LithoMap

Rock identification comparison

Amphibolite vs. gabbro: how to tell them apart

Amphibolite and gabbro are dark crystalline rocks that can share plagioclase and mafic minerals. Amphibolite is metamorphic and commonly contains aligned elongate amphibole; gabbro is igneous with a generally random interlocking texture dominated by pyroxene and plagioclase.

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Dark fine-grained amphibolite hand specimen composed mostly of hornblende and plagioclase with minor biotite
AmphiboliteAmphibole-rich metamorphic rockPhoto: Nessa Eull · CC0 1.0
Dark-gray medium-grained gabbro with pale plagioclase and dark mafic minerals
GabbroMafic intrusive igneous rockPhoto: Mike Davis · CC0 1.0
Quick answer

The most useful distinction

Look for fabric and mineral shape. Persistent alignment of shiny elongate amphibole favors amphibolite; randomly oriented blocky pyroxene and plagioclase favor gabbro.

Side-by-side evidence

Amphibolite and Gabbro comparison table

PropertyAmphiboliteGabbroHow to use it
OriginMetamorphic recrystallization, commonly of mafic rockSlow crystallization of mafic magma undergroundTexture records the contrasting process.
FabricMay be foliated or lineated, though some is massiveUsually massive with random crystal orientation; igneous layering can occurA pervasive metamorphic alignment is strong evidence.
Dominant dark mineralAmphibole, commonly hornblendePyroxene, commonly with olivine or amphibole alterationCrystal habit helps where grains are large enough.
Pale mineralPlagioclase grains or patchesInterlocking plagioclasePlagioclase occurs in both and cannot distinguish them alone.
Field sequence

A practical identification sequence

  1. 1

    Examine a fresh surface with a hand lens.

  2. 2

    Look for a repeated direction in elongate dark grains.

  3. 3

    Compare prismatic amphibole with blockier pyroxene cautiously.

  4. 4

    Use contacts and regional mapping to test metamorphic versus intrusive context.

Limits and cautions

Where the distinction gets difficult

  • Massive amphibolite and altered gabbro may require thin-section identification.
  • Igneous layering is not automatically metamorphic foliation.
  • Dark color and density alone cannot separate the rocks.

Use several independent observations. This comparison supports preliminary field identification, not laboratory confirmation, appraisal, proof of origin, or collecting permission.

Common questions

Amphibolite vs. gabbro FAQ

Can gabbro become amphibolite?

Yes. Metamorphism can recrystallize gabbro into amphibolite, although basalt and other mafic rocks are also common precursors.

Is all amphibolite foliated?

No. Amphibolite ranges from strongly foliated to nearly massive.

Can gabbro contain hornblende?

Yes, as a primary or alteration mineral. Mineral abundance, fabric, and field context must be evaluated together.

Source basis

Authoritative sources

LithoMap wrote this comparison in original language from the physical properties and geological concepts supported by these public scientific references.

Editorial review: September 17, 2026.

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