Sarmientite is a very rare secondary iron arsenate mineral found in the oxidized zones of arsenic-rich ore deposits. It typically forms attractive acicular or fibrous clusters that are easily recognized by their vibrant yellow to orange-yellow color.
Is this sarmientite?
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 sarmientite with a known reference. Sarmientite 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: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: yellow orange, light yellow, light orange.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular crystals, fibrous, crusts.
Often confused with
Sarmientite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Sarmientite leaves white, Scorodite leaves greenish white.
How to tell apart: Sarmientite is noticeably harder (Mohs 3.5 vs. 2.5); streak differs, so Sarmientite leaves white, Pharmacosiderite leaves yellowish green; luster reads vitreous on Sarmientite and adamantine to greasy on Pharmacosiderite.
Often found alongside sarmientite
Minerals reported to co-occur with sarmientite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe³⁺₂(AsO₄)(SO₄)(OH)·5H₂O
- Mohs hardness
- 3.5
- Density
- 2.58 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular crystals, fibrous, crusts
- Cleavage
- Perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare mineral in oxidized pyritic veins in diabase
Where rockhounds find sarmientite
Classic worldwide localities
- Santa Elena mine, Quebrada de La Alcaparrosa, between San Juan and Calingasta, San Juan Province, Argentina
Field-hunting tip
Where it forms: A rare mineral in oxidized pyritic veins in diabase. If you start seeing fibroferrite, copiapite, botryogen in float, you are in the right ground. Typical habit: acicular crystals, fibrous, crusts. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify sarmientite?+
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Is sarmientite safe to handle?+
What rocks look like sarmientite?+
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Find sarmientite on the map
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