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 1.5-2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Gillulyite leaves a reddish-brown streak.
- 3Read the lusterHold the specimen under a strong light. Gillulyite typically shows a adamantine luster.
- 4Match the color rangeCompare against the expected color range: dark red, brownish-red.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: tabular to prismatic crystals, often as small aggregates or crusts.
Often confused with
Gillulyite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Gillulyite leaves reddish-brown, Realgar leaves orange-red; luster reads adamantine on Gillulyite and resinous on Realgar.
How to tell apart: Streak differs — Gillulyite leaves reddish-brown, Orpiment leaves yellow; luster reads adamantine on Gillulyite and resinous 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
- Tl₂As₈S₁₃
- Mohs hardness
- 1.5-2
- Density
- 3.5 g/cm³
- Colors
- Streak
- Reddish-brown
- Luster
- Adamantine
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Tabular to Prismatic Crystals, Often as Small Aggregates or Crusts
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Sedimentary Gold Deposits
- Typical price
- $100-500 for small thumbnail specimens
Where rockhounds find gillulyite
Classic worldwide localities
- Carlin Trend, Nevada, USA
Field-hunting tip
Look in sedimentary gold deposits country — that is the host setting where gillulyite typically forms. If you start seeing realgar, orpiment, pyrite in float, you are in the right ground. Field specimens usually show a tabular to prismatic crystals, often as small aggregates or crusts habit, so 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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