Macfallite is a rare manganese silicate mineral primarily found in the Keweenaw Peninsula of Michigan. It typically occurs as small, radiating sprays of acicular crystals or fibrous aggregates within cavities of basaltic lava. Collectors often look for its distinctive brownish color associated with other copper-region secondary minerals.
Is this macfallite?
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 macfallite with a known reference. Macfallite sits at Mohs 5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Macfallite leaves a brown with a reddish tint streak.
- 3Read the lusterHold the specimen under a strong light. Macfallite typically shows a silky to subadamantine luster.
- 4Match the color rangeCompare against the expected color range: brown, reddish-brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular crystals, radiating sprays, radial fibrous aggregates.
Often confused with
Macfallite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Macfallite leaves brown with a reddish tint, Prehnite leaves white; luster reads silky to subadamantine on Macfallite and vitreous on Prehnite.
How to tell apart: Streak differs, so Macfallite leaves brown with a reddish tint, Datolite leaves white; luster reads silky to subadamantine on Macfallite and vitreous on Datolite.
Often found alongside macfallite
Minerals reported to co-occur with macfallite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂Mn³⁺₃Si₃O₁₀(OH)₃
- Mohs hardness
- 5.5
- Density
- 3.37 g/cm³
- Colors
- Streak
- Brown with A Reddish Tint
- Luster
- Silky to Subadamantine
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular Crystals, Radiating Sprays, Radial Fibrous Aggregates
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Hydrothermal Veins in Basaltic Lavas
Where rockhounds find macfallite
Classic worldwide localities
- Copper Falls Mine, Michigan, USA
- Clark Mine, Michigan, USA
- Phoenix Mine, Michigan, USA
Field-hunting tip
Look in hydrothermal veins in basaltic lavas country, the host setting where macfallite typically forms. If you start seeing pumpellyite, datolite, prehnite in float, you are in the right ground. Field specimens usually show a acicular crystals, radiating sprays, radial fibrous aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find macfallite on the map
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