Grunerite is a cummingtonite-subgroup amphibole known for its association with iron-rich metamorphic rocks. It often presents in fibrous or radiating columnar masses and is most famous for its fibrous variety, amosite, historically used as a commercial form of asbestos.
Is this grunerite?
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 grunerite with a known reference. Grunerite 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: colorless.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous, silky when fibrous.
- 4Match the color rangeCompare against the expected color range: brown, ashen, brownish green, dark gray.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: columnar, fibrous, massive.
Often confused with
Grunerite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Grunerite leaves colorless, Cummingtonite leaves grayish white.
How to tell apart: Streak differs, so Grunerite leaves colorless, Actinolite leaves white.
How to tell apart: Streak differs, so Grunerite leaves colorless, Anthophyllite leaves white or grayish.
Often found alongside grunerite
Minerals reported to co-occur with grunerite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₇Si₈O₂₂(OH)₂
- Mohs hardness
- 5-6
- Density
- 3.40-3.60 g/cm³
- Colors
- Streak
- Colorless
- Luster
- Vitreous, silky when fibrous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Columnar, fibrous, massive
- Cleavage
- Perfect on {110}, with intersections of ~56° and ~124°
- Rarity
- Common
- Uses
- Collector, Scientific study
- Host rock
- Common in medium- to high-grade metamorphosed iron formations
Where rockhounds find grunerite
Classic worldwide localities
- Michigamme, Marquette Co., Michigan, USA
- Loch Duich, Ross-shire, and Glen Beag, Glenelg, Inverness-shire, Scotland
- Botallack mine, St. Just, Cornwall, England
- Vila Real, Portugal
- Strömshult, near Tunaberg, Sweden
- Penge area, Transvaal, South Africa
Field-hunting tip
Where it forms: Common in medium- to high-grade metamorphosed iron formations. If you start seeing magnetite, hematite, hedenbergite in float, you are in the right ground. Typical habit: columnar, fibrous, massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify grunerite?+
Where is grunerite found?+
Is grunerite safe to handle?+
What rocks look like grunerite?+
What minerals are found with grunerite?+
What kind of rock does grunerite form in?+
What is grunerite used for?+
Find grunerite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
