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Formula: NaFe3+Mg2Fe3+2(PO4)4(OH)2.8H2O
Hydrated phosphate, jahnsite subgroup,
jahnsite group
Crystal system: Monoclinic
Specific gravity: 2.58 measured, 2.608 calculated for the empirical formula
Hardness: 4
Streak: White
Colour: Yellow, orange-red
Solubility: Crystals slowly dissolve in cold dilute hydrochloric acid
Environments
Pegmatites
Hydrothermal environments
Localities
At the type locality, the Tip Top Mine, Fourmile, Custer Mining District, Custer County, South Dakota, USA, all
known specimens of jahnsite-(NaFeMg) came from a single football-sized rock, consisting mainly of
heterosite and presumed to be a fragment of highly altered and oxidised
triphylite. This rock was collected from the dump of the mine. Vugs in
the heterosite contain the jahnsite-(NaFeMg) in association with
several other secondary phosphate minerals including
leucophosphite, dufrénite,
barbosalite,
rockbridgeite,
mitridatite and ushkovite.
All of these species were presumably formed as late-stage hydrothermal decomposition products of
triphylite. The jahnsite-(NaFeMg) crystals appear to be the latest
of the species to form.
The Tip Top mine exploits a complex granitic
pegmatite, and has produced commercial quantities of
muscovite,
microcline perthite,
beryl,
montebrasite-amblygonite,
spodumene and
columbite-tantalite, but is
best known by mineralogists for its complex assemblages of unusual
secondary phosphate minerals.
Crystals of jahnsite-(NaFeMg) occur as isolated twinned individuals and in subparallel to divergent
intergrowths. They are typically up to 0.5 × 0.1 × 0.1 mm3, but some crystals reach 1 mm in length. The
crystals are yellow with orange-red bands near their terminations. The lustre is vitreous and most crystals exhibit
good transparency. The colour banding observed is a visual clue to chemical zonation. The central portion of
crystals, between their bases and terminations and amounting to roughly 80% of their volumes, corresponds to
jahnsite-(NaFeMg)
(AM 93.5.940-945).
Jahnsite-(NaFeMg) from the Tip Top Mine -
Image
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