Afwillite is a rare calcium silicate hydrate often found as colorless to white prismatic crystals or radial clusters. It is primarily known to collectors from its classic occurrences in the Dutoitspan diamond mine and high-temperature metamorphic skarns. Its specific cleavage and unique mineral associations are key for identifying this relatively obscure species.
Is this afwillite?
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 afwillite with a known reference. Afwillite sits at Mohs 3-4, 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: monoclinic. Typical habit: massive, prismatic.
Often confused with
Afwillite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Afwillite is noticeably harder (Mohs 3-4 vs. 1.5-2); luster reads vitreous on Afwillite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.
How to tell apart: Okenite is the harder of the two (Mohs 5 vs. 3-4).
How to tell apart: Prehnite is the harder of the two (Mohs 6-6.5 vs. 3-4).
Often found alongside afwillite
Minerals reported to co-occur with afwillite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₃Si₂O₄(OH)₆
- Mohs hardness
- 3-4
- Density
- 2.630 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Massive, prismatic
- Cleavage
- Perfect on {101}; good on {100}
- Rarity
- Rare
- Uses
- Collector, Research
- Host rock
- A product of contact metamorphism of limestones
Where rockhounds find afwillite
Classic worldwide localities
- Crestmore, Riverside Co., California, USA
- Dutoitspan diamond mine, Kimberley, and the Wessels mine, near Kuruman, Cape Province, South Africa
- Scawt Hill, near Larne, Co. Antrim, Ireland
- Schellkopf, near Brenk, Eifel district, and on the Zeilberg, near Maroldsweisach, Bavaria, Germany
- Campomorto, Montalto di Castro, Lazio, Italy
- Nahal Ayalon, Israel
Field-hunting tip
Where it forms: A product of contact metamorphism of limestones. If you start seeing apophyllite, natrolite, thaumasite in float, you are in the right ground. Typical habit: massive, prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify afwillite?+
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What rocks look like afwillite?+
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Find afwillite on the map
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