Ettringite is a delicate, hydrated calcium aluminum sulfate mineral typically found as transparent to translucent prismatic crystals. It is most famous for its occurrence in the Ettringen area of the Eifel volcanic field and in the unique metamorphic assemblages of the Franklin mining district. Collectors should handle these specimens with extreme care, as they are soft and may dehydrate if exposed to low-humidity environments for long periods.
Is this ettringite?
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 ettringite with a known reference. Ettringite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ettringite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Ettringite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: colorless, white, yellow, orange.
- 5Look at form & habitCrystal system: trigonal. Typical habit: prismatic crystals, acicular, fibrous, radiating clusters.
Often confused with
Ettringite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside ettringite
Minerals reported to co-occur with ettringite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O
- Mohs hardness
- 2
- Density
- 1.77-1.80 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Trigonal
- Crystal habit
- Prismatic Crystals, Acicular, Fibrous, Radiating Clusters
- Cleavage
- Perfect Prismatic
- Rarity
- Uncommon
- Uses
- Collector, Geological Research
- Host rock
- Metamorphosed Limestone Inclusions, Thermal Metamorphic Zones
- Typical price
- $20-150 for thumbnail to cabinet specimens
Where rockhounds find ettringite
Classic worldwide localities
- Ettringen, Germany
- Franklin, New Jersey, USA
- Mont Saint-Hilaire, Quebec, Canada
- Hatrurim Basin, Israel
Field-hunting tip
Look in metamorphosed limestone inclusions, thermal metamorphic zones country — that is the host setting where ettringite typically forms. If you start seeing calcite, gypsum, afwillite in float, you are in the right ground. Field specimens usually show a prismatic crystals, acicular, fibrous, radiating clusters habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find ettringite on the map
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