Calomel is a rare secondary mercury chloride mineral typically found in the oxidation zones of mercury-bearing ore deposits. It is known for its high density and characteristic adamantine luster, often appearing as crusts or small, poorly formed crystals associated with native mercury and cinnabar.
Is this calomel?
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 calomel with a known reference. Calomel sits at Mohs 1.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: pale yellowish white.
- 3Read the lusterHold the specimen under a strong light. Luster: adamantine.
- 4Match the color rangeCompare against the expected color range: white, yellowish gray, gray, yellowish white, brown.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: earthy, prismatic.
Often confused with
Calomel vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Cerussite is the harder of the two (Mohs 3-3.5 vs. 1.5); streak differs, so Calomel leaves pale yellowish white, Cerussite leaves white.
How to tell apart: Anglesite is the harder of the two (Mohs 2.5-3 vs. 1.5); streak differs, so Calomel leaves pale yellowish white, Anglesite leaves white.
How to tell apart: Streak differs, so Calomel leaves pale yellowish white, Phosgenite leaves white.
Often found alongside calomel
Minerals reported to co-occur with calomel. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Hg₂Cl₂
- Mohs hardness
- 1.5
- Density
- 7.15 g/cm³
- Streak
- Pale yellowish white
- Luster
- Adamantine
- Transparency
- Transparent to translucent
- Crystal system
- Tetragonal
- Crystal habit
- Earthy, prismatic
- Cleavage
- Good on {110}, uneven; imperfect on {011}
- Fluorescence
- Dark red under SW and LW UV
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- An uncommon secondary mineral formed through the alteration of other mercury minerals
Where rockhounds find calomel
1 mapped spotsClassic worldwide localities
- Palo Alto, San Mateo Co., and the Redington mine, Napa Co., California, USA
- Sunflower district, Maricopa Co., Arizona, USA
- McDermitt mine, Humboldt Co., Nevada, USA
- Terlingua, Brewster Co., Texas, USA
- Alva, Serbia
- Almadén, Ciudad Real Province, Spain
Field-hunting tip
Where it forms: An uncommon secondary mineral formed through the alteration of other mercury minerals. If you start seeing mercury, amalgam, cinnabar in float, you are in the right ground. Typical habit: earthy, prismatic. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Texas. Start trip planning there.
Common questions
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Find calomel on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
