Montroydite is a rare mercury oxide that occurs primarily as orange-red acicular needles or earthy coatings in mercury-rich deposits. It is best identified by its extreme density and close association with other mercury minerals like cinnabar in limestone environments.
Is this montroydite?
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 montroydite with a known reference. Montroydite 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-brown.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to adamantine.
- 4Match the color rangeCompare against the expected color range: red, brownish red, brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: crystalline.
Often confused with
Montroydite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Montroydite leaves yellow-brown, Cinnabar leaves scarlet; luster reads vitreous to adamantine on Montroydite and adamantine, inclining to metallic when dark; dull in friable material on Cinnabar.
How to tell apart: Kleinite is the harder of the two (Mohs 3.5-4 vs. 2.5); streak differs, so Montroydite leaves yellow-brown, Kleinite leaves sulfur-yellow; luster reads vitreous to adamantine on Montroydite and adamantine to greasy on Kleinite.
Often found alongside montroydite
Minerals reported to co-occur with montroydite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- HgO
- Mohs hardness
- 2.5
- Density
- 11.23 g/cm³
- Colors
- Streak
- Yellow-brown
- Luster
- Vitreous to adamantine
- Transparency
- Opaque, transparent on thin edges
- Crystal system
- Orthorhombic
- Crystal habit
- Crystalline
- Cleavage
- {001}, perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In hydrothermal mercury deposits
Where rockhounds find montroydite
Classic worldwide localities
- Terlingua, Brewster Co., Texas, USA
- McDermitt mine, Opalite district, Humboldt Co., Nevada, USA
- Huahuaxtla, Guerrero, and at El Doktor, Queretaro, Mexico
- Khaydarkan, Fergana Valley, Alai Range, Kyrgyzstan
Field-hunting tip
Where it forms: In hydrothermal mercury deposits. If you start seeing mercury, cinnabar, metacinnabar in float, you are in the right ground. Typical habit: crystalline. Train your eye for that shape before scanning the outcrop.
Common questions
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Find montroydite on the map
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