Formula: Mn2+MnMg2Fe3+2(PO4)4(OH)2.8H2O
Phosphate, jahnsite subgroup, jahnsite group, forms a series with jahnsite-(CaMnMg), manganese-bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.625 calculated
Hardness: 4
Streak: Very pale yellow
Colour: Yellow to honey-coloured or greenish yellow
Luminescence: No fluorescence under UV


Jahnsite-(MnMnMg) is a relatively new mineral, approved in 2018.


At the type locality, the Sapucaia mine, Sapucaia do Norte, Galiléia, Minas Gerais, Brazil, jahnsite-(MnMnMg) was found in the dumps of the Sapucaia pegmatite. Phosphates occur in the inner wall zone and inner zone of the pegmatite and form two different phosphate mineral associations, depending on the intensity of oxidation of the genetic environment.
Association I displays stronger oxidising conditions and shows the most varied association of phosphates, dominated by lipscombite, and consisting mainly of ferrisicklerite, heterosite, variscite, rockbridgeite, barbosalite, phosphosiderite, hureaulite, and minerals belonging to the jahnsite group. Jahnsite-(MnMnMg)-jahnsite-(CaMnMg) occurs as rims or intergrowths with ferrisicklerite, or as acicular yellow crystals.
The type specimen of jahnsite-(MnMnMg) was found as single crystals or groups of crystals perched on albite crystals in hydrothermal cavities hosted by a metasomatic unit of the pegmatite composed of large lamellar aggregates of albite variety cleavelandite. The jahnsite-(MnMnMg) occurs in close association with frondelite, leucophosphite and mangangordonite as prisms up to 250 microns in length (CM 57.3.363-370).

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