Armstrongite is a rare calcium zirconium silicate typically found in hyper-alkaline pegmatites and igneous complexes. It usually forms as small, colorless to pale pink platy crystals or granular masses that require careful identification due to its similarity to other zirconium silicates.
Is this armstrongite?
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 armstrongite with a known reference. Armstrongite sits at Mohs 4.6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: brownish white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: light brown, reddish brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: aggregates, bladed, spherical.
Often confused with
Armstrongite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Armstrongite leaves brownish white, Eudialyte leaves white.
How to tell apart: Catapleiite is the harder of the two (Mohs 5.5-6 vs. 4.6); streak differs, so Armstrongite leaves brownish white, Catapleiite leaves white to pale yellow; luster reads vitreous on Armstrongite and weakly vitreous or dull on Catapleiite.
Often found alongside armstrongite
Minerals reported to co-occur with armstrongite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaZrSi₆O₁₅·2H₂O
- Mohs hardness
- 4.6
- Density
- 2.562-2.593 g/cm³
- Colors
- Streak
- Brownish white
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Aggregates, bladed, spherical
- Cleavage
- Perfect on {001}, good on {100}
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In schlieren of alkalic granite pegmatite, at the contact of arfvedsonite-bearing granite with xenoliths of felsic volcanic rock
Where rockhounds find armstrongite
Classic worldwide localities
- Khan-Bogdo granitic massif, southern Gobi desert, Mongolia
- Strange Lake alkaline complex, southeast of Lac Brisson, Quebec and Labrador, Newfoundland, Canada
Field-hunting tip
Where it forms: In schlieren of alkalic granite pegmatite, at the contact of arfvedsonite-bearing granite with xenoliths of felsic volcanic rock. If you start seeing elpidite, quartz, microcline in float, you are in the right ground. Typical habit: aggregates, bladed, spherical. Train your eye for that shape before scanning the outcrop.
Common questions
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Find armstrongite on the map
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