Oneillite is a complex silicate mineral primarily occurring in alkaline igneous complexes. It forms as thin, brittle tabular crystals within pegmatite pockets, typically found alongside other rare zirconium and titanium minerals.

Hardness

5-6

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent to translucent

mineralrareLomonosovite Group family

Oneillite

Hardness: 5-6
Luster: Vitreous
Streak: White
Transparency: Transparent to translucent

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Is this oneillite?

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 oneillite with a known reference. Oneillite sits at Mohs 5-6, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: yellowish brown.
5
Look at form & habit
Crystal system: trigonal. Typical habit: anhedral grains.

Often confused with

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

mineral

Lomonosovite

Mohs 3-4

How to tell apart: Oneillite is noticeably harder (Mohs 5-6 vs. 3-4); streak differs, so Oneillite leaves white, Lomonosovite leaves light rose-cinnamon.

How to tell apart: Oneillite is noticeably harder (Mohs 5-6 vs. 2-3).

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Often found alongside oneillite

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

All properties

Chemical formula: Na₁₅Ca₃Mn₃Fe₃Zr₃Nb(Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl)₂
Mohs hardness: 5-6
Density: 3.20 g/cm³
Colors: Yellowish brown
Streak: White
Luster: Vitreous
Transparency: Transparent to translucent
Crystal system: Trigonal
Crystal habit: Anhedral grains
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Along the contact between a vein of albite and its host nepheline syenite in an alkaline intrusive complex

Where rockhounds find oneillite

Classic worldwide localities

Poudrette quarry, Mont Saint-Hilaire, Quebec, Canada

Field-hunting tip

Where it forms: Along the contact between a vein of albite and its host nepheline syenite in an alkaline intrusive complex. If you start seeing albite, sodalite, pyrite in float, you are in the right ground. Typical habit: anhedral grains. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify oneillite?+

Mohs hardness is 5-6. Luster: vitreous. The streak is white. Common colors include yellowish brown.

Where is oneillite found?+

Notable localities include Poudrette quarry, Mont Saint-Hilaire, Quebec, Canada.

What rocks look like oneillite?+

Oneillite is most often confused with Lomonosovite, Murmanite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with oneillite?+

Oneillite commonly co-occurs with Albite, Sodalite, Pyrite, Aegirine. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does oneillite form in?+

Where it forms: Along the contact between a vein of albite and its host nepheline syenite in an alkaline intrusive complex. Working float back to the host body is the standard way to chase a fresh occurrence.

What is oneillite used for?+

Oneillite is used in collector.

Find oneillite on the map

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