Tan aggregates of the rare phosphate mineral lacroixite in amblygonite from this Colorado locality (Mica Load, Canon City Area, Fremont County, Colorado, United States of America) and associated to very nice purple lepidolite. This combination was once known as natromontebrasite, the name has now been discredited as it is lacroixite and amblygonite. Ex Ralph E. Merrill collection. — Photo: David Hospital · CC BY-SA 4.0
Lacroixite
Lacroixite is an IMA-recognized mineral species in the phosphate class, with the chemical formula NaAl(PO₄)F.
It crystallizes in the monoclinic crystal system (space group C2/c). It is a member of the Titanite structural group.
Key properties: Mohs hardness 4.5; specific gravity 3.29; Vitreous, pearly on cleavages luster; optically biaxial.
IMA symbol Lac, grandfathered status — known before the IMA’s founding, named in 1914.
Found in druses in granite (Greifensteine, Germany); an alteration product rimming natromontebrasite in a complex granite pegmatite (Rusororo, Rwanda).
It is found associated with Morinite, viitaniemiite, apatite, childrenite, roscherite, feldspar, tourmaline, quartz (Greifensteine, Germany); montebrasite, scorzalite.
The mineral was named to honor François Antoine Alfred Lacroix (1863–1948), French mineralogist, Natural History Museum, Paris, France.