Jahnsite-(MnMnMg) is a rare phosphate mineral typically found as secondary crusts or small, well-formed prismatic crystals in complex granitic pegmatites. Collectors should look for it in phosphate-rich zones where it often forms associated with other rare-earth and phosphate minerals during hydrothermal alteration.
Is this jahnsite-(mnmnmg)?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch jahnsite-(mnmnmg) with a known reference. Jahnsite-(MnMnMg) sits at Mohs ~4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: very pale yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: yellow, honey-colored, greenish yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals.
Often confused with
Jahnsite-(MnMnMg) vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Whitlockite is the harder of the two (Mohs 5 vs. ~4); streak differs, so Jahnsite-(MnMnMg) leaves very pale yellow, Whitlockite leaves white.
How to tell apart: Streak differs, so Jahnsite-(MnMnMg) leaves very pale yellow, Fairfieldite leaves white; luster reads vitreous on Jahnsite-(MnMnMg) and pearly to subadamantine, vitreous on the {001} cleavage on Fairfieldite.
How to tell apart: Jahnsite-(MnMnMg) is noticeably harder (Mohs ~4 vs. 3); streak differs, so Jahnsite-(MnMnMg) leaves very pale yellow, Laueite leaves white.
Often found alongside jahnsite-(mnmnmg)
Minerals reported to co-occur with jahnsite-(mnmnmg). Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- MnMnMg₂Fe₂(PO₄)₄(OH)₂·8H₂O
- Mohs hardness
- ~4
- Density
- 2.625 g/cm³
- Colors
- Streak
- Very pale yellow
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic crystals
- Cleavage
- Good on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A late-stage alteration product of primary triphylite in a metasomatic unit of zoned granitic pegmatite
Where rockhounds find jahnsite-(mnmnmg)
Classic worldwide localities
- Tip Top Mine, South Dakota
- Palermo Mine, New Hampshire
- Mangualde, Portugal
Field-hunting tip
Where it forms: A late-stage alteration product of primary triphylite in a metasomatic unit of zoned granitic pegmatite. If you start seeing frondelite, leucophosphite, mangangordonite in float, you are in the right ground. Typical habit: prismatic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify jahnsite-(mnmnmg)?+
Where is jahnsite-(mnmnmg) found?+
What rocks look like jahnsite-(mnmnmg)?+
What minerals are found with jahnsite-(mnmnmg)?+
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