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 3.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: reddish brown.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: dark red, lighter red orange.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: aggregates, anhedral grains, platy.
Often confused with
Sewardite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Duftite is the harder of the two (Mohs 4.5 vs. 3.5); streak differs, so Sewardite leaves reddish brown, Duftite leaves pale green to white; luster reads vitreous on Sewardite and dull to vitreous on fractures on Duftite.
How to tell apart: Conichalcite is the harder of the two (Mohs 4.5 vs. 3.5); streak differs, so Sewardite leaves reddish brown, Conichalcite leaves green.
How to tell apart: Streak differs, so Sewardite leaves reddish brown, Austinite leaves white; luster reads vitreous on Sewardite and subadamantine to silky in aggregates 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
- 3.5
- Density
- 4.16 g/cm³
- Colors
- Dark redLighter red orange
- Streak
- Reddish brown
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Aggregates, anhedral grains, platy
- Cleavage
- Imperfect on {100}; imperfect on {011}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Intergrown with ferrilotharmeyerite and another unknown. (Ca-dominant analogue of carminite; structure)
Where rockhounds find sewardite
Classic worldwide localities
- 31st Level, Tsumeb mine, Tsumeb, Namibia (type locality)
Field-hunting tip
Where it forms: Intergrown with ferrilotharmeyerite and another unknown. (Ca-dominant analogue of carminite; structure). If you start seeing tsumcorite, arseniosiderite, goethite in float, you are in the right ground. Typical habit: aggregates, anhedral grains, platy. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify sewardite?+
Where is sewardite found?+
Is sewardite safe to handle?+
What rocks look like sewardite?+
What minerals are found with sewardite?+
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Find sewardite on the map
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