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Formula: (K,Ca,Na)(Fe,Mg,Al)8(Si,Al)12(O,OH)36.nH2O
Phyllosilicate (sheet silicate)
Crystal System: Triclinic
Specific gravity: 2.59 to 2.96 measured, 2.667 calculated
Hardness: 3 to 4
Streak: White, grey
Colour: Red, brown, black, dark green, dark reddish brown, golden brown
Common impurities: Ti,Mn,Ca,Na,K,H2O
Environments:
Stilpnomelane can be an important constituent of low-grade,
regionally
metamorphosed slate and
schist,
and it also occurs in banded iron formations. It
is a mineral of the prehnite-pumpellyite,
greenschist and
blueschist facies.
In the Kanto mountains, Japan, the minerals associated with stilpnomelane depend on the metamorphic grade.
At low grade metamorphism stilpnomelane is associated with quartz,
albite, calcite,
graphite and hematite.
At medium grade metamorphism the assemblages are
(1) stilpnomelane and chlorite,
and
(2) lawsonite, stilpnomelane and
sericite.
At the highest grade metamorphism the assemblages are
(1) muscovite,
chlorite and stilpnomelane,
(2) chlorite,
epidote, garnet and
stilpnomelane,
(3) chlorite, garnet,
muscovite and stilpnomelane.
Localities
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Stilpnomelane is occasionally found as bronze coloured flakes in the
skarn, associated with
actinolite,
diopside, fluorite and
magnetite. It replaces
actinolite
and occurs as infillings of cavities in the skarn
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At Gjerdingselva, Lunner, Viken, Norway, tiny crystals of genthelvite
have been observed in miarolitic cavities in the southwestern part of the pluton. At this locality, the mineral
content of the cavities includes aegirine (partly decomposed),
fluorite, hematite,
zircon and stilpnomelane
(CM 58.367-379)>
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