Macdonaldite is a rare barium silicate mineral typically found in metamorphic environments associated with sanbornite. Collectors look for its characteristic bladed, white to colorless radial crystal sprays that exhibit a bright white fluorescence under short-wave ultraviolet light.
Is this macdonaldite?
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 macdonaldite with a known reference. Macdonaldite sits at Mohs 3.5-4, 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: satiny to vitreous.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular.
Often confused with
Macdonaldite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside macdonaldite
Minerals reported to co-occur with macdonaldite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- BaCa₄Si₁₆O₃₆(OH)₂(H₂O)₆·4H₂O
- Mohs hardness
- 3.5-4
- Density
- 2.27 g/cm³
- Streak
- White
- Luster
- Satiny to vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular
- Cleavage
- Perfect on {010}, good on {001}, poor or a fracture on {100}
- Fluorescence
- Bright white under SW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As veinlets, fracture coatings, and disseminated in a sanbornite-quartz-bearing metamorphic rock
Where rockhounds find macdonaldite
Classic worldwide localities
- Big Creek-Rush Creek area, Fresno Co., and on Trumbull Peak, near Incline, Mariposa Co., California, USA
Field-hunting tip
Where it forms: As veinlets, fracture coatings, and disseminated in a sanbornite-quartz-bearing metamorphic rock. If you start seeing sanbornite, quartz 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 macdonaldite?+
Where is macdonaldite found?+
What rocks look like macdonaldite?+
What minerals are found with macdonaldite?+
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What is macdonaldite used for?+
Find macdonaldite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
