Ferro-actinolite is an iron-rich member of the amphibole group found primarily in metamorphosed iron-rich sedimentary rocks. It often occurs as dark green to black elongated prismatic crystals or in radiating fibrous masses, which require careful handling due to potential asbestiform characteristics.
Is this ferro-actinolite?
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-actinolite with a known reference. Ferro-actinolite 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: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: green, blackish green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals, acicular, fibrous.
Often confused with
Ferro-actinolite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside ferro-actinolite
Minerals reported to co-occur with ferro-actinolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- □Ca₂Mg₂.₅₋₀Fe₂.₅₋₅Si₈O₂₂(OH)₂
- Mohs hardness
- 5-6
- Density
- 3.24-3.48 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic crystals, acicular, fibrous
- Cleavage
- Perfect on {110}, intersecting at 56° and 124°; parting on {100}
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- In iron-rich greenschist and blueschist facies metamorphic rocks; in metamorphosed iron formations; as part of a vein assemblage cutting Ca-Fe-Si tactites
Where rockhounds find ferro-actinolite
Classic worldwide localities
- Jackson County Iron Formation, Wisconsin, and at Cumberland, Providence Co., Rhode Island, USA
- Black Perry Mountain, New South Wales, Australia
- Prieska, Cape Province, South Africa
Field-hunting tip
Where it forms: In iron-rich greenschist and blueschist facies metamorphic rocks; in metamorphosed iron formations; as part of a vein assemblage cutting Ca-Fe-Si tactites. If you start seeing hedenbergite, andradite, ilvaite in float, you are in the right ground. Typical habit: prismatic crystals, acicular, fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ferro-actinolite?+
Where is ferro-actinolite found?+
Is ferro-actinolite safe to handle?+
What rocks look like ferro-actinolite?+
What minerals are found with ferro-actinolite?+
What kind of rock does ferro-actinolite form in?+
What is ferro-actinolite used for?+
Find ferro-actinolite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
