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.

Hardness

2.5-3

Mohs

Luster

Pearly

Streak

White

Transparency

Transparent to translucent

mineralrare

Fontarnauite

Hardness: 2.5-3
Luster: Pearly
Streak: White
Transparency: Transparent to translucent

Is this fontarnauite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch fontarnauite with a known reference. Fontarnauite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: pearly.
4
Match the color range
Compare against the expected color range: colorless.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: tabular crystals.

Often confused with

Fontarnauite vs. its common look-alikes, and how to tell them apart in the field.

mineral

Gypsum

Mohs 1.5-2

How to tell apart: Luster reads pearly on Fontarnauite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.

How to tell apart: Luster reads pearly on Fontarnauite and vitreous to resinous, may be earthy on Borax.

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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: Na₂Sr(SO₄)[B₅O₈(OH)](H₂O)₂
Mohs hardness: 2.5-3
Density: 2.37 g/cm³
Colors: Colorless
Streak: White
Luster: Pearly
Transparency: Transparent to translucent
Crystal system: Monoclinic
Crystal habit: Tabular crystals
Cleavage: Perfect on {010}
Rarity: Rare
Uses: Collector
Host rock: An early diagenetic phase in borate-bearing marls and tuffs

Where rockhounds find fontarnauite

Classic worldwide localities

Kütahya-Emet 2 and 188 (Doğanlar) boreholes, Emet (or Emet-Hisarcık) basin, near Doğanlar, Kttahya Province, Western Anatolia, Turkey

Field-hunting tip

Where it forms: An early diagenetic phase in borate-bearing marls and tuffs. If you start seeing probertite, glauberite, celestine in float, you are in the right ground. Typical habit: tabular crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify fontarnauite?+

Mohs hardness is 2.5-3. Luster: pearly. The streak is white. Common colors include colorless.

Where is fontarnauite found?+

Notable localities include Kütahya-Emet 2 and 188 (Doğanlar) boreholes, Emet (or Emet-Hisarcık) basin, near Doğanlar, Kttahya Province, Western Anatolia, Turkey.

What rocks look like fontarnauite?+

Fontarnauite is most often confused with Gypsum, Borax. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with fontarnauite?+

Fontarnauite commonly co-occurs with Probertite, Glauberite, Celestine, Kaliborite replacing probertite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does fontarnauite form in?+

Where it forms: An early diagenetic phase in borate-bearing marls and tuffs. Working float back to the host body is the standard way to chase a fresh occurrence.

What is fontarnauite used for?+

Fontarnauite is used in collector.

Find fontarnauite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.