Gillulyite is a rare thallium-arsenic sulfosalt mineral primarily found in sediment-hosted gold deposits. It is typically identified by its dark red color and association with other arsenic sulfides, requiring professional analysis for positive identification due to its similarity to realgar.
Is this gillulyite?
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 gillulyite with a known reference. Gillulyite sits at Mohs 2-2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: bright brick-red.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to metallic if tarnished.
- 4Match the color rangeCompare against the expected color range: dark red, red, maroon, blue.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic.
Often confused with
Gillulyite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Gillulyite leaves bright brick-red, Realgar leaves red-orange to red; luster reads vitreous to metallic if tarnished on Gillulyite and resinous to greasy on Realgar.
How to tell apart: Streak differs, so Gillulyite leaves bright brick-red, Orpiment leaves pale lemon-yellow; luster reads vitreous to metallic if tarnished on Gillulyite and resinous, pearly on cleavage surface on Orpiment.
Often found alongside gillulyite
Minerals reported to co-occur with gillulyite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Tl₂As₇.₅Sb₀.₃S₁₃
- Mohs hardness
- 2-2.5
- Density
- 4.02 g/cm³
- Streak
- Bright brick-red
- Luster
- Vitreous to metallic if tarnished
- Transparency
- Opaque, translucent in thin fragments
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic
- Cleavage
- Perfect on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In a sediment-hosted disseminated gold deposit, in organic-rich unoxidized carbonates
Where rockhounds find gillulyite
Classic worldwide localities
- Mercur gold deposit, southern Oquirrh Mountains, about 56 km southwest of Salt Lake City, Tooele Co., USA (type locality)
Field-hunting tip
Where it forms: In a sediment-hosted disseminated gold deposit, in organic-rich unoxidized carbonates. If you start seeing orpiment, realgar, lorandite in float, you are in the right ground. Typical habit: prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
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Find gillulyite on the map
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