Glass-(La)

Chemical formula: PbCuLa(PO₄)₂(OH)

Glass-(La) is a rare mineral from the whitlockite group, a lanthanide phosphate, characterized by distinctive, fine, vitreous crystals.

## Characteristics Glass-(La) is a mineral belonging to the whitlockite group. It forms very small, prismatic or acicular crystals, rarely exceeding 1 mm in length. They usually occur as radial or tangled aggregates. The crystals exhibit a vitreous luster and are most often colorless or white. ## Physical Properties This mineral is characterized by a hardness of 4.5 on the Mohs scale. Its crystals are brittle, and the density calculated based on the chemical formula and unit cell parameters is approximately 4.98 g/cm³. The luster is vitreous. ## Colors and Varieties Glass-(La) is usually colorless or white. No colored varieties have been distinguished. ## History and Name The mineral's name honors Alvin J. Glass (1911–1971), an American mineralogist and chemist who worked for the U.S. Geological Survey. The -(La) suffix in the name indicates the dominance of lanthanum among the rare earth elements in its chemical composition. The mineral was first described in 1956. ## Applications Due to its rarity and microscopic crystal sizes, Glass-(La) has no industrial applications and is solely an object of interest for collectors specializing in rare minerals and for scientists.

Properties

Mohs hardness: 4.5
Luster: Vitreous
Streak: White
Density: 4.98
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Amateur identification of Glass-(La) is practically impossible due to the microscopic size of the crystals and their resemblance to many other secondary minerals. Definitive identification requires advanced analytical methods, such as X-ray spectroscopy (XRD, EDS). ## Distinguishing from Similar Minerals It can be confused with other minerals from the whitlockite group, such as whitlockite-(Ce) or whitlockite-(Y), as well as other acicular phosphates or silicates. Differentiation is possible only based on chemical analysis of the rare earth element composition. ## Crystal Forms Glass-(La) forms elongated, prismatic or acicular crystals, often terminated by a pyramid. They usually occur as radial or haphazardly tangled clusters and aggregates.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation zones of ore deposits where rare earth element minerals are present. It forms as a result of the weathering of primary minerals, such as monazite or bastnäsite, in the presence of phosphorus-rich solutions. ## Mineral Associations It most often co-occurs with quartz, fluorite, barite, monazite, bastnäsite, as well as other secondary phosphates and arsenates. ## Localities The most important and classic locality for this mineral is the Cap-Garonne mine in Le Pradet, Var, France. This is its type locality. In addition, its occurrence has been reported in several other places around the world, including the Clara mine in the Black Forest (Germany) and the Old Dominion mine in Globe (Arizona, USA).

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens are those with well-formed, sharp crystals forming aesthetic, radial aggregates on a contrasting rock matrix. Due to their microscopic size, the quality of the specimen as seen under magnification is crucial. The size of the aggregates and the abundance of their occurrence on the specimen also increase its value. ## Popular Localities Specimens from the type locality – the Cap-Garonne mine in France – are by far the most sought after by collectors. They are considered classic and of the best quality for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using compressed air to remove dust. Due to the brittleness and small size of the crystals, contact with water or brushes is not recommended, as it can lead to damage of the delicate aggregates. ## What to Avoid Avoid contact with acids and other chemicals. The crystals are brittle, so they should be protected from impacts, vibrations, and sudden temperature changes. Ultrasonic cleaners should not be used. ## Storage It is recommended to store specimens in stable, sealed containers (micromount type) that protect them from dust, mechanical damage, and moisture. Avoid exposure to direct sunlight, although there is no evidence of its light sensitivity.