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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless, white, light green.
- 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. 1.5-2); luster reads vitreous on Talmessite and feeble pearly to silky on Picropharmacolite.
How to tell apart: Luster reads vitreous on Talmessite and resinous on Adelite.
How to tell apart: Luster reads vitreous on Talmessite and subadamantine to silky in aggregates on Austinite.
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
- 3.421 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Triclinic
- Crystal habit
- Radial, spherical, or drusy aggregates of tiny crystals
- Cleavage
- Perfect on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare secondary species formed typically in the oxidized zone of some hydrothermal mineral deposits, an alteration product of realgar, orpiment, or Cu–Ni arsenides
Where rockhounds find talmessite
Classic worldwide localities
- Gold Hill mine, Tooele Co., Utah: in a cave in Peach Springs Canyon, Mohave Co., Arizona, USA
- Talmessi mine, 35 km west of Anarak, Iran
- Arhbar (Aghbar) and Aı̈t Ahmane mines, Bou Azzer district, Morocco
- Tsumeb, Namibia
- Gomez Palacio, Durango, Mexico
- Cross Creek, near Savona, British Columbia, Canada
Field-hunting tip
Where it forms: A rare secondary species formed typically in the oxidized zone of some hydrothermal mineral deposits, an alteration product of realgar, orpiment, or Cu–Ni arsenides. If you start seeing gaitite, erythrite, annabergite in float, you are in the right ground. Typical habit: radial, spherical, or drusy aggregates of tiny crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify talmessite?+
Where is talmessite found?+
Is talmessite safe to handle?+
What rocks look like talmessite?+
What minerals are found with talmessite?+
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What is talmessite used for?+
Find talmessite on the map
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