Tweddillite is a rare manganese-bearing member of the epidote group, characterized by its dark, often near-opaque prismatic habit. It is primarily known from the highly mineralized Wessels Mine in South Africa, where it occurs within manganese-rich hydrothermal assemblages.
Is this tweddillite?
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 tweddillite with a known reference. Tweddillite sits at Mohs 6-6.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Tweddillite leaves a brownish black streak.
- 3Read the lusterHold the specimen under a strong light. Tweddillite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: black, brownish black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals, anhedral grains.
Often confused with
Tweddillite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside tweddillite
Minerals reported to co-occur with tweddillite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaSrMn²⁺Al₂ (Si₂O₇)(SiO₄)O(OH)
- Mohs hardness
- 6-6.5
- Density
- 3.84 g/cm³
- Colors
- Streak
- Brownish Black
- Luster
- Vitreous
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic Crystals, Anhedral Grains
- Cleavage
- Good On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Hydrothermal Manganese Deposits
- Typical price
- $50-500 thumbnail
Where rockhounds find tweddillite
Classic worldwide localities
- Wessels Mine, Kalahari Manganese Field, South Africa
Field-hunting tip
Look in hydrothermal manganese deposits country, the host setting where tweddillite typically forms. If you start seeing hausmannite, bixbyite, braunite in float, you are in the right ground. Field specimens usually show a prismatic crystals, anhedral grains habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find tweddillite on the map
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