalite, biotite, osumilite and iron oxide dust. The osumilite crystallised from the rhyolite/dacite magma in the final stage of consolidation (AM 41.104-116).

Alteration

Osumilite breaks down retrogressively to cordierite, mica and quartz (Dana).

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