Tranquillityite is a complex silicate mineral first discovered in lunar basalt samples returned from the Apollo 11 mission. It is a rare accessory mineral found in specific iron-rich igneous rocks, appearing as tiny, dark, non-fluorescent grains that often contain measurable amounts of radioactive elements.
Is this tranquillityite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch tranquillityite with a known reference. Tranquillityite sits at Mohs 5-6 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Tranquillityite leaves a brown streak.
- 3Read the lusterHold the specimen under a strong light. Tranquillityite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: dark brown, reddish brown, opaque black.
- 5Look at form & habitCrystal system: trigonal. Typical habit: anhedral to subhedral grains.
Often confused with
Tranquillityite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Zircon is the harder of the two (Mohs 7.5 vs. 5-6); streak differs — Tranquillityite leaves brown, Zircon leaves white; luster reads submetallic on Tranquillityite and adamantine on Zircon.
How to tell apart: Streak differs — Tranquillityite leaves brown, Baddeleyite leaves white.
How to tell apart: Streak differs — Tranquillityite leaves brown, Manaccanite leaves black.
Often found alongside tranquillityite
Minerals reported to co-occur with tranquillityite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Fe²⁺, Ca, Mg, Y)₈(Zr, Nb)₂(Ti, Fe³⁺)₃Si₃O₂₄
- Mohs hardness
- 5-6
- Density
- 4.8-5.0 g/cm³
- Streak
- Brown
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Anhedral to Subhedral Grains
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific Research
- Host rock
- Lunar Basalt, Terrestrial Mafic Igneous Rocks
- Typical price
- n/a (extremely limited market availability)
Where rockhounds find tranquillityite
Classic worldwide localities
- Moon (Sea of Tranquility)
- Pilbara Craton, Western Australia
Field-hunting tip
Look in lunar basalt, terrestrial mafic igneous rocks country — that is the host setting where tranquillityite typically forms. If you start seeing fayalite, pyroxene, plagioclase in float, you are in the right ground. Field specimens usually show a anhedral to subhedral grains habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find tranquillityite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
