Sewardite is a rare calcium-iron arsenate mineral primarily known from the legendary Tsumeb Mine in Namibia. It typically occurs as small, dark green platy crystals or crusts coating other minerals within the oxidation zone of arsenic-rich ore bodies.
Is this sewardite?
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 sewardite with a known reference. Sewardite sits at Mohs 3-4 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Sewardite leaves a light green streak.
- 3Read the lusterHold the specimen under a strong light. Sewardite typically shows a sub-adamantine luster.
- 4Match the color rangeCompare against the expected color range: dark green, yellow-green.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy crystals, crusts, radial aggregates.
Often confused with
Sewardite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Luster reads sub-adamantine on Sewardite and vitreous on Duftite.
How to tell apart: Luster reads sub-adamantine on Sewardite and vitreous on Conichalcite.
How to tell apart: Streak differs — Sewardite leaves light green, Austinite leaves white; luster reads sub-adamantine on Sewardite and vitreous on Austinite.
Often found alongside sewardite
Minerals reported to co-occur with sewardite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaFe³⁺₂(AsO₄)₂(OH)₂
- Mohs hardness
- 3-4
- Density
- 6.12 g/cm³
- Colors
- Streak
- Light Green
- Luster
- Sub-adamantine
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy Crystals, Crusts, Radial Aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Oxidized Hydrothermal Lead-zinc-copper Deposits
- Typical price
- $50-300 per specimen
Where rockhounds find sewardite
Classic worldwide localities
- Tsumeb Mine, Namibia
Field-hunting tip
Look in oxidized hydrothermal lead-zinc-copper deposits country — that is the host setting where sewardite typically forms. If you start seeing tsumcorite, arseniosiderite, goethite in float, you are in the right ground. Field specimens usually show a platy crystals, crusts, radial aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find sewardite on the map
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