Fontarnauite

Chemical formula: (Na,K)₂(Sr,Ca)(S⁶⁺O₄)[B₅O₈(OH)]·₂H₂O

Fontarnauite is a rare, hydrated borosulfate of sodium, potassium, strontium, and calcium, forming colorless or light brown, tabular crystals.

## Characteristics Fontarnauite is a complex, hydrated borosulfate belonging to the sulfate group. This mineral typically forms small, tabular or bladed crystals, often grouped into radial or rosette aggregates. Its appearance is subtle, and specimens are usually colorless, white, or take on shades of light brown. Due to its rarity and small crystal size, it is primarily of interest to specialized collectors and scientists. ## Physical Properties Fontarnauite is a relatively soft mineral, with a Mohs hardness of 2.5-3, meaning it can be scratched by a copper wire. Its density is approximately 2.37 g/cm³. Crystals exhibit a vitreous luster and are transparent to translucent. Its splintery fracture is characteristic. ## Colors and Varieties This mineral does not exhibit significant color variation. It occurs mainly as colorless, white, or light brown crystals. No named color varieties or commercial varieties have been distinguished. ## History and Name Fontarnauite was officially recognized as a new mineral by the International Mineralogical Association (IMA) in 2009. Its name honors Ramon Fontarnau i Vileta (born 1949), a chemist from the Jaume Almera Institute of Earth Sciences in Barcelona (Spain), in recognition of his contributions to the analytical chemistry of minerals, including borates. It was discovered in samples from drill cores in the Emet region of Turkey. ## Uses Due to its extreme rarity, small crystal size, and low hardness, fontarnauite has no industrial or jewelry applications. Its significance is purely scientific and collectible.

Properties

Mohs hardness: 2.5-3
Luster: Vitreous
Streak: White
Density: 2.37
Cleavage: {010}
Fracture: Splintery
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Fontarnauite is difficult to identify visually without specialized equipment. It is distinguished by its low hardness (2.5-3), vitreous luster, and occurrence in the form of small, tabular crystals, often in radial aggregates. Its genesis in evaporitic borate deposits is characteristic. Final identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other borates and sulfates occurring in the same environment, such as gypsum, colemanite, or hydroboracite. It differs from gypsum by its slightly higher hardness and different crystal habit. From other borates, it can be distinguished based on detailed optical and physical properties, but in practice, this is impossible without laboratory analysis. ## Crystal Forms Fontarnauite crystals are usually small, with a tabular or bladed habit. They often form fan-shaped, radial, or rosette aggregates and clusters.

Geological environment

## Genesis Fontarnauite is a mineral of evaporitic origin. It forms in lacustrine sediments rich in boron, sodium, strontium, and sulfates, under arid or semi-arid climatic conditions. It crystallizes from solutions remaining after water evaporation. ## Mineral Associations This mineral co-occurs with other borates and sulfates typical of evaporitic deposits. In its type locality (Emet, Turkey), it was found in association with colemanite, hydroboracite, celestine, calcite, and gypsum. ## Localities The only confirmed and described occurrence of fontarnauite in the world is its type locality: the vicinity of Doğanlar village in the Emet district (Kütahya province, Turkey). The mineral was identified there in drill cores from a borate deposit.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a fontarnauite specimen is primarily assessed based on the size and development of its crystals and the aesthetics of its aggregates. The most desirable specimens are those with well-formed, sharp, transparent crystals forming visually attractive, radial or rosette clusters. Association with other minerals is also important, as it can enhance the aesthetic and scientific value of the specimen. Purity (lack of inclusions) and absence of damage are crucial. ## Popular Localities The only source of fontarnauite specimens is its type locality in Turkey. The material comes from drill cores, which makes access to it very limited, and specimens on the collector's market are extremely rare.

Care and storage

## Cleaning Fontarnauite specimens should be cleaned with the utmost care, using only a soft brush to remove dust. Due to its low hardness and potential solubility, contact with water, especially hot water and chemical agents, should be avoided. ## What to Avoid Ultrasonic cleaners, all acids, and detergents must be absolutely avoided. The mineral is soft and brittle, so it must be protected from impacts, scratching, and vibrations. Exposure to high temperatures, which can lead to the loss of water from its structure and crystal damage, should also be avoided. ## Storage It is recommended to store fontarnauite specimens in stable conditions, in closed collector boxes or display cases, away from dust, moisture, and direct sunlight. It is best to keep it in a separate container to avoid contact with harder minerals that could damage it.