Mejillonesite is a very rare phosphate mineral typically found as fibrous, colorless crusts or powdery aggregates in arid environments. It occurs primarily in guano-rich phosphate deposits, specifically identified at its type locality in Mejillones, Chile. Because it is highly localized and rare, it is primarily of interest to systematic mineral collectors and researchers.
Is this mejillonesite?
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 mejillonesite with a known reference. Mejillonesite sits at Mohs 4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous aggregates.
Often confused with
Mejillonesite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Mejillonesite is noticeably harder (Mohs 4 vs. 2-2.5); luster reads vitreous on Mejillonesite and nearly vitreous to pearly on cleavages on Bobierrite.
How to tell apart: Mejillonesite is noticeably harder (Mohs 4 vs. 1.5-2).
Often found alongside mejillonesite
Minerals reported to co-occur with mejillonesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaMg₂(PO₃OH)(PO₄)(OH)₄·H₂O
- Mohs hardness
- 4
- Density
- 2.36 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous aggregates
- Cleavage
- Perfect on {100}, good on {010} and {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Formed at the contact between granite and what is now a fine-grained, completely altered siliceous layer at the base of a Pliocene (ca. 2.5 Ma) coastal bird guano deposit
Where rockhounds find mejillonesite
Classic worldwide localities
- Mejillones, Antofagasta, Chile
Field-hunting tip
Where it forms: Formed at the contact between granite and what is now a fine-grained, completely altered siliceous layer at the base of a Pliocene (ca. 2.5 Ma) coastal bird guano deposit. If you start seeing bobierrite, opal, clinoptilolite-na in float, you are in the right ground. Typical habit: fibrous aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify mejillonesite?+
Where is mejillonesite found?+
What rocks look like mejillonesite?+
What minerals are found with mejillonesite?+
What kind of rock does mejillonesite form in?+
What is mejillonesite used for?+
Find mejillonesite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
