Talmessite is a rare hydrated calcium magnesium arsenate that typically occurs as attractive, delicate spherical or radial clusters in arid, arsenic-rich deposits. It is highly sought after by collectors for its vibrant green colors and intricate crystal habits. Because it contains arsenic, specimens should be handled with care and stored safely.
Is this talmessite?
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 talmessite with a known reference. Talmessite sits at Mohs 5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Talmessite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Talmessite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: colorless, white, pale green, apple green, pale yellow.
- 5Look at form & habitCrystal system: triclinic. Typical habit: radial, spherical, or drusy aggregates of tiny crystals.
Often confused with
Talmessite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Talmessite is noticeably harder (Mohs 5 vs. 2.5); luster reads vitreous on Talmessite and silky on Picropharmacolite.
How to tell apart: Talmessite is noticeably harder (Mohs 5 vs. 4).
Often found alongside talmessite
Minerals reported to co-occur with talmessite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂Mg(AsO₄)₂·2H₂O
- Mohs hardness
- 5
- Density
- 3.47 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Radial, Spherical, Or Drusy Aggregates of Tiny Crystals
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Hydrothermal Veins in Serpentinized Rocks
- Typical price
- $20-150 per specimen depending on size and crystal quality
Where rockhounds find talmessite
Classic worldwide localities
- Talmessi Mine, Iran
- Bou Azzer, Morocco
- Mapimi, Mexico
Field-hunting tip
Look in hydrothermal veins in serpentinized rocks country — that is the host setting where talmessite typically forms. If you start seeing arsenates, calcite, dolomite in float, you are in the right ground. Field specimens usually show a radial, spherical, or drusy aggregates of tiny crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find talmessite on the map
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