Antofagastaite is a rare hydrated sodium-calcium sulfate mineral typically found as delicate, needle-like or fibrous crystal aggregates in arid evaporite settings. Collectors look for these fragile formations within the hyper-arid regions of the Atacama Desert in Chile, where it occurs as a secondary precipitate.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Antofagastaite

Hardness: 2
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this antofagastaite?

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 antofagastaite with a known reference. Antofagastaite sits at Mohs 2, 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: white, colorless.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: fibrous, acicular, prismatic.

Often confused with

Antofagastaite 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 vitreous on Antofagastaite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.

Often found alongside antofagastaite

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

All properties

Chemical formula: Na₂Ca(SO₄)₂·1.5H₂O
Mohs hardness: 2
Density: 2.42 g/cm³
Colors: White Colorless
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Fibrous, acicular, prismatic
Cleavage: Distinct on (001) and a second direction visible in thin section
Rarity: Rare
Uses: Collector
Host rock: evaporite deposits

Where rockhounds find antofagastaite

Classic worldwide localities

Antofagasta, Chile

Field-hunting tip

Where it forms: evaporite deposits. If you start seeing halite, anhydrite, gypsum in float, you are in the right ground. Typical habit: fibrous, acicular, prismatic. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify antofagastaite?+

Mohs hardness is 2. Luster: vitreous. The streak is white. Common colors include white, colorless.

Where is antofagastaite found?+

Notable localities include Antofagasta, Chile.

What rocks look like antofagastaite?+

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

What minerals are found with antofagastaite?+

Antofagastaite commonly co-occurs with halite, anhydrite, gypsum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does antofagastaite form in?+

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

What is antofagastaite used for?+

Antofagastaite is used in collector.

Find antofagastaite on the map

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