Monazite-(Sm) is a very rare rare-earth phosphate mineral belonging to the monazite group. It typically occurs as small crystals in complex pegmatites and is highly prized by collectors for its chemical specificity, despite being mildly radioactive due to thorium impurities.

Hardness

5-5.5

Mohs

Luster

Vitreous to greasy

Streak

White

Transparency

Transparent to translucent

mineralrareMonazite Group family

Monazite-(Sm)

Hardness: 5-5.5
Luster: Vitreous to greasy
Streak: White
Transparency: Transparent to translucent

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Is this monazite-(sm)?

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 monazite-(sm) with a known reference. Monazite-(Sm) sits at Mohs 5-5.5, 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 to greasy.
4
Match the color range
Compare against the expected color range: yellowish.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: platy, subhedral crystals.

Often confused with

Monazite-(Sm) vs. its common look-alikes, and how to tell them apart in the field.

mineral

Monazite-(Ce)

Mohs 5-5.5

How to tell apart: Streak differs, so Monazite-(Sm) leaves white, Monazite-(Ce) leaves grayish white; luster reads vitreous to greasy on Monazite-(Sm) and resinous, waxy, vitreous to adamantine on Monazite-(Ce).

How to tell apart: Zircon is the harder of the two (Mohs 7.5 vs. 5-5.5).

Often found alongside monazite-(sm)

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

All properties

Chemical formula: SmPO₄
Mohs hardness: 5-5.5
Density: 5.478 g/cm³
Colors: Yellowish
Streak: White
Luster: Vitreous to greasy
Transparency: Transparent to translucent
Crystal system: Monoclinic
Crystal habit: Platy, subhedral crystals
Cleavage: One good direction
Rarity: Rare
Uses: Collector
Host rock: From the inner intermediate zone of a lepidolite-subtype granitic pegmatite

Where rockhounds find monazite-(sm)

Classic worldwide localities

Annie Claim no. 3, near Greer Lake, near the Winnipeg River, southeastern Manitoba, Canada
Shimo-ono ilmenite-series granitic pegmatite, Ibaraki, Japan

Field-hunting tip

Where it forms: From the inner intermediate zone of a lepidolite-subtype granitic pegmatite. If you start seeing quartz, albite, lithian muscovite in float, you are in the right ground. Typical habit: platy, subhedral crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify monazite-(sm)?+

Mohs hardness is 5-5.5. Luster: vitreous to greasy. The streak is white. Common colors include yellowish.

Where is monazite-(sm) found?+

Notable localities include Annie Claim no. 3, near Greer Lake, near the Winnipeg River, southeastern Manitoba, Canada; Shimo-ono ilmenite-series granitic pegmatite, Ibaraki, Japan.

Is monazite-(sm) safe to handle?+

Monazite-(Sm) contains trace uranium or thorium. Contains thorium and uranium trace elements; store in a shielded container or away from living areas and handle with care, avoiding inhalation of dust.

What rocks look like monazite-(sm)?+

Monazite-(Sm) is most often confused with Monazite-(Ce), Zircon, Xenotime. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with monazite-(sm)?+

Monazite-(Sm) commonly co-occurs with Quartz, Albite, Lithian muscovite, Manganocolumbite, Zircon, Allanite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does monazite-(sm) form in?+

Where it forms: From the inner intermediate zone of a lepidolite-subtype granitic pegmatite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is monazite-(sm) used for?+

Monazite-(Sm) is used in collector.

Find monazite-(sm) on the map

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