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 5-5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: brown with a reddish tint.
- 3Read the lusterHold the specimen under a strong light. Luster: silky to subadamantine.
- 4Match the color rangeCompare against the expected color range: brown, maroon, reddish brown, pink.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular, aggregates, twinning common.
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, weak pearly on {001} 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, rarely subresinous on fracture surface 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(SiO₄)(Si₂O₇)(OH)₃
- Mohs hardness
- 5-5.5
- Density
- 3.43 g/cm³
- Colors
- Streak
- Brown with a reddish tint
- Luster
- Silky to subadamantine
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular, aggregates, twinning common
- Cleavage
- Perfect on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In abundance, replacing calcite in fissures and in lenses in basalt (Manganese Lake, Michigan, USA); replacing braunite under low-temperature metamorphic conditions (Faggiona, Italy)
Where rockhounds find macfallite
Classic worldwide localities
- Manganese Lake, Copper Harbor, Keweenaw Co., Michigan, USA
- Cerchiara mine, Faggiona, La Spezia, Liguria, Italy
Field-hunting tip
Where it forms: In abundance, replacing calcite in fissures and in lenses in basalt (Manganese Lake, Michigan, USA); replacing braunite under low-temperature metamorphic conditions (Faggiona, Italy). If you start seeing manganite, braunite, orientite in float, you are in the right ground. Typical habit: acicular, aggregates, twinning common. 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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