Postite is a very rare magnesium aluminum sulfate mineral typically found as white, delicate acicular crystals or efflorescent crusts. It is most famous for its occurrence in the Post Mine in Utah, forming due to post-mining sulfate activity on mineralized mine walls.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Postite

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

No image

Is this postite?

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 postite with a known reference. Postite 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. Postite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Postite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, colorless.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: acicular crystals, efflorescent crusts.

Often confused with

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

Often found alongside postite

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

All properties

Chemical formula: Mg(H₂O)₆Al₂(SO₄)₄·16H₂O
Mohs hardness: 2
Density: 1.74 g/cm³
Colors: White Colorless
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Acicular Crystals, Efflorescent Crusts
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Mine Tunnel Walls in Acidic Environments
Typical price: $20-100 per specimen

Where rockhounds find postite

Classic worldwide localities

Post Mine, Utah, USA

Field-hunting tip

Look in mine tunnel walls in acidic environments country — that is the host setting where postite typically forms. If you start seeing alunogen, epsomite, gypsum in float, you are in the right ground. Field specimens usually show a acicular crystals, efflorescent crusts habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify postite?+

Mohs hardness is 2. It typically shows a vitreous luster. The streak is white. Common colors include white, colorless.

Where is postite found?+

Notable localities include Post Mine, Utah, USA.

How much is postite worth?+

Typical asking prices fall in the range of $20-100 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like postite?+

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

What minerals are found with postite?+

Postite commonly co-occurs with alunogen, epsomite, gypsum, pickeringite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does postite form in?+

Postite typically forms in mine tunnel walls in acidic environments. Working float back to the host body is the standard way to chase a fresh occurrence.

What is postite used for?+

Postite is used in collector.

Find postite on the map

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