Ferro-hornblende is a complex iron-rich amphibole mineral often found in igneous and metamorphic terrains. Collectors identify it by its characteristic prismatic crystal habit and perfect cleavage angles, which distinguish it from pyroxenes like augite.
Is this ferro-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 ferro-hornblende with a known reference. Ferro-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. Streak: grayish white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: green, greenish brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals, massive, fibrous.
Often confused with
Ferro-hornblende vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Ferro-hornblende leaves grayish white, Actinolite leaves white.
How to tell apart: Streak differs, so Ferro-hornblende leaves grayish white, Arfvedsonite leaves deep bluish gray, gray-green.
How to tell apart: Streak differs, so Ferro-hornblende leaves grayish white, Augite leaves gray-green.
Often found alongside ferro-hornblende
Minerals reported to co-occur with ferro-hornblende. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- □Ca₂(Fe₄Al)(Si₇Al)O₂₂(OH)₂
- Mohs hardness
- 5-6
- Density
- 3.12-3.30 g/cm³
- Colors
- Streak
- Grayish white
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic crystals, massive, fibrous
- Cleavage
- Perfect on {110}, with intersections at 56° and 124°; may show partings on {100}, {010}
- Rarity
- Common
- Uses
- Collector, Petrological research
- Host rock
- From granites, granodiorites, and metabasalts; common in amphibolites and schists
Where rockhounds find ferro-hornblende
Classic worldwide localities
- Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- Edwards, Pierrepont, and Gouverneur, St. Lawrence Co., New York, USA
- Monte Somma and Vesuvius, Campania, Italy
- Pargas, Finland
- Kragerö, Arendal, and around the Langesundsfjord, Norway
- Bancroft, Pakenham, and Eganville, Ontario, Canada
Field-hunting tip
Where it forms: From granites, granodiorites, and metabasalts; common in amphibolites and schists. If you start seeing hedenbergite, biotite, epidote in float, you are in the right ground. Typical habit: prismatic crystals, massive, fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ferro-hornblende?+
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What rocks look like ferro-hornblende?+
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Find ferro-hornblende on the map
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