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.

Hardness

Mohs

Luster

Earthy

Streak

Reddish-brown

Transparency

Opaque

mineralrare

Macaulayite

Hardness: 2
Luster: Earthy
Streak: Reddish-brown
Transparency: Opaque

Is this macaulayite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch macaulayite with a known reference. Macaulayite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Macaulayite leaves a reddish-brown streak.
3
Read the luster
Hold the specimen under a strong light. Macaulayite typically shows a earthy luster.
4
Match the color range
Compare against the expected color range: brick-red.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: platy aggregates.

Often confused with

Macaulayite vs. its common look-alikes — and how to tell them apart in the field.

mineral

Iron Ore

Mohs 5-6.5

How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 2); streak differs — Macaulayite leaves reddish-brown, Iron Ore leaves reddish-brown to black; luster reads earthy on Macaulayite and metallic to submetallic on Iron Ore.

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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: Fe₈Si₈O₂₀(OH)₈
Mohs hardness: 2
Density: 2.8 g/cm³
Colors: Brick-red
Streak: Reddish-brown
Luster: Earthy
Transparency: Opaque
Crystal system: Monoclinic
Crystal habit: Platy Aggregates
Cleavage: Perfect
Rarity: Rare
Uses: Collector
Host rock: Weathered Granite
Typical price: $50-300 per specimen

Where rockhounds find macaulayite

Classic worldwide localities

Macaulay, Scotland

Field-hunting tip

Look in weathered granite country — that is the host setting where macaulayite typically forms. If you start seeing kaolinite, hematite, quartz in float, you are in the right ground. Field specimens usually show a platy aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify macaulayite?+

Mohs hardness is 2. It typically shows a earthy luster. The streak is reddish-brown. Common colors include brick-red.

Where is macaulayite found?+

Notable localities include Macaulay, Scotland.

How much is macaulayite worth?+

Typical asking prices fall in the range of $50-300 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like macaulayite?+

Macaulayite is most often confused with Iron Ore. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with macaulayite?+

Macaulayite commonly co-occurs with kaolinite, hematite, quartz. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does macaulayite form in?+

Macaulayite typically forms in weathered granite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is macaulayite used for?+

Macaulayite is used in collector.

Find macaulayite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.