Sideromelane Identification Guide

What Sideromelane Looks Like

Sideromelane is a pure, transparent variety of basaltic (mafic) volcanic glass. Unlike most basaltic glass, it lacks the tiny iron-oxide crystallites that darken its cousin tachylite, so it appears clear.

• Color: honey-brown, yellow-brown, pale amber, sometimes greenish — translucent to transparent in thin fragments
• Luster: vitreous (glassy)
• Habit: angular glass shards, droplets, and grains; commonly found as the glassy fragments in hyaloclastite and tuff
• Setting clue: frequently rimmed or partly altered to yellow-orange palagonite

Step-by-Step Field-ID Checklist

1. Check transparency in thin chips. Sideromelane transmits light with a warm brown tint — true tachylite is opaque black.
2. Look for glassy conchoidal fracture. Smooth, curved, shell-like breaks with sharp edges confirm glass.
3. Examine the setting. It usually occurs as shards within fragmental volcanic deposits (hyaloclastite) formed where lava met water or ice.
4. Look for palagonite alteration. A yellow-brown, waxy or clay-like rind on the glass is a strong indicator.
5. Hardness check. It scratches glass marginally (~5–6) and feels like a tough natural glass.

Key Diagnostic Tests

• Mohs hardness: about 5–6.
• Streak: white to pale (glass).
• Cleavage/fracture: none; conchoidal fracture.
• Specific gravity: roughly 2.7–2.9 — denser than felsic obsidian.
• Composition: basaltic (low silica), which is why it is darker/browner and slightly denser than rhyolitic obsidian.
• Acid: no fizz.

Common Look-Alikes and How to Tell Them Apart

• Tachylite: the opaque, black, crystallite-rich basaltic glass — the same composition but cloudy black, whereas sideromelane is clear honey-brown. Transparency in thin chips is the separator.
• Obsidian: obsidian is felsic (rhyolitic) glass, usually deep black and glossy, with higher silica and slightly lower density; sideromelane is browner, found in water-lava deposits, and basaltic.
• Amber: amber is organic, very soft (2–2.5), warm to the touch, and floats in saltwater; sideromelane is much harder and denser.
• Palagonite: this is the altered product of sideromelane — dull, waxy, yellow-orange and clay-like rather than glassy; often coexists on the same sample.
• Brown bottle glass: man-made glass has bubbles, mold marks, and no volcanic deposit context.

The key combination is transparent honey-brown glass + conchoidal fracture + basaltic/hyaloclastite setting + palagonite rinds.

Where Sideromelane Is Found

Sideromelane forms during hydrovolcanic (phreatomagmatic) and subglacial eruptions where hot basaltic lava is quenched rapidly by water or ice. Classic occurrences are in Iceland (subglacial volcanoes and tuyas), Hawaii (littoral and submarine eruptions), and other oceanic and ice-covered volcanic regions, typically within hyaloclastite, palagonite tuff, and pillow-lava breccias.