Palagonite Identification Guide

What Palagonite Looks Like

Palagonite is not a single mineral but an alteration product - a yellow, orange-brown, to greenish-brown waxy or earthy material formed when basaltic volcanic glass (sideromelane) reacts with water. It is most common in hyaloclastite and palagonite tuff, where glassy ash fragments are cemented and rimmed by this altered material. It ranges from a translucent honey-yellow "gel" palagonite to a dull, opaque, clay-rich "fibrous" palagonite.

• Color: yellow, amber, orange-brown, olive to dark brown
• Transparency: translucent (gel type) to opaque (fibrous type)
• Luster: waxy, resinous, to dull/earthy
• Habit: rinds and matrix around glassy basalt fragments in tuff/breccia

Step-by-Step Field ID Checklist

1. Look at the host rock. Palagonite occurs in fragmental volcanic rock (hyaloclastite/tuff) with angular glassy clasts.
2. Spot the rinds. Glass shards often have yellow-brown altered margins.
3. Check softness. Palagonite is soft - it can be scratched by a knife or even a fingernail in clay-rich form.
4. Feel the texture. Earthy, sometimes crumbly; gel type is waxy.
5. Note water absorption. Earthy palagonite may stick to the tongue (clay-like).
6. Examine color zoning from fresh dark glass to yellow alteration.

Key Diagnostic Tests

• Mohs hardness: low and variable, roughly 2-5; commonly knife-scratchable.
• Streak: yellow-brown.
• Fracture: earthy to conchoidal in glassy parts; no true cleavage.
• Density: lower than fresh basalt due to hydration and porosity.
• Reaction: absorbs water; clay-rich forms feel sticky/plastic when wet.

Common Look-Alikes and How to Tell Them Apart

• Fresh basalt glass (sideromelane/tachylite): darker, harder, glassy; palagonite is the yellow-brown altered rim around it.
• Limonite/goethite: iron oxides are harder and not glass-derived; palagonite occurs specifically with volcanic glass clasts.
• Bole/clay alteration: general clay lacks the association with glassy volcanic shards.
• Bentonite tuff: white-to-pale altered ash, not the amber-brown of palagonite.

Where It Is Found

Palagonite is abundant where basaltic lava erupted into water or ice - Iceland (the type region, the name comes from Palagonia, Sicily), Hawaii, oceanic islands, and subglacial volcanic complexes. It forms in hyaloclastite ridges, tuff cones, and pillow-lava breccias.