Hilgardite is a rare borate mineral primarily found in marine evaporite deposits. It is best known for its exceptional fluorescence under UV light and often occurs as small, sharp, wedge-shaped crystals in halite-rich environments.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent to translucent

mineralrare

Hilgardite

Hardness: 5
Luster: Vitreous
Streak: White
Transparency: Transparent to translucent

Is this hilgardite?

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 hilgardite with a known reference. Hilgardite sits at Mohs 5, 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: vitreous.
4
Match the color range
Compare against the expected color range: colorless.
5
Look at form & habit
Crystal system: triclinic. Typical habit: tabular crystals, wedge-shaped.

Often confused with

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

mineral

Danburite

Mohs 7-7.25

How to tell apart: Danburite is the harder of the two (Mohs 7-7.25 vs. 5).

How to tell apart: Boracite is the harder of the two (Mohs 7-7.5 vs. 5).

Compare →

Often found alongside hilgardite

Minerals reported to co-occur with hilgardite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: Ca₂B₅O₉Cl·H₂O
Mohs hardness: 5
Density: 2.67-2.71 g/cm³
Colors: Colorless
Streak: White
Luster: Vitreous
Transparency: Transparent to translucent
Crystal system: Triclinic
Crystal habit: Tabular crystals, wedge-shaped
Cleavage: On {010}, perfect; on {100}, good
Fluorescence: Bright yellow to orange under UV light
Rarity: Rare
Uses: Collector
Host rock: In marine evaporite deposits

Where rockhounds find hilgardite

Classic worldwide localities

Choctaw salt dome, Iberville Parish, Louisiana, and in Wayne Co., Mississippi, USA
Louann Salt Formation, Clarke Co., Alabama, USA
Penobsquis and Salt Springs evaporite deposits, near Sussex, New Brunswick, Canada
Königshall-Hindenburg potash mine, Reyershausen, near Göttingen, Lower Saxony, Germany
Boulby potash mine, northwest of Whitby, Yorkshire, England
Chelkar salt dome, Ak-saı̆ Valley, Uralsk district, Kazakhstan

Field-hunting tip

Where it forms: In marine evaporite deposits. If you start seeing boracite, anhydrite, danburite in float, you are in the right ground. Typical habit: tabular crystals, wedge-shaped. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify hilgardite?+

Mohs hardness is 5. Luster: vitreous. The streak is white. Common colors include colorless.

Where is hilgardite found?+

Notable localities include Choctaw salt dome, Iberville Parish, Louisiana, and in Wayne Co., Mississippi, USA; Louann Salt Formation, Clarke Co., Alabama, USA; Penobsquis and Salt Springs evaporite deposits, near Sussex, New Brunswick, Canada; Königshall-Hindenburg potash mine, Reyershausen, near Göttingen, Lower Saxony, Germany; Boulby potash mine, northwest of Whitby, Yorkshire, England.

What rocks look like hilgardite?+

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

What minerals are found with hilgardite?+

Hilgardite commonly co-occurs with Boracite, Anhydrite, Danburite, Dolomite, Magnesite, Pyrite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does hilgardite form in?+

Where it forms: In marine evaporite deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is hilgardite used for?+

Hilgardite is used in collector.

Find hilgardite on the map

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