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 3-3.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Gianellaite leaves a yellow streak.
- 3Read the lusterHold the specimen under a strong light. Gianellaite typically shows a adamantine luster.
- 4Match the color rangeCompare against the expected color range: yellow, orange-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.
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
- Hg₂SO₄
- Mohs hardness
- 3-3.5
- Density
- 7.7-7.8 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Adamantine
- 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
- Mercury-bearing Hydrothermal Veins and Altered Limestone
- Typical price
- $100-500 per specimen depending on size and quality
Where rockhounds find gianellaite
Classic worldwide localities
- Terlingua District, Texas, USA
Field-hunting tip
Look in mercury-bearing hydrothermal veins and altered limestone country — that is the host setting where gianellaite typically forms. If you start seeing cinnabar, eglestonite, terlinguaite in float, you are in the right ground. Field specimens usually show a small octahedral crystals, sometimes as coatings habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find gianellaite on the map
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