Gianellaite is a rare mercury sulfate mineral discovered in the mercury mines of the Terlingua district in Texas. It typically presents as brilliant yellow, transparent octahedra that are highly sought after by collectors of rare mercury species due to their intense fluorescence.
Is this gianellaite?
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 gianellaite with a known reference. Gianellaite sits at Mohs ~3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: light yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: yellow, straw yellow.
- 5Look at form & habitCrystal system: cubic. Typical habit: small octahedral crystals, sometimes as coatings.
Often confused with
Gianellaite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Gianellaite leaves light yellow, Eglestonite leaves yellow or greenish yellow, rapidly becoming black; luster reads vitreous on Gianellaite and brilliant adamantine to resinous on Eglestonite.
How to tell apart: Streak differs, so Gianellaite leaves light yellow, Terlinguaite leaves lemon-yellow, turning olive-green; luster reads vitreous on Gianellaite and brilliant adamantine on Terlinguaite.
Often found alongside gianellaite
Minerals reported to co-occur with gianellaite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- [(NHg₂)₂](SO₄)(H₂O)x
- Mohs hardness
- ~3
- Density
- 7.19 g/cm³
- Colors
- Streak
- Light yellow
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Cubic
- Crystal habit
- Small octahedral crystals, sometimes as coatings
- Cleavage
- None
- Fluorescence
- Bright yellow under SW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare secondary mineral coating fracture surfaces in oxidized portions of a hydrothermal mercury deposit, the nitrogen likely derived from decaying organic matter (Mariposa mine, Texas, USA)
Where rockhounds find gianellaite
Classic worldwide localities
- Perry pit, Mariposa mine, Terlingua district, Brewster Co., Texas, USA
- Clear Creek claim, near the Clear Creek mercury mine, New Idria district, San Benito Co., California, USA
Field-hunting tip
Where it forms: A rare secondary mineral coating fracture surfaces in oxidized portions of a hydrothermal mercury deposit, the nitrogen likely derived from decaying organic matter (Mariposa mine, Texas, USA). If you start seeing cinnabar, terlinguaite, montroydite in float, you are in the right ground. Typical habit: small octahedral crystals, sometimes as coatings. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify gianellaite?+
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Is gianellaite safe to handle?+
What rocks look like gianellaite?+
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Find gianellaite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
