Tuhualite is an extremely rare cyclosilicate mineral typically found in volcanic rocks. It is most famous for its distinctive deep violet-blue color and tabular habit, primarily occurring in specific peralkaline rhyolite deposits.
Is this tuhualite?
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 tuhualite with a known reference. Tuhualite 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: reddish gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: blue violet, dark blue, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals, massive.
Often confused with
Tuhualite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Osumilite is the harder of the two (Mohs 5-5.5 vs. 3-4); streak differs, so Tuhualite leaves reddish gray, Osumilite leaves blue-gray.
How to tell apart: Milarite is the harder of the two (Mohs 5.5-6 vs. 3-4); streak differs, so Tuhualite leaves reddish gray, Milarite leaves white.
How to tell apart: Cordierite is the harder of the two (Mohs 7-7.5 vs. 3-4); streak differs, so Tuhualite leaves reddish gray, Cordierite leaves white.
Often found alongside tuhualite
Minerals reported to co-occur with tuhualite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaFeFeSi₆O₁₅
- Mohs hardness
- 3-4
- Density
- 2.89 g/cm³
- Streak
- Reddish gray
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular crystals, massive
- Cleavage
- Three good, on {100}, {001}, and {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A primary igneous mineral and in vesicles of some silicic lavas, as comendites and pantellerites
Where rockhounds find tuhualite
Classic worldwide localities
- Mayor Island, near Opo Bay, New Zealand
Field-hunting tip
Where it forms: A primary igneous mineral and in vesicles of some silicic lavas, as comendites and pantellerites. If you start seeing alkalic feldspar, quartz, aegirine in float, you are in the right ground. Typical habit: tabular crystals, massive. Train your eye for that shape before scanning the outcrop.
Common questions
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Find tuhualite on the map
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