Macaulayite is a rare, brick-red iron silicate mineral discovered in the weathered granite of Scotland. It typically occurs as fine, platy aggregates that are often found associated with kaolinite and hematite. Due to its extreme rarity and very small grain size, it is primarily a target for specialized mineral collectors.
Is this macaulayite?
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 macaulayite with a known reference. Macaulayite sits at Mohs 2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: light red.
- 3Read the lusterHold the specimen under a strong light. Luster: earthy.
- 4Match the color rangeCompare against the expected color range: blood red.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: earthy.
Often confused with
Macaulayite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Hematite is the harder of the two (Mohs 5-6 vs. 2); streak differs, so Macaulayite leaves light red, Hematite leaves cherry-red or reddish brown; luster reads earthy on Macaulayite and metallic or submetallic to dull on Hematite.
How to tell apart: Goethite is the harder of the two (Mohs 5-5.5 vs. 2); streak differs, so Macaulayite leaves light red, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads earthy on Macaulayite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
Often found alongside macaulayite
Minerals reported to co-occur with macaulayite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₂₄Si₄O₄₃(OH)₂
- Mohs hardness
- 2
- Density
- 4.41 g/cm³
- Colors
- Blood red
- Streak
- Light red
- Luster
- Earthy
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Earthy
- Cleavage
- {001}, strongly suspected
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A secondary mineral formed in deeply weathered granite
Where rockhounds find macaulayite
Classic worldwide localities
- Bennachie, near Inverurie, Aberdeenshire, Scotland
Field-hunting tip
Where it forms: A secondary mineral formed in deeply weathered granite. If you start seeing kaolinite, illite in float, you are in the right ground. Typical habit: earthy. Train your eye for that shape before scanning the outcrop.
Common questions
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Find macaulayite on the map
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