Fontarnauite is a rare sulfate-borate mineral discovered in the potash-bearing evaporite deposits of Spain. It typically forms thin, transparent, tabular crystals that are found in association with other salts such as halite and sylvite.
Is this fontarnauite?
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 fontarnauite with a known reference. Fontarnauite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Fontarnauite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Fontarnauite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: tabular crystals.
Often confused with
Fontarnauite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside fontarnauite
Minerals reported to co-occur with fontarnauite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Na,K)₅Ca₂(SO₄)₄(B₅O₉)·3H₂O
- Mohs hardness
- 2
- Density
- 2.12 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Tabular Crystals
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Evaporite Deposits
- Typical price
- $50-300 per specimen
Where rockhounds find fontarnauite
Classic worldwide localities
- Sallent, Spain
- Cardona, Spain
Field-hunting tip
Look in evaporite deposits country — that is the host setting where fontarnauite typically forms. If you start seeing halite, sylvite, anhydrite in float, you are in the right ground. Field specimens usually show a tabular crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find fontarnauite on the map
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