Sigloite

Chemical formula: Fe³⁺Al₂(PO₄)₂(OH)₃·7H₂O

Sigloite is a rare hydrated iron and aluminum phosphate, forming small, platy crystals with a vitreous or pearly luster.

## Characteristics Sigloite is a rare phosphate mineral, chemically classified as a hydrated iron and aluminum phosphate. It occurs as very small, thin, platy crystals, rarely exceeding 1 mm. These crystals often form rosette-like or radial aggregates. Due to its small size, it is a mineral primarily valued by specialized collectors (micromounters). ## Physical Properties This mineral is characterized by a relatively low hardness, approximately 3 on the Mohs scale. It has a vitreous luster, and a pearly luster on cleavage surfaces. It is transparent to translucent. Its density is approximately 2.46 g/cm³. ## Colors and Varieties Sigloite is typically colorless, white, or pale yellow. No varieties have been distinguished. ## History and Name The mineral's name comes from its discovery locality – the Siglo Veinte mine (which means "Twentieth Century" in Spanish) in the Potosí Department, Bolivia. It was first described in 1978 by a team of mineralogists: Augustin A. Huidobro, Eugene E. Foord, and Charles W. Maxwell.

Properties

Mohs hardness
3
Color
Light brown, straw-yellow, pale brownish orange
Luster
Vitreous, Pearly
Streak
White
Density
2.35
Cleavage
On {010}, perfect; on {001}, good
Fracture
Micaceous
Transparency
Translucent
Crystal system
Triclinic

Diagnostic features

## Identification Amateur identification of sigloite is practically impossible due to the microscopic size of its crystals and their resemblance to many other secondary phosphate minerals. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from Similar Minerals Sigloite can be confused with other rare phosphates found in the same environment, such as paravauxite, vauxite, or childrenite. Differentiation is based on crystallographic and chemical analysis, as visual characteristics (color, habit) are often very similar. ## Crystal Forms Sigloite forms very small, thin, platy crystals with a hexagonal or octagonal outline. These crystals often occur as radial or rosette-like clusters and aggregates.

Geological environment

## Genesis Sigloite is a secondary mineral, formed in the oxidation zones of hydrothermal tin and tungsten ore deposits. It forms as a result of the alteration of primary phosphate minerals in the presence of aluminum-rich solutions. ## Mineral Associations This mineral co-occurs with other phosphates, such as vauxite, paravauxite, quartz, pyrite, cassiterite, wolframite, and childrenite. It most commonly grows on quartz crystals or other minerals. ## Localities The most important and classic locality for sigloite is the Siglo Veinte mine in Llallagua, Potosí Department, Bolivia, where it was discovered. This is the main locality from which specimens available on the collector's market originate.

Rarity

Very rare

For collectors

## Quality Criteria For micromineral collectors (micromounters), the most desirable specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates. Specimens where sigloite is clearly distinct and contrasts with the host rock (most often quartz) are also highly valued. Crystal size, even if a fraction of a millimeter, is important – the larger the crystals, the more valuable the specimen. ## Popular Localities The only commercially and historically significant source of sigloite specimens is its type locality – the Siglo Veinte mine in Bolivia. Specimens from other locations are practically unheard of on the market.

Care and storage

## Cleaning Sigloite specimens should be cleaned with the utmost care, using only compressed air to remove dust. Due to its fragility and the small size of its crystals, any mechanical contact, including brushes, should be avoided. Wet cleaning is not recommended due to the risk of damaging delicate aggregates. ## What to Avoid Avoid contact with water and chemicals. The mineral is very soft and brittle, so it must be protected from impacts, scratches, and vibrations. It should not be heated or exposed to ultrasound. ## Storage It is recommended to store sigloite specimens exclusively in specialized micromount boxes, which protect them from dust, mechanical damage, and sudden changes in humidity.

External references

Sources

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