Mottanaite-(Ce) is an extremely rare member of the hellandite group of silicates often found in miarolitic cavities within granitic pegmatites. It typically appears as brownish, prismatic crystals with a distinctive vitreous luster that can be confused with other rare-earth minerals like gadolinite.

Hardness

5.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrareHellandite Group family

Mottanaite-(Ce)

Hardness: 5.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this mottanaite-(ce)?

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 mottanaite-(ce) with a known reference. Mottanaite-(Ce) sits at Mohs 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.
4
Match the color range
Compare against the expected color range: pale brown.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: anhedral grains, tabular.

Often confused with

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

mineral

Gadolinite-(Y)

Mohs 6.5-7

How to tell apart: Gadolinite-(Y) is the harder of the two (Mohs 6.5-7 vs. 5.5); streak differs, so Mottanaite-(Ce) leaves white, Gadolinite-(Y) leaves greenish gray.

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Often found alongside mottanaite-(ce)

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

All properties

Chemical formula: Ca₄Ce₂Al(Be₁.₅□₀.₅)[B₄Si₄O₂₂]O₂
Mohs hardness: 5.5
Density: 3.61 g/cm³
Colors: Pale brown
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Anhedral grains, tabular
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: In miarolitic cavities and voids in feldspathoid-bearing alkali-syenitic pyroclastic ejecta

Where rockhounds find mottanaite-(ce)

Classic worldwide localities

Sabatini volcanic complex, Latium, Italy (type locality)

Field-hunting tip

Where it forms: In miarolitic cavities and voids in feldspathoid-bearing alkali-syenitic pyroclastic ejecta. If you start seeing britholite-, sanidine, plagioclase in float, you are in the right ground. Typical habit: anhedral grains, tabular. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify mottanaite-(ce)?+

Mohs hardness is 5.5. Luster: vitreous. The streak is white. Common colors include pale brown.

Where is mottanaite-(ce) found?+

Notable localities include Sabatini volcanic complex, Latium, Italy (type locality).

Is mottanaite-(ce) safe to handle?+

Mottanaite-(Ce) contains beryllium. Contains thorium as a common minor element; handle with care and wash hands after contact.

What rocks look like mottanaite-(ce)?+

Mottanaite-(Ce) is most often confused with Gadolinite-(Y). A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with mottanaite-(ce)?+

Mottanaite-(Ce) commonly co-occurs with Britholite-, Sanidine, Plagioclase, Nepheline, Biotite, Clinopyroxene. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does mottanaite-(ce) form in?+

Where it forms: In miarolitic cavities and voids in feldspathoid-bearing alkali-syenitic pyroclastic ejecta. Working float back to the host body is the standard way to chase a fresh occurrence.

What is mottanaite-(ce) used for?+

Mottanaite-(Ce) is used in collector.

Find mottanaite-(ce) on the map

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