Hornblende Schist Identification Guide
Identify hornblende schist by its dark foliated fabric, aligned shiny amphibole crystals, and medium-grade metamorphic origin.
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What Hornblende Schist Looks Like
Hornblende schist is a medium-grade metamorphic rock dominated by aligned crystals of hornblende, a dark amphibole, giving it a foliated (schistose) fabric. It typically forms from the metamorphism of basalt, gabbro, or other mafic rocks.
- Color: dark green to black, often with lighter plagioclase streaks.
- Luster: the hornblende grains are vitreous and shiny/reflective, giving the foliation surfaces a sparkle.
- Texture: schistose — visibly foliated with parallel, elongated, bladed amphibole crystals you can usually see with the naked eye.
- Habit: medium- to coarse-grained; may show wavy or planar foliation, sometimes with garnet porphyroblasts.
Field-ID Checklist
- Confirm a dark, foliated rock that splits or aligns along planes of shiny bladed crystals.
- Identify hornblende: black-green elongated prisms with two cleavages meeting at ~56 and 124 degrees.
- Check that mica is NOT dominant — schists rich in flaky mica are mica schist, not hornblende schist.
- Note any garnet, plagioclase, or quartz segregations.
- Test hardness — hornblende is 5–6 (scratches glass with effort, harder than a knife is borderline).
Key Diagnostic Tests
- Mohs hardness: hornblende ~5–6.
- Cleavage: amphibole two-direction cleavage at ~56/124 degrees (key to separating it from pyroxene at ~90 degrees).
- Streak: pale gray to gray-brown/white.
- Foliation: schistose alignment of platy/bladed minerals is diagnostic of a schist.
- Density: relatively heavy (~2.9–3.2) due to mafic mineralogy.
- Acid/magnetism: non-effervescent; generally non-magnetic unless magnetite is present.
Common Look-Alikes
- Amphibolite: same mineralogy (hornblende + plagioclase) but massive/weakly foliated rather than strongly schistose; hornblende schist is more obviously layered/aligned.
- Mica schist: also foliated but dominated by flaky, soft, splitting mica rather than hard bladed hornblende.
- Biotite gneiss: has banded light/dark layers (gneissic banding) rather than pervasive schistosity.
- Pyroxenite/dark igneous rock: lacks foliation and shows ~90-degree pyroxene cleavage instead of amphibole's 56/124.
- Slate/phyllite: much finer-grained, with dull-to-silky sheen and no visible bladed crystals.
The fingerprint: dark, schistose, shiny, hornblende-dominated rock with amphibole's diagnostic cleavage angles.
Where It Is Found
Hornblende schist occurs in regionally metamorphosed terranes worldwide — mountain belts and shield areas such as the Appalachians, Scottish Highlands, Scandinavian and Canadian Shields, and the Alps — wherever mafic protoliths were metamorphosed at medium grade.
Frequently asked questions
How can you tell if a rock is hornblende schist?
Look for a dark green-to-black, foliated rock whose shiny surfaces are made of aligned bladed hornblende crystals. Confirm hornblende by its two cleavages meeting at about 56 and 124 degrees and a hardness near 5–6. The strong schistose (layered, aligned) fabric distinguishes it from massive amphibolite.
What does hornblende schist look like?
It looks like a dark, sparkly, layered rock with parallel rows of elongated black-green amphibole crystals, sometimes streaked with lighter feldspar and dotted with garnet.
Hornblende schist vs amphibolite — what is the difference?
They share the same minerals (hornblende and plagioclase), but hornblende schist is strongly foliated and aligned, while amphibolite is more massive with little or no foliation.
How is hornblende schist different from mica schist?
Hornblende schist is built of hard, bladed dark amphibole, whereas mica schist is dominated by soft, flaky, easily split mica that gives a much more lustrous, sheety surface.