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Formula: (Na,Ca)Mn2+(Mg,Fe3+)2Fe3+2(PO4)4(OH)2.8H2O
Hydrated phosphate containing hydroxyl, jahnsite subgroup,
jahnsite group,
manganese-bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.68 measured, 2.684 calculated
Hardness: 4
Streak: Very pale yellow
Colour: Light orange to orange-yellow
Luminescence: No fluorescence under UV
Solubility: Slowly soluble in dilute hydrochloric acid at room temperature
Environments
Jahnsite-(NaMnMg) is a low temperature, secondary
mineral formed by alteration of primary phosphates in
zoned granitic
pegmatites
(HOM).
Localities
There are two co-type localities, the White Rock No. 2 pegmatite, White Rock Feldspar Mine, Old Boolcoomata Station,
Bimbowrie Conservation Park, Olary Province, South Australia, and the Sapucaia mine, Sapucaia do Norte, Galiléia, Minas
Gerais, Brazil.
At the type localities jahnsite-(NaMnMg) occurs in the
pegmatites as zoned yellow prismatic crystals with
orange-red bands near their terminations. Associated minerals include
ushkovite, rockbridgeite,
mitridatite,
leucophosphite, heterosite
and dufrénite
(Mindat).
The White Rock No. 2 pegmatite, White Rock Feldspar Mine, Old Boolcoomata Station, Bimbowrie Conservation Park, Olary
Province, South Australia, belongs to the
beryl-columbite-phosphate-rare
element type of pegmatite. The
pegmatite has been mined for
feldspar, both albite and
microcline, and beryl. It is
mineralogically zoned, characterised by the occurrence of late-stage phosphate nodules between the
quartz core and intermediate
feldspar-rich zone.
Jahnsite-(NaMnMg) occurs here in seams in a matrix comprising triplite,
fluorapatite and quartz.
Associated minerals are phosphosiderite,
ushkovite, strunzite and
bermanite.
Triplite–zwieselite was formed by
metasomatic alteration of magmatic fluorapatite and has been transformed
by hydrothermal alteration and weathering, in an oxidising, low-temperature, low-pH (acid) environment, to produce a
complex, microcrystalline intergrowth of secondary phosphate
minerals.
On the cotype specimen, ushkovite and
phosphosiderite are the earliest-formed minerals,
bermanite and strunzite overgrow
ushkovite and
phosphosiderite, and jahnsite-(NaMnMg) overgrows
ushkovite. The presence of
bermanite, in particular, indicates a very oxidising environment and is
consistent with some of the Mn in jahnsite-(NaMnMg) from the White Rock occurrence being Mn3+
(CM 56.6.871-882).
At the Sapucaia mine, Sapucaia do Norte, Galiléia, Minas Gerais, Brazil, the
pegmatite is a zoned
pegmatite that has been mined since the 1920s for
muscovite, beryl and
feldspar, but is best known mineralogically for its complex phosphate mineral
assemblages. Jahnsite-(NaMnMg) was found in vugs in matrix consisting mostly of
frondelite and albite in
association with meurigite-K,
phosphosiderite and
strengite.
Jahnsite-(NaMnMg) crystals are found growing directly on surficially altered
albite blades, which presumably are the source of the sodium. It is suggested
that there were two triphylite alteration sequences at Sapucaia leading to
two different secondary phosphate mineral assemblages.
Assemblage I involves oxidising conditions, while assemblage II does not.
Jahnsite-series phases occur in assemblage I, consistent with the presence of
essential Fe3+ in jahnsites. By contrast,
whiteite series phases of the
jahnsite group occur in assemblage II, consistent with the presence of Al
in place of Fe3+ in the structure.
Sodium is a minor constituent in the jahnsites from Sapucaia and, indeed,
sodium is at most only a minor constituent in the vast majority of jahnsites
worldwide. The only other approved jahnsite species with essential sodium is
jahnsite-(NaFeMg)
(CM 56.6.871-882).
At the Tip Top Mine, Fourmile, Custer Mining District, Custer county, South Dakota, USA, jahnsite-(NaMnMg) was
previously reported forming small zones in crystals that were mostly
jahnsite-(NaFeMg)
(CM 56.6.871-882).
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