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, 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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