Molysite is a rare iron chloride mineral typically found as a sublimation product around volcanic fumaroles. It is extremely deliquescent, meaning it rapidly absorbs moisture from the air and dissolves into a liquid, making it a challenging mineral to preserve in a dry collection.
Is this molysite?
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 molysite with a known reference. Molysite 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: brownish yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: yellow, brownish red, purple red.
- 5Look at form & habitCrystal system: trigonal. Typical habit: encrustations.
Often confused with
Molysite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Erythrosiderite is the harder of the two (Mohs 2.5 vs. 1.5); streak differs, so Molysite leaves brownish yellow, Erythrosiderite leaves yellowish-orange.
How to tell apart: Streak differs, so Molysite leaves brownish yellow, Halite leaves white.
Often found alongside molysite
Minerals reported to co-occur with molysite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- FeCl₃
- Mohs hardness
- 1.5
- Density
- 2.90 g/cm³
- Streak
- Brownish yellow
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Trigonal
- Crystal habit
- Encrustations
- Cleavage
- Perfect on {0001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A volcanic sublimate
Where rockhounds find molysite
Classic worldwide localities
- Found on the Augustine volcano, Augustine Island, near the Kenai Peninsula, Alaska, USA
- Vesuvius, Campania, Italy
- Larderello, Val di Cecina, Tuscany, Italy
- Vulcano, Lipari Islands, Italy
- Tolbachik fissure volcano, Kamchatka Peninsula, Russia
Field-hunting tip
Where it forms: A volcanic sublimate. If you start seeing tridymite, hematite, anhydrite in float, you are in the right ground. Typical habit: encrustations. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify molysite?+
Where is molysite found?+
Is molysite safe to handle?+
What rocks look like molysite?+
What minerals are found with molysite?+
What kind of rock does molysite form in?+
What is molysite used for?+
Find molysite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
