Holtite is a very rare tantalum-rich borosilicate mineral first discovered in Australian pegmatites. It typically occurs as yellowish to brownish fibrous or prismatic crystals embedded in quartz or muscovite matrices. Because it is chemically related to dumortierite but contains significant tantalum, it is highly sought after by advanced mineral collectors.
Is this holtite?
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 holtite with a known reference. Holtite sits at Mohs 8.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: light buff.
- 3Read the lusterHold the specimen under a strong light. Luster: resinous, vitreous, dull when weathered.
- 4Match the color rangeCompare against the expected color range: beige white, brown, greenish, greenish brown, light brown white.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: blocky, pseudo hexagonal.
Often confused with
Holtite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Holtite leaves light buff, Dumortierite leaves white; luster reads resinous, vitreous, dull when weathered on Holtite and vitreous to dull on Dumortierite.
How to tell apart: Holtite is noticeably harder (Mohs 8.5 vs. 6.5-7.5); streak differs, so Holtite leaves light buff, Sillimanite leaves white; luster reads resinous, vitreous, dull when weathered on Holtite and vitreous to subadamantine on Sillimanite.
Often found alongside holtite
Minerals reported to co-occur with holtite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Ta₀.₆□₀.₄)Al₆BSi₃O₁₈
- Mohs hardness
- 8.5
- Density
- 3.60-3.90 g/cm³
- Colors
- Streak
- Light buff
- Luster
- Resinous, vitreous, dull when weathered
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Blocky, pseudo hexagonal
- Cleavage
- Good on {001}, may be parting
- Fluorescence
- Bright yellow under SW UV; dull orange under LW UV
- Rarity
- Rare
- Uses
- Collector, Mineralogical study
- Host rock
- As coatings on stibiotantalite and replacing tantalite in a pegmatite (Greenbushes, Western Australia); in pegmatites cutting amphibolites (Voron'i massif, Russia); in alluvial gravels derived from pegmatite
Where rockhounds find holtite
Classic worldwide localities
- Bunbury Gully, Greenbushes, Western Australia
- Mount Vasin-Myl’k, Voron'i massif, Kola Peninsula, Russia
- Szklary, Lower Silesia, Poland
Field-hunting tip
Where it forms: As coatings on stibiotantalite and replacing tantalite in a pegmatite (Greenbushes, Western Australia); in pegmatites cutting amphibolites (Voron'i massif, Russia); in alluvial gravels derived from pegmatite. If you start seeing stibiotantalite, tantalite, tourmaline in float, you are in the right ground. Typical habit: blocky, pseudo hexagonal. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify holtite?+
Where is holtite found?+
What rocks look like holtite?+
What minerals are found with holtite?+
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Find holtite on the map
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