Poldervaartite is a rare calcium silicate hydroxide mineral primarily known for its delicate, radiating sprays of acicular crystals. It is most famous for specimens from the Kalahari Manganese Field in South Africa, where it often forms spherical aggregates on host rocks like calcite or hausmannite.
Is this poldervaartite?
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 poldervaartite with a known reference. Poldervaartite sits at Mohs 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 on fractures to subvitreous on crystal faces.
- 4Match the color rangeCompare against the expected color range: colorless, milky white.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: sprays of acicular crystals, spherical aggregates.
Often confused with
Poldervaartite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads vitreous on fractures to subvitreous on crystal faces on Poldervaartite and silky, subvitreous on Pectolite.
How to tell apart: Prehnite is the harder of the two (Mohs 6-6.5 vs. 5).
Often found alongside poldervaartite
Minerals reported to co-occur with poldervaartite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca(Ca,Mn)(SiO₃OH)(OH)
- Mohs hardness
- 5
- Density
- 2.91 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous on fractures to subvitreous on crystal faces
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Sprays of acicular crystals, spherical aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In a pocket in massive manganese ores in a bedded manganese ore deposit
Where rockhounds find poldervaartite
Classic worldwide localities
- Wessels mine, near Kuruman, Cape Province, South Africa
Field-hunting tip
Where it forms: In a pocket in massive manganese ores in a bedded manganese ore deposit. If you start seeing braunite, hausmannite, henritermierite in float, you are in the right ground. Typical habit: sprays of acicular crystals, spherical aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify poldervaartite?+
Where is poldervaartite found?+
What rocks look like poldervaartite?+
What minerals are found with poldervaartite?+
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Find poldervaartite on the map
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