Walstromite is a rare barium calcium silicate mineral most notably found in the sanbornite-bearing metamorphic rocks of Fresno County, California. It is highly prized by collectors for its brilliant reddish-orange fluorescence under short-wave ultraviolet light. It typically occurs as small, colorless, platy crystals or granular masses embedded within larger mineral assemblages.
Is this walstromite?
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 walstromite with a known reference. Walstromite sits at Mohs ~3.5, 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: subvitreous, pearly on cleavages.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: granular.
Often confused with
Walstromite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Sanbornite is the harder of the two (Mohs 5 vs. ~3.5); luster reads subvitreous, pearly on cleavages on Walstromite and pearly on Sanbornite.
How to tell apart: Luster reads subvitreous, pearly on cleavages on Walstromite and vitreous on Fresnoite.
Often found alongside walstromite
Minerals reported to co-occur with walstromite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- BaCa₂(Si₃O₉)
- Mohs hardness
- ~3.5
- Density
- 3.67 g/cm³
- Streak
- White
- Luster
- Subvitreous, pearly on cleavages
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
- Crystal habit
- Granular
- Cleavage
- Perfect on {011}, {010}, and {100}
- Fluorescence
- Pale pink under SW UV; pinkish orange under LW UV
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- Disseminated in a sanbornite-quartz gneissic metamorphic rock, especially where the quartz content is high
Where rockhounds find walstromite
Classic worldwide localities
- Rush Creek and Big Creek areas, Fresno Co., and on Trumbull Peak, near Incline, Mariposa Co., California, USA
- Gurim Anticline and Zuk Tamrur, Negev Desert, Israel
- El Rosario and La Madrelena claim, Baja California Norte, Mexico
Field-hunting tip
Where it forms: Disseminated in a sanbornite-quartz gneissic metamorphic rock, especially where the quartz content is high. If you start seeing sanbornite, quartz, wollastonite in float, you are in the right ground. Typical habit: granular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify walstromite?+
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What rocks look like walstromite?+
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Find walstromite on the map
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