Mejillonesite is a very rare phosphate mineral typically found as fibrous, yellow 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 3 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Mejillonesite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Mejillonesite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: yellow, yellowish-brown.
- 5Look at form & habitCrystal system: monoclinic. 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 3 vs. 2); luster reads dull on Mejillonesite and pearly on Bobierrite.
How to tell apart: Mejillonesite is noticeably harder (Mohs 3 vs. 1.5-2); streak differs — Mejillonesite leaves white, Vivianite leaves white to light blue; luster reads dull on Mejillonesite and vitreous on Vivianite.
All properties
- Chemical formula
- Na₂Mg₂(OH)₂(PO₄)₂·4H₂O
- Mohs hardness
- 3
- Density
- 2.8-2.9 g/cm³
- Colors
- Streak
- White
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous Aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Phosphate Deposits in Guano Layers
- Typical price
- n/a (rare research mineral)
Where rockhounds find mejillonesite
Classic worldwide localities
- Mejillones, Antofagasta, Chile
Field-hunting tip
Look in phosphate deposits in guano layers country — that is the host setting where mejillonesite typically forms. If you start seeing guano-derived minerals in float, you are in the right ground. Field specimens usually show a fibrous aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find mejillonesite on the map
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