Rankamaite is a very rare tantalum-niobium oxide mineral typically found as fine, acicular, or fibrous aggregates in complex pegmatites. Due to its scarcity and small crystal size, it is a highly sought-after species for advanced mineral collectors specializing in rare earth or niobium-tantalum minerals. It is primarily identified by its occurrence in specific pegmatitic environments associated with other tantalum-bearing species.

Hardness

3-4

Mohs

Luster

Silky

Streak

White

Transparency

Semitransparent

mineralrare

Rankamaite

Hardness: 3-4
Luster: Silky
Streak: White
Transparency: Semitransparent

Is this rankamaite?

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 rankamaite with a known reference. Rankamaite 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: white.
3
Read the luster
Hold the specimen under a strong light. Luster: silky.
4
Match the color range
Compare against the expected color range: white, creamy white.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: acicular, fibrous, massive.

Often confused with

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

mineral

Wodginite

Mohs 5.5

How to tell apart: Wodginite is the harder of the two (Mohs 5.5 vs. 3-4); streak differs, so Rankamaite leaves white, Wodginite leaves brown; luster reads silky on Rankamaite and submetallic on Wodginite.

How to tell apart: Tantalite is the harder of the two (Mohs 6 vs. 3-4); streak differs, so Rankamaite leaves white, Tantalite leaves black to reddish-brown; luster reads silky on Rankamaite and submetallic to resinous on Tantalite.

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Often found alongside rankamaite

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

All properties

Chemical formula: (Na,K)₃(Ta,Nb,Al)₁₁(O,OH)₃₁
Mohs hardness: 3-4
Density: 5.5 g/cm³
Colors: WhiteCreamy white
Streak: White
Luster: Silky
Transparency: Semitransparent
Crystal system: Orthorhombic
Crystal habit: Acicular, fibrous, massive
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Apparently a replacement of simpsonite, the result of intense sodium metasomatism in some Li-Ta-Nb-bearing granite pegmatites

Where rockhounds find rankamaite

Classic worldwide localities

Mumba, Kivu, Zaire (type locality)

Field-hunting tip

Where it forms: Apparently a replacement of simpsonite, the result of intense sodium metasomatism in some Li-Ta-Nb-bearing granite pegmatites. If you start seeing simpsonite, microlite, cassiterite in float, you are in the right ground. Typical habit: acicular, fibrous, massive. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify rankamaite?+

Mohs hardness is 3-4. Luster: silky. The streak is white. Common colors include white, creamy white.

Where is rankamaite found?+

Notable localities include Mumba, Kivu, Zaire (type locality).

What rocks look like rankamaite?+

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

What minerals are found with rankamaite?+

Rankamaite commonly co-occurs with Simpsonite, Microlite, Cassiterite, Beryl, Manganotantalite, Muscovite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does rankamaite form in?+

Where it forms: Apparently a replacement of simpsonite, the result of intense sodium metasomatism in some Li-Ta-Nb-bearing granite pegmatites. Working float back to the host body is the standard way to chase a fresh occurrence.

What is rankamaite used for?+

Rankamaite is used in collector.

Find rankamaite on the map

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