Amphibolite Identification Guide
Identify amphibolite, a dark medium-grade metamorphic rock of hornblende and plagioclase, by its mineralogy, weak foliation, and field setting.
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What Amphibolite Looks Like
Amphibolite is a metamorphic rock dominated by amphibole (usually hornblende) plus plagioclase feldspar. It is dark green to greenish-black or black, often with a salt-and-pepper look where pale feldspar speckles the dark amphibole. Grain size is medium to coarse, so individual mineral grains are visible to the naked eye. Many amphibolites show a weak to moderate foliation or lineation — the stubby black hornblende crystals line up, giving a streaky or combed appearance — but it is far less platy than schist or slate.
Step-by-Step Field-ID Checklist
- Assess overall color — dominantly dark green/black with scattered white-to-gray feldspar.
- Identify hornblende — look for shiny black, elongate, blade- or prism-shaped grains; under a hand lens you may see two cleavages meeting at roughly 56°/124°.
- Identify plagioclase — white to gray, blocky, with flat reflective cleavage faces, sometimes faint striations.
- Check fabric — note any alignment of the dark minerals (foliation/lineation) without strong platy splitting.
- Look for accessory minerals — red garnet, green epidote, or shiny black mica may be present.
Key Diagnostic Tests
- Hardness: the rock as a whole is hard (~5.5–6, dominated by amphibole and feldspar); it scratches glass.
- Hornblende cleavage angle: ~60° and ~120° — diagnostic, distinguishing amphibole from pyroxene (~90°).
- No acid reaction (separates it from carbonate-bearing rocks).
- Density: moderately high (~2.9–3.1) due to abundant iron-magnesium amphibole.
- Generally non-magnetic, though magnetite accessories may give a slight response.
Common Look-Alikes
- Basalt / diabase (igneous): fine-grained and unfoliated; basalt shows no mineral alignment and lacks the coarse, aligned hornblende blades.
- Gabbro: also dark and coarse but igneous and unfoliated, and rich in pyroxene (~90° cleavage) rather than aligned hornblende (~60°/120°).
- Hornblende schist: more strongly foliated and platy, splitting more readily; the boundary with amphibolite is gradational.
- Greenschist: lower-grade, finer, softer, and greener due to chlorite/actinolite rather than coarse hornblende.
Where It Is Found
Amphibolite forms in medium- to high-grade regional metamorphism of mafic igneous rocks (basalt, gabbro) or of certain calcareous sediments. It is common in the cores of mountain belts, ancient shield terranes, and metamorphic complexes worldwide — for example the Appalachians, the Scottish Highlands, the Canadian and Baltic shields, and the Adirondacks. It is widely quarried as a tough crushed-stone aggregate and dimension stone.
Frequently asked questions
How do you identify amphibolite?
Look for a dark green-to-black, medium-to-coarse metamorphic rock made mostly of black hornblende and white-gray plagioclase, often with weak alignment of the dark grains. Hornblende's roughly 60/120-degree cleavage confirms the amphibole.
What is the difference between amphibolite and basalt?
Both are dark, but basalt is a fine-grained volcanic rock with no visible alignment, while amphibolite is a coarser metamorphic rock with visible hornblende crystals that are often aligned into a foliation or lineation.
Amphibolite vs gabbro — how can I tell them apart?
Gabbro is igneous, unfoliated, and rich in pyroxene with about 90-degree cleavage; amphibolite is metamorphic, often shows aligned grains, and is rich in hornblende with 60/120-degree cleavage.
Does amphibolite react with acid?
No. Amphibolite is a silicate rock and does not fizz with dilute hydrochloric acid, which helps separate it from carbonate rocks like marble.
Amphibolite identified by the community
Recent Amphibolite specimens identified with Rock Identifier.
