Lacroixite

Chemical formula: NaAl(PO<sub>4</sub>)F

A rare sodium and aluminum phosphate, valued by collectors for its well-formed crystals with a vitreous luster and greenish color.

## Characteristics Lacroixite is a rare mineral from the phosphate group, chemically a sodium and aluminum fluorophosphate. It most often forms imperfectly developed, prismatic crystals of small size, occurring as granular aggregates or radial groups. Its appearance is usually inconspicuous, and its value for collectors lies in its rarity and specific chemical composition. ## Physical Properties This mineral is characterized by a hardness of approximately 4.5 on the Mohs scale. It has a vitreous luster, which can be greasy on fracture surfaces. It is brittle, and its density is about 3.13 g/cm³. Crystals are usually translucent, rarely transparent. ## Colors and Varieties Lacroixite occurs in a limited color range. The most common shades are greenish-yellow, pale green, as well as colorless or white. No named varieties of this mineral are distinguished. ## History and Name The mineral was first described in 1914 by the Czech mineralogist František Slavík. He named it in honor of the eminent French mineralogist and geologist, Alfred Lacroix (1863–1948), who was a professor at the National Museum of Natural History in Paris. Type specimens come from Greifenstein in the Ore Mountains (Saxony, Germany). ## Uses Due to its extreme rarity and small crystal sizes, lacroixite has no industrial application. It is a mineral of purely scientific and collector's interest, sought after by advanced collectors of rare phosphate minerals.

Properties

Mohs hardness
4.5
Luster
Vitreous
Streak
White
Density
3.13
Cleavage
Good on {110}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Field identification of lacroixite is practically impossible without advanced research methods. In a collection, it is recognized based on its specific appearance (prismatic crystals, often in radial aggregates), greenish color, and vitreous luster. Confirmation of the specimen's origin locality is crucial. ## Distinguishing from Similar Minerals Lacroixite can be confused with other rare pegmatite phosphates, such as amblygonite or montebrasite, from which it differs by lower hardness and specific crystal form. However, definitive differentiation requires chemical analysis (EDS) or X-ray analysis (XRD). ## Crystal Forms It forms short, prismatic crystals with a square or rectangular cross-section, often terminated by flat faces. Crystals rarely occur individually; they usually form granular, massive, or radial aggregates embedded in the host rock.

Geological environment

## Genesis Lacroixite is a hydrothermal mineral, crystallizing in the late stages of the evolution of lithium- and phosphorus-rich granitic pegmatites. It forms as a result of metasomatic alterations of other phosphates, such as amblygonite-montebrasite. ## Mineral Associations It most often co-occurs with minerals typical of granitic pegmatites, such as quartz, albite, muscovite, as well as with other phosphates: amblygonite, montebrasite, apatite, wardite, wavellite, and tourmaline. ## Localities The most important and classic locality, from which the best specimens originate, is Greifenstein in Saxony (Germany). Other known localities include Forcel de la Vaca (Pyrenees, Spain), the Foote Mine in North Carolina (USA), as well as occurrences in Brazil (Minas Gerais region) and Rwanda.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals of a distinct, greenish color, set on a contrasting rock matrix, are most valued by collectors. Origin from a classic locality, such as Greifenstein, is also of great importance. Due to the mineral's rarity, even small but well-defined crystals are considered valuable. ## Popular Localities Specimens from Greifenstein, Germany, are historically the most important and most desired. In recent years, interesting specimens from Spain and Brazil have also appeared on the market, but the German locality remains the benchmark for this mineral.

Care and storage

## Cleaning Cleaning is recommended only with a soft brush to remove dust. The use of water, especially with detergents, is inadvisable due to the risk of chemical reaction and damage to delicate crystals. ## What to Avoid Avoid contact with all chemicals, including acids and bases. The mineral is brittle and sensitive to impacts and sudden temperature changes. It should not be cleaned in ultrasonic cleaners. ## Storage Lacroixite specimens should be stored in stable conditions, in separate, padded collector boxes to prevent abrasions and mechanical damage. They should be protected from dust and moisture.

Sources

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