Graulichite-(La)

Chemical formula: LaFe<sup>3+</sup><sub>3</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>

Graulichite-(La) is a rare lanthanide and iron arsenate, forming small, yellowish-brown trigonal crystals.

## Characteristics Graulichite-(La) is a mineral from the arsenate group, belonging to the extensive alunite supergroup. It occurs as very small, tabular or rhombohedral crystals, usually not exceeding several tens of micrometers. These crystals often form rosette-like or spherical aggregates. Due to their size, visual features are difficult to observe without the use of a microscope. ## Physical Properties The mineral is characterized by a Mohs hardness of approximately 4.5. It has a vitreous to dull luster. It is a brittle mineral with an uneven fracture. The density calculated based on the chemical formula and unit cell parameters is 4.35 g/cm³. ## Colors and Varieties Graulichite-(La) ranges in color from yellow to yellowish-brown. No varieties have been distinguished. ## History and Name The mineral is named in honor of the Belgian mineralogist Jean-Marie Graulich (1920-1999) for his contributions to the mineralogy of Belgium. The -(La) suffix in the name indicates the dominant rare earth element – lanthanum. The mineral was approved by the International Mineralogical Association (IMA) in 2002 (IMA2002-031). It was first described by F. Haterter et al. in a 2003 publication based on material from Oumlil-East in Morocco. ## Uses Due to its rarity and microscopic crystal sizes, graulichite-(La) has no industrial applications. It is solely an object of scientific and collecting interest for specialists accumulating rare minerals and microminerals.

Properties

Mohs hardness
4.5
Luster
Vitreous to dull
Streak
Light yellow
Density
4.35
Cleavage
None
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Trigonal

Diagnostic features

## Identification Identification of graulichite-(La) is almost exclusively possible using advanced analytical methods, such as energy-dispersive spectroscopy (EDS/EDX) to confirm chemical composition (presence of La, Fe, and As) and X-ray diffraction (XRD) to determine crystal structure. Visually, in the field or in a collection, it is practically impossible to unequivocally identify. Suspicions may arise from small, yellowish-brown, tabular crystals in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals This mineral can be confused with other arsenates from the alunite supergroup, such as beudantite, segnitite, hidalgoite, or philipsbornite, and especially with its analogues containing other rare earth elements (graulichite-(Ce), graulichite-(Y)). Differentiation requires chemical analysis to determine the dominant element. ## Crystal Forms It forms very small, tabular crystals with a hexagonal or rhombohedral outline. These crystals often combine into rosette-like, spherulitic, or crusty aggregates.

Geological environment

## Genesis Graulichite-(La) is a secondary mineral, forming in the oxidation zones (gossans) of hydrothermal ore deposits rich in arsenic and rare earth elements. It forms under low-temperature conditions as a result of the weathering of primary sulfides and arsenides. ## Mineral Associations It co-occurs with minerals typical of oxidation zones, such as quartz, goethite, hematite, jarosite, as well as with other arsenates, e.g., mimetite, scorodite, beudantite, segnitite, and other minerals from the alunite supergroup. ## Localities The most important confirmed localities for this mineral are: - Oumlil-East, Bou Azzer, Morocco - type locality. - Clara Mine, Wolfach, Black Forest, Germany. - Cap Garonne Mine, Le Pradet, Var, France. - Kamareza Mine, Laurium, Greece.

Rarity

Very rare

For collectors

## Quality Criteria As a micromineral, graulichite-(La) is primarily evaluated based on the quality and development of crystals visible under a microscope. Specimens with sharp, well-formed crystals creating aesthetic, isolated groups or rosettes on a contrasting substrate (matrix) are most valued. The intensity and purity of the color are also important. Due to the need for analytical confirmation, specimens with an accompanying analysis result (e.g., EDS) have higher value. ## Popular Localities The most known and valued specimens, including the type material, come from Oumlil in the Bou Azzer region of Morocco. Specimens from European localities, such as the Clara Mine in Germany or Cap Garonne in France, are also sought after by specialists.

Care and storage

## Cleaning Specimens of graulichite-(La) are extremely delicate due to the microscopic size of the crystals. Mechanical cleaning is not recommended. If dust removal is necessary, compressed air can be used from a safe distance, or the specimen can be very carefully rinsed in distilled water and then left to dry completely. ## What to Avoid Avoid contact with acids and other chemicals that can damage arsenates. Do not subject the specimen to ultrasound. Due to its brittleness, protect it from impacts and vibrations. ## Storage The safest storage method is to place the specimen in a sealed "micromount" box, which protects it from dust, mechanical damage, and sudden changes in humidity. Avoid exposure to direct sunlight, although there is no evidence of fading.

Sources

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