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