Postite is a very rare magnesium aluminum sulfate mineral typically found as golden yellow, 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.
Is this postite?
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 postite with a known reference. Postite 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. Postite leaves a yellow streak.
- 3Read the lusterHold the specimen under a strong light. Postite typically shows a subadamantine luster.
- 4Match the color rangeCompare against the expected color range: golden yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular crystals, efflorescent crusts.
Often confused with
Postite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Postite leaves yellow, Epsomite leaves white; luster reads subadamantine on Postite and vitreous on Epsomite.
How to tell apart: Streak differs, so Postite leaves yellow, Halotrichite leaves white; luster reads subadamantine on Postite and vitreous on Halotrichite.
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
- 2.23 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Subadamantine
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular Crystals, Efflorescent Crusts
- Cleavage
- Perfect ‖ [001] Likely On {100} and {010}, Good On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Mine Tunnel Walls in Acidic Environments
Where rockhounds find postite
Classic worldwide localities
- Post Mine, Utah, USA
Field-hunting tip
Look in mine tunnel walls in acidic environments country, 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?+
Where is postite found?+
What rocks look like postite?+
What minerals are found with postite?+
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Find postite on the map
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