Originally discovered in lunar samples returned by Apollo 11, Armalcolite is a titanium-rich mineral named after astronauts Armstrong, Aldrin, and Collins. On Earth, it is found primarily in volcanic rocks and kimberlites, typically occurring as microscopic grains associated with ilmenite. It is highly prized by mineral collectors due to its extraterrestrial fame and rarity.
Is this armalcolite?
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 armalcolite with a known reference. Armalcolite sits at Mohs 6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: gray.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: gray.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: granular.
Often confused with
Armalcolite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Armalcolite leaves gray, Ilmenite leaves black to reddish brown.
How to tell apart: Streak differs, so Armalcolite leaves gray, Pseudobrookite leaves reddish brown to ocher-yellow.
How to tell apart: Streak differs, so Armalcolite leaves gray, Hematite leaves cherry-red or reddish brown.
Often found alongside armalcolite
Minerals reported to co-occur with armalcolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- MgTi₂O₅
- Mohs hardness
- 6
- Density
- 4.94 g/cm³
- Colors
- Streak
- Gray
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Granular
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- Formed typically at low pressures and high temperatures; in Ti-rich basalts and microbreccias of lunar samples; in salic volcanics; rarely in granite pegmatite; in ultramafic rocks, lamproites, kimberlites
Where rockhounds find armalcolite
Classic worldwide localities
- Knippa quarry, near Uvalde, Uvalde Co., Texas, and at Smoky Butte, near Jordan, Garfield Co., Montana, USA
- Descartes site, Moon
- El Toro cinder cone, near San Luis Potosí, Mexico
- Nördlinger Reis crater, Bavaria, Germany
- Jumilla, Murcia Province, and Cancarix, Albacete Province, Spain
- Disko Island, Greenland
Field-hunting tip
Where it forms: Formed typically at low pressures and high temperatures; in Ti-rich basalts and microbreccias of lunar samples; in salic volcanics; rarely in granite pegmatite; in ultramafic rocks, lamproites, kimberlites. If you start seeing ilmenite, fe–ti oxides, iron in float, you are in the right ground. Typical habit: granular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify armalcolite?+
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Find armalcolite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
