Eglestonite is a rare mercury oxychloride mineral that is most famous for its occurrence in the Terlingua mining district of Texas. It typically presents as small, brilliant yellow-orange or yellow-brown crystals that quickly darken to a dull brownish-black upon exposure to light, making fresh, bright specimens highly sought after by collectors.
Is this eglestonite?
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 eglestonite with a known reference. Eglestonite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: yellow or greenish yellow, rapidly becoming black.
- 3Read the lusterHold the specimen under a strong light. Luster: brilliant adamantine to resinous.
- 4Match the color rangeCompare against the expected color range: yellow, brown, brownish yellow, dark brown, orange.
- 5Look at form & habitCrystal system: cubic. Typical habit: dodecahedral crystals, massive, crusts.
Often confused with
Eglestonite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Eglestonite is noticeably harder (Mohs 2.5 vs. 1.5); streak differs, so Eglestonite leaves yellow or greenish yellow, rapidly becoming black, Calomel leaves pale yellowish white; luster reads brilliant adamantine to resinous on Eglestonite and adamantine on Calomel.
How to tell apart: Streak differs, so Eglestonite leaves yellow or greenish yellow, rapidly becoming black, Terlinguaite leaves lemon-yellow, turning olive-green.
Often found alongside eglestonite
Minerals reported to co-occur with eglestonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Hg₆OCl₃(OH)
- Mohs hardness
- 2.5
- Density
- 8.309-8.45 g/cm³
- Streak
- Yellow or greenish yellow, rapidly becoming black
- Luster
- Brilliant adamantine to resinous
- Transparency
- Translucent
- Crystal system
- Cubic
- Crystal habit
- Dodecahedral crystals, massive, crusts
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- An oxidation product of other mercury minerals in mercury deposits
Where rockhounds find eglestonite
Classic worldwide localities
- Terlingua, Brewster Co., Texas, USA
- Crawford and Jackfork, Pike Co., Arkansas, USA
- McDermitt and Cordero mercury mines, Opalite district, and at the Cahill mine, Poverty Peak district, Humboldt Co., Nevada, USA
- Denio district, Harney Co., Oregon, USA
- Huahuaxtla, Guerrero, and Guadalćazar, San Luis Potosi, Mexico
- Monarch cinnabar mine, Transvaal, South Africa
Field-hunting tip
Where it forms: An oxidation product of other mercury minerals in mercury deposits. If you start seeing calomel, mercury, terlinguaite in float, you are in the right ground. Typical habit: dodecahedral crystals, massive, crusts. Train your eye for that shape before scanning the outcrop.
Common questions
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Find eglestonite on the map
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