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 2-2.5, 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.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: trigonal. Typical habit: acicular.
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)₁₂·27H₂O
- Mohs hardness
- 2-2.5
- Density
- 1.77 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to opaque
- Crystal system
- Trigonal
- Crystal habit
- Acicular
- Cleavage
- On {1010}, perfect
- Rarity
- Uncommon
- Uses
- Collector, Geological research
- Host rock
- In metamorphosed limestone near igneous contacts or in xenoliths; as weathering crusts on larnite rocks (Hatrurim Formation, Israel)
Where rockhounds find ettringite
Classic worldwide localities
- Crestmore quarry, Riverside Co., California, USA
- Lucky Cuss mine, Tombstone, Cochise Co., Arizona, USA
- Ettringer-Bellerberg volcano, near Mayen, and Schellkopf near Brenk, Eifel district, Germany
- Zeilberg quarry, Maroldsweisach, Bavaria, Germany
- Boisséjour, near Clermont-Ferrand, Puy-de-Dôme, France
- Scawt Hill, near Larne, Co. Antrim, Ireland
Field-hunting tip
Where it forms: In metamorphosed limestone near igneous contacts or in xenoliths; as weathering crusts on larnite rocks (Hatrurim Formation, Israel). If you start seeing portlandite, afwillite, hydrocalumite in float, you are in the right ground. Typical habit: acicular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ettringite?+
Where is ettringite found?+
What rocks look like ettringite?+
What minerals are found with ettringite?+
What kind of rock does ettringite form in?+
What is ettringite used for?+
Find ettringite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
