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. Grandaite leaves a reddish brown streak.
- 3Read the lusterHold the specimen under a strong light. Grandaite typically shows a waxy to vitreous luster.
- 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, Grossularite Garnet leaves white; luster reads waxy to vitreous on Grandaite and vitreous on Grossularite Garnet.
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 to 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
- Skarn
Where rockhounds find grandaite
Classic worldwide localities
- dumps of the Valletta mine, Piedmont, Italy
Field-hunting tip
Look in skarn country, the host setting where grandaite typically forms. If you start seeing calcite, diopside, vesuvianite in float, you are in the right ground. Field specimens usually show a platy tablets; thin fan-like masses habit, so 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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