Magnesio-hornblende is a common member of the amphibole group, typically occurring as dark, elongated prismatic crystals in igneous and metamorphic rocks. It is most easily identified in the field by its distinct cleavage angles and vitreous luster. It is a major constituent of many common rock types but is usually valued by collectors for its well-formed, terminated crystals.
Is this magnesio-hornblende?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch magnesio-hornblende with a known reference. Magnesio-hornblende sits at Mohs 5-6 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Magnesio-hornblende leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Magnesio-hornblende typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: dark green, brownish-green, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals, elongated, bladed, massive.
Often confused with
Magnesio-hornblende vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Luster reads vitreous on Magnesio-hornblende and vitreous to silky on Actinolite.
How to tell apart: Streak differs — Magnesio-hornblende leaves white, Augite leaves grayish white.
How to tell apart: Streak differs — Magnesio-hornblende leaves white, Ferro-hornblende leaves grayish white.
Often found alongside magnesio-hornblende
Minerals reported to co-occur with magnesio-hornblende. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂(Mg₄Al)(Si₇Al)O₂₂(OH)₂
- Mohs hardness
- 5-6
- Density
- 3.1-3.3 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic Crystals, Elongated, Bladed, Massive
- Cleavage
- Perfect in Two Directions At 56 and 124 Degrees
- Rarity
- Common
- Uses
- Collector, Petrology Study
- Host rock
- Igneous and Metamorphic Rocks Like Granite, Diorite, And Amphibolite
- Typical price
- $5-30 for typical crystal specimens
Where rockhounds find magnesio-hornblende
Classic worldwide localities
- Vesuvius, Italy
- Franklin, New Jersey, USA
- Norwegian Highlands, Norway
- Ontario, Canada
Field-hunting tip
Look in igneous and metamorphic rocks like granite, diorite, and amphibolite country — that is the host setting where magnesio-hornblende typically forms. If you start seeing quartz, plagioclase, biotite in float, you are in the right ground. Field specimens usually show a prismatic crystals, elongated, bladed, massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find magnesio-hornblende on the map
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