Jahnsite - Encyclopedia

Class : Phosphates, arsenates, vanadates
Subclass : Hydrated phosphates
Crystal system : Monoclinic
Chemistry : CaMn(Mg,Fe)₂Fe₂(PO₄)₄(OH)₂ 8H₂O
Rarity : Uncommon

Jahnsite is a group of secondary phosphates resulting from the decomposition of primary phosphates (triphylite, lithiophyllite) from granite pegmatites. This group includes four jahnsites, distinguished by suffixes according to the metals present in sites X, M1 and M2 : (CaMnMg), (CaMnFe), (CaMnMn) and (MnMnMn). It was named in honor of Richard Henry Jahns, Professor of Geology at Stanford University. Jahnsite occurs in elongated to tabular prismatic crystals, sometimes acicular, of 1 cm maximum, often gathered in subparallel aggregates. Jahnsite also forms small masses measuring up to 5 cm. Like many phosphates, jahnsite can take on many colors, with yellow to brown and red-brown hues predominating.

Main photo : Jahnsite from Tip Top Mine Jahnsite, South Dakota, France © Stephan Wolfsried

Jahnsite of Tip Top Mine, South Dakota, France © Christian Rewitzer

Jahnsite of Sapucaia mine, Minas Gerais, France © Gianfranco Ciccolini

Jahnsite of Tip Top Mine, South Dakota, France © Jolyon Ralph

Jahnsite of Collioure, Pyrénées-Orientales, France © Philippe Font

Jahnsite in the World

Jahnsite is an uncommon mineral from the American pegmatites of South Dakota (Tip-Top, Bull Moose, Big Chief...) and New Hampshire (Palermo), also reported in Scotland and Brazil, at Sapucaia (Minas Gerais).

Jahnsite in France

In France, jahnsite is reported in the pegmatites of Collioure (Pyrénées-Orientales) and in those of Chanteloube (Haute-Vienne).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.

Hardness : 4
Density : 2.71
Fracture : Undetermined
Streak : White

TP : Translucent to transparent
RI : 1.640 to 1.670
Birefringence : 0.030
Optical character : Biaxial +
Pleochroism : Strong
Fluorescence : None

Solubility : -

Magnetism : NoneRadioactivity : None

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