Cummingtonite is a monoclinic amphibole that typically forms fibrous or columnar masses in metamorphic environments. It is most easily identified by its characteristic brownish-grey color and its tendency to occur in radiating or bladed habit. It is found in metamorphosed iron-rich deposits and regional metamorphic rocks.
Is this cummingtonite?
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 cummingtonite with a known reference. Cummingtonite 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: white, black, blackish brown, blackish green, gray.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: columnar, fibrous, granular.
Often confused with
Cummingtonite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Cummingtonite leaves grayish white, Anthophyllite leaves white or grayish.
How to tell apart: Streak differs, so Cummingtonite leaves grayish white, Tremolite leaves white.
How to tell apart: Streak differs, so Cummingtonite leaves grayish white, Actinolite leaves white.
Often found alongside cummingtonite
Minerals reported to co-occur with cummingtonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- □Mg₂(Mg₅)Si₈O₂₂(OH)₂
- Mohs hardness
- 5-6
- Density
- 3.1-3.6 g/cm³
- Streak
- Grayish white
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Columnar, fibrous, granular
- Cleavage
- Good on {110}, intersecting at 54° and 126°
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- Commonly in medium-grade regionally metamorphosed rocks; characteristic of metamorphosed iron formations; a late-stage mineral in some gabbros and norites; rarely in silicic volcanic rocks
Where rockhounds find cummingtonite
1 mapped spotsClassic worldwide localities
- Cummington, Hampshire Co., Massachusetts, USA
- Homestake mine, Lead, Pennington Co., South Dakota, USA
- Jackson County Iron Formation, Wisconsin, USA
- Wabush Iron Formation, Labrador City, Labrador, Newfoundland, Canada
- Miyamori district, Iwate Prefecture, and the Hitachi mine, Ibaragi Prefecture, Japan
- Garabal Hill-Glen Fyne complex, Scotland
Field-hunting tip
Where it forms: Commonly in medium-grade regionally metamorphosed rocks; characteristic of metamorphosed iron formations; a late-stage mineral in some gabbros and norites; rarely in silicic volcanic rocks. If you start seeing hornblende, anthophyllite, actinolite in float, you are in the right ground. Typical habit: columnar, fibrous, granular. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Wisconsin. Start trip planning there.
Common questions
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Find cummingtonite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
