Bytownite is a calcium-rich member of the plagioclase feldspar group, representing the compositional range between labradorite and anorthite. Collectors identify it by its specific gravity and optical properties often associated with mafic intrusive rocks like gabbros.
Is this bytownite?
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 bytownite with a known reference. Bytownite sits at Mohs 6-6.5, 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: vitreous, pearly on cleavages.
- 4Match the color rangeCompare against the expected color range: white, gray, yellowish, colorless, gray.
- 5Look at form & habitCrystal system: triclinic. Typical habit: crystalline, granular, striated.
Often confused with
Bytownite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside bytownite
Minerals reported to co-occur with bytownite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Na,Ca)(Al,Si)₄O₈
- Mohs hardness
- 6-6.5
- Density
- 2.72-2.74 g/cm³
- Streak
- White
- Luster
- Vitreous, pearly on cleavages
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
- Crystal habit
- Crystalline, granular, striated
- Cleavage
- Perfect on {001}, good on {010}, imperfect on {110}
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- A rock-forming mineral typical of mafic intrusive igneous rocks, as gabbros and anorthosites; as phenocrysts in basalts; rare in metamorphic rocks
Where rockhounds find bytownite
Classic worldwide localities
- Stillwater complex, Montana, USA
- Cornwall, Lebanon Co., and Phoenixville, Chester Co., Pennsylvania, USA
- Lakeview, Lake Co., Oregon, USA
- Lucky Cuss mine, Tombstone, Cochise Co., Arizona, USA
- Elsewhere in Canada, from the Shawmere anorthosite, Foleyet Township, Ontario, and on Yamaska Mountain, near Abbotsford, Quebec
- Rhum Island, Scotland
Field-hunting tip
Where it forms: A rock-forming mineral typical of mafic intrusive igneous rocks, as gabbros and anorthosites; as phenocrysts in basalts; rare in metamorphic rocks. If you start seeing pyroxenes, olivine in float, you are in the right ground. Typical habit: crystalline, granular, striated. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify bytownite?+
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Find bytownite on the map
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