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. Streak: brownish black.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 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
- CaSrMn₂Al(Si₂O₇)(SiO₄)O(OH)
- Mohs hardness
- 6-6.5
- Density
- 3.78 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
Where rockhounds find tweddillite
Classic worldwide localities
- Wessels Mine, Kalahari Manganese Field, South Africa
Field-hunting tip
Where it forms: hydrothermal manganese deposits. If you start seeing hausmannite, bixbyite, braunite in float, you are in the right ground. Typical habit: prismatic crystals, anhedral grains. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tweddillite?+
Where is tweddillite found?+
What rocks look like tweddillite?+
What minerals are found with tweddillite?+
What kind of rock does tweddillite form in?+
What is tweddillite used for?+
Find tweddillite on the map
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