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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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