Eggletonite is a rare manganese silicate mineral that typically appears as brown to reddish-brown tabular crystals or massive grains. It is structurally related to the pyrosmalite group and is most famous for its occurrences in specific alkaline igneous environments. Collectors prize it primarily for its distinct crystal habits and association with rare-earth mineral assemblages.

Hardness

3-4

Mohs

Luster

Vitreous

Streak

Light brown

Transparency

Semitransparent

mineralrarePyrosmalite Group family

Eggletonite

Hardness: 3-4
Luster: Vitreous
Streak: Light brown
Transparency: Semitransparent

Is this eggletonite?

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 eggletonite with a known reference. Eggletonite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: light brown.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: golden brown.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: acicular, prismatic, radial.

Often confused with

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

mineral

Friedelite

Mohs 4-5

How to tell apart: Streak differs, so Eggletonite leaves light brown, Friedelite leaves pale rose.

Compare →

Often found alongside eggletonite

Minerals reported to co-occur with eggletonite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: (Na,K,Ca)₂Mn₆(Si,Al)₁₀O₂₄(OH)₄·7-11H₂O
Mohs hardness: 3-4
Density: 2.76 g/cm³
Colors: Golden brown
Streak: Light brown
Luster: Vitreous
Transparency: Semitransparent
Crystal system: Monoclinic
Crystal habit: Acicular, prismatic, radial
Cleavage: Perfect on {001}
Rarity: Rare
Uses: Collector
Host rock: In miarolitic pockets in nepheline syenite pegmatite

Where rockhounds find eggletonite

Classic worldwide localities

Big Rock quarry, Granite Mountain, near Little Rock, Pulaski Co., Arkansas, USA

Field-hunting tip

Where it forms: In miarolitic pockets in nepheline syenite pegmatite. If you start seeing albite, natrolite, apophyllite in float, you are in the right ground. Typical habit: acicular, prismatic, radial. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify eggletonite?+

Mohs hardness is 3-4. Luster: vitreous. The streak is light brown. Common colors include golden brown.

Where is eggletonite found?+

Notable localities include Big Rock quarry, Granite Mountain, near Little Rock, Pulaski Co., Arkansas, USA.

What rocks look like eggletonite?+

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

What minerals are found with eggletonite?+

Eggletonite commonly co-occurs with Albite, Natrolite, Apophyllite, Pyrophanite, Kupletskite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does eggletonite form in?+

Where it forms: In miarolitic pockets in nepheline syenite pegmatite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is eggletonite used for?+

Eggletonite is used in collector.

Find eggletonite on the map

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