Jahnsite-(NaMnMg)

jahnsite-(NaMnMg)

jahnsite-(NaFeMg)

ushkovite

rockbridgeite

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Formula: (Na,Ca)Mn2+(Mg,Fe3+)2Fe3+2(PO4)4(OH)2.8H2O
Hydrated phosphate containing hydroxyl, jahnsite group, manganese-bearing mineral
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

Pegmatites

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. Triplitezwieselite 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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