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Formula: Na3CaFe2+11(PO4)9
Anhydrous normal phosphate, fillowite group
Crystal System: Trigonal
Specific gravity: 3.35 measured, 3.41 calculated
Hardness: 4½
Streak: Light brown
Colour: Dark brown
Environments
Pegmatites
Metamorphic environments
Johnsomervilleite is an accessory mineral in podiform metamorphic segregations in
kyanite-sillimanite grade
gneiss (Webmin).
Associated minerals include albite,
bermanite,
hureaulite, jahnsite,
microcline,
phosphosiderite,
quartz, tourmaline,
triphylite and vivianite
(Mindat).
Localities
At the Sapucaia mine, Sapucaia do Norte, Galiléia, Minas Gerais, Brazil, johnsomervilleite occurs as a
primary mineral in a complex
granite
pegmatite in
staurolite grade
mica schist. Associated minerals
include
triphylite, frondelite,
hureaulite, bermanite,
jahnsite, rockbridgeite,
phosphosiderite, vivianite,
autunite, zircon,
tourmaline, microcline,
albite and quartz
(HOM).
The type locality is Loch Quoich, Caol and Mallaig, Highland, Scotland, UK. Here johnsomervilleite occurs as
one of several
primary accessory minerals forming clusters in podiform
metamorphic segregations
in kyanite-sillimanite grade
gneiss (HOM). The segregation pods consist of
quartz, muscovite,
plagioclase, almandine -
spessartine garnet and
apatite together with rare, very small clusters of
graftonite, jahnsite,
phosphosiderite,
rockbridgeite,
mitridatite, vivianite and
johnsomervilleite. Johnsomervilleite usually occurs in close association with
graftonite or vivianite. It is
assumed that
fairly high temperatures and pressures were operative during the formation of the
primary phosphate assemblage
(MM 43.331.833-836, HOM).
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