Grandaite is a rare strontium aluminum arsenate, typically bright orange to salmon to brown. It occurs in a small Fe-Mn-As deposit in metamorphosed quartzites (high-P, low-T blueschist facies). Associated minerals include aegirine, adelite and barite. It was first described from the dumps of the Valletta mine, Cuneo province, Piedmont, Italy.
Is this grandaite?
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 grandaite with a known reference. Grandaite 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: reddish brown.
- 3Read the lusterHold the specimen under a strong light. Luster: waxy to vitreous.
- 4Match the color rangeCompare against the expected color range: bright orange, salmon, brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy tablets; thin fan-like masses.
Often confused with
Grandaite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Grandaite leaves reddish brown, Grossular leaves white; luster reads waxy to vitreous on Grandaite and vitreous to resinous on Grossular.
How to tell apart: Streak differs, so Grandaite leaves reddish brown, Andradite leaves white; luster reads waxy to vitreous on Grandaite and adamantine to resinous, dull on Andradite.
Often found alongside grandaite
Minerals reported to co-occur with grandaite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Sr₂Al(AsO₄)₂(OH)
- Mohs hardness
- 6-6.5
- Density
- 4.378 g/cm³
- Colors
- Streak
- Reddish brown
- Luster
- Waxy to vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Platy tablets; thin fan-like masses
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- From a small Fe-Mn-As deposit in metamorphosed quartzites (high-P, low-T blueschist facies)
Where rockhounds find grandaite
Classic worldwide localities
- Found on the dumps of the Valletta mine, Valletta Valley, Canosio municipality, Maira Valley, Cuneo province, Piedmont, Italy
Field-hunting tip
Where it forms: From a small Fe-Mn-As deposit in metamorphosed quartzites (high-P, low-T blueschist facies). If you start seeing aegirine, adelite, barite in float, you are in the right ground. Typical habit: platy tablets; thin fan-like masses. Train your eye for that shape before scanning the outcrop.
Common questions
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Find grandaite on the map
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