Lacroixite is a scarce phosphate mineral primarily found in complex granitic pegmatites. It is most easily identified by its distinctive yellow to greenish hues and association with other phosphate minerals, though it is often mistaken for amblygonite without chemical analysis.
Is this lacroixite?
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 lacroixite with a known reference. Lacroixite sits at Mohs 4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous, pearly on cleavages.
- 4Match the color rangeCompare against the expected color range: colorless, gray, gray white, light green, light yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: rarely as small prismatic crystals, commonly massive or granular.
Often confused with
Lacroixite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside lacroixite
Minerals reported to co-occur with lacroixite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaAlFPO₄
- Mohs hardness
- 4.5
- Density
- 3.29 g/cm³
- Streak
- White
- Luster
- Vitreous, pearly on cleavages
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Rarely as small prismatic crystals, commonly massive or granular
- Cleavage
- Indistinct on {111} and {111}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In druses in granite (Greifensteine, Germany); an alteration product rimming natromontebrasite in a complex granite pegmatite (Rusororo, Rwanda)
Where rockhounds find lacroixite
Classic worldwide localities
- Strickland quarry, Portland, Middlesex Co., Connecticut, and in the Mount Rubellite quarry, Hebron, Oxford Co., Maine, USA
- Lázně Kynzvart, near Mariánské Lázně (Königswart, near Marienbad), and Ječlov, near Jihlava, Czech Republic
- Montebras, Creuse, France
- Rusororo and Buranga pegmatites, near Gatumba, Rwanda
Field-hunting tip
Where it forms: In druses in granite (Greifensteine, Germany); an alteration product rimming natromontebrasite in a complex granite pegmatite (Rusororo, Rwanda). If you start seeing morinite, viitaniemiite, apatite in float, you are in the right ground. Typical habit: rarely as small prismatic crystals, commonly massive or granular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify lacroixite?+
Where is lacroixite found?+
What rocks look like lacroixite?+
What minerals are found with lacroixite?+
What kind of rock does lacroixite form in?+
What is lacroixite used for?+
Find lacroixite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
