Tachylite Identification Guide
Identifying tachylite, a black basaltic volcanic glass, by its dark color, glassy fracture, association with basalt, and how to distinguish it from obsidian.
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What Tachylite Looks Like
Tachylite (also spelled tachylyte) is a basaltic volcanic glass, the rapidly quenched, mafic counterpart to obsidian. It is typically black to dark brown or greenish-black, with a glassy to resinous or pitchy (subvitreous) luster rather than the bright glass shine of obsidian. Because it forms from iron- and magnesium-rich basaltic magma, it is opaque even in thin chips and often looks duller and more pitch-like. It occurs as chilled margins of basalt flows, dikes, and pillow lavas, and may contain tiny crystals (microlites) giving it a slightly textured look.
Key Visual Cues
- Black to dark brown or greenish-black
- Glassy to pitchy/resinous, subvitreous luster
- Opaque even in thin pieces
- Found as chilled margins of basalt flows and dikes
Step-by-Step Field ID Checklist
- Check the color and luster. Dark, dull-to-pitchy glassy material in a basaltic setting suggests tachylite.
- Look for conchoidal fracture. Like all glass, it breaks with smooth, curved, sharp-edged surfaces.
- Test hardness. It is roughly Mohs 5 to 6; it may just scratch glass and is scratched by quartz.
- Check the streak. Grey to brownish streak.
- Note the geologic context. It occurs at the margins of basalt flows, dikes, and pillow lavas.
- Assess opacity. Tachylite stays opaque in thin chips, unlike many obsidians.
Diagnostic Tests
- Mohs hardness: ~5 to 6.
- Streak: grey to brown.
- Fracture: conchoidal, glassy, with sharp edges.
- Specific gravity: ~2.7 to 2.9, slightly higher than obsidian due to iron-rich composition.
- Composition: basaltic (mafic) glass, the key distinction from rhyolitic obsidian.
- Acid: inert to hydrochloric acid.
Common Look-Alikes and How to Tell Them Apart
- Obsidian: obsidian is rhyolitic (felsic) glass, usually more brightly glassy, often translucent at thin edges, and slightly less dense (~2.35 to 2.6). Tachylite is basaltic, duller/pitchier, opaque, and denser; geologic context (basalt vs rhyolite) is decisive.
- Basalt (crystalline): ordinary basalt is fine-grained but crystalline, not glassy, and lacks conchoidal glassy fracture; tachylite is glass.
- Pitchstone: pitchstone is a duller volcanic glass with higher water content and a resinous look but is generally more felsic; composition and context separate them.
- Black chert/flint: chert is harder (7), opaque microcrystalline quartz, and not associated with fresh lava margins.
- Slag (man-made glass): slag often has gas bubbles, frothy texture, and unnatural colors; tachylite occurs in a natural volcanic context.
Where Tachylite Is Found
Tachylite forms wherever basaltic magma is quenched quickly, especially the chilled glassy margins of basalt dikes, sills, lava flows, and pillow lavas erupted underwater. It is common in basaltic volcanic provinces worldwide, including Hawaii, Iceland, the Hebrides and other parts of Scotland, and many oceanic and continental flood-basalt regions. Look for it as a thin dark glassy selvage at the contact between a basalt body and the rock it intruded or flowed over.
Frequently asked questions
How can you tell if it's tachylite?
Tachylite is a dark, opaque, glassy-to-pitchy basaltic glass that breaks with conchoidal fracture, has a hardness around 5 to 6, and occurs at the chilled margins of basalt flows and dikes. Its basaltic composition, dull pitchy luster, opacity, and slightly higher density distinguish it from obsidian.
What does tachylite look like?
It looks like a black to dark brown or greenish-black volcanic glass with a pitchy, resinous, or subvitreous luster, opaque even in thin chips, typically forming a glassy rind on basalt.
Tachylite vs obsidian, what is the difference?
Both are volcanic glass, but obsidian is rhyolitic (felsic), brighter, often translucent at the edges, and a bit less dense, while tachylite is basaltic (mafic), duller and pitchier, opaque, denser, and found with basalt rather than rhyolite.
Is tachylite the same as basalt?
No. Basalt is a crystalline fine-grained volcanic rock, while tachylite is the glassy, non-crystalline form of basaltic magma created when it cools too fast to crystallize, typically along chilled flow and dike margins.
Where is tachylite found?
It forms at the rapidly quenched margins of basalt flows, dikes, sills, and pillow lavas in basaltic volcanic regions such as Hawaii, Iceland, and Scotland.