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Formula: CaMn2+Mg2Fe3+2(PO4)4(OH)2.8H2O
Hydrated phosphate containing hydroxyl,
jahnsite subgroup,
jahnsite group,
manganese-bearing
mineral
Crystal System: Monoclinic
Specific gravity: 2.706 to 2.718 measured, 2.715 calculated
Hardness: 4
Streak: White
Colour: Brown, golden brown, purplish brown, yellow, yellow-orange, greenish yellow
Luminescence: Not fluorescent under UV
Solubility: Soluble in dilute hydrochloric acid
Environments
Jahnsite-(CaMnMg) occurs in granitic
pegmatites.
Localities
At the type locality, the Tip Top mine, Fourmile, Custer Mining District, Custer county, South Dakota, USA,
jahnsite-(CaMnMg) occurs as a late stage product in corroded
triphylite - heterosite -
ferrisicklerite -
rockbridgeite masses. Associated minerals include
leucophosphite, hureaulite,
collinsite, laueite,
vivianite, triphylite,
tavorite, rockbridgeite and
robertsite.
The jahnsite-(CaMnMg) forms tabular short to long prismatic crystals with vitreous to subadamantine lustre
of a nut-brown, purplish brown, yellow, yellow-orange, or greenish yellow colour. Persistent striations parallel
to the prism distinguish it from hureaulite and
leucophosphite
(AM 59.48-59, Mindat).
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