Stillwellite-(Ce) is a rare borosilicate mineral typically found in complex rare-earth element deposits. It usually occurs as trigonal prismatic crystals or massive, irregular grains within metasomatized rocks, often associated with other rare earth minerals.
Is this stillwellite-(ce)?
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 stillwellite-(ce) with a known reference. Stillwellite-(Ce) sits at Mohs ~6.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: resinous.
- 4Match the color rangeCompare against the expected color range: red brown, pale pink.
- 5Look at form & habitCrystal system: trigonal. Typical habit: massive, rhombohedral crystals.
Often confused with
Stillwellite-(Ce) vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Stillwellite-(Ce) leaves white, Allanite leaves gray; luster reads resinous on Stillwellite-(Ce) and submetallic on Allanite.
How to tell apart: Stillwellite-(Ce) is noticeably harder (Mohs ~6.5 vs. 5-5.5).
Often found alongside stillwellite-(ce)
Minerals reported to co-occur with stillwellite-(ce). Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CeBSiO₅
- Mohs hardness
- ~6.5
- Density
- 4.57-4.60 g/cm³
- Streak
- White
- Luster
- Resinous
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
- Crystal habit
- Massive, rhombohedral crystals
- Cleavage
- One imperfect
- Rarity
- Rare
- Uses
- Collector, Scientific study
- Host rock
- Locally abundant as a metasomatic replacement of metamorphosed calcareous sediments (Mary Kathleen mine, Australia); in alkalic pegmatites in syenite in an alkalic massif (Dara-i-Pioz massif, Tajikistan)
Where rockhounds find stillwellite-(ce)
Classic worldwide localities
- Mineville, Essex Co., New York, USA
- Mary Kathleen mine, about 55 km east of Mt. Isa, Queensland, Australia
- Desmont mine, near Wilberforce, Ontario, and at Mont Saint-Hilaire, Quebec, Canada
- Dara-i-Pioz massif, Alai Range, Tien Shan, Tajikistan
- Inagli massif, 30 km west of Aldan, Yakutia, Russia
- Found around the Langesundsfjord, Norway
Field-hunting tip
Where it forms: Locally abundant as a metasomatic replacement of metamorphosed calcareous sediments (Mary Kathleen mine, Australia); in alkalic pegmatites in syenite in an alkalic massif (Dara-i-Pioz massif, Tajikistan). If you start seeing allanite, garnet, uraninite in float, you are in the right ground. Typical habit: massive, rhombohedral crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify stillwellite-(ce)?+
Where is stillwellite-(ce) found?+
What rocks look like stillwellite-(ce)?+
What minerals are found with stillwellite-(ce)?+
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Find stillwellite-(ce) on the map
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