Tunellite

Chemical formula: SrB<sub>6</sub>O<sub>9</sub>(OH)<sub>2</sub>·3H<sub>2</sub>O

Tunellite is a rare hydrated strontium borate, forming colorless, vitreous crystals and aggregates, prized by collectors of borate minerals.

## Characteristics Tunellite is a rare mineral from the borate group, chemically classified as a hydrated strontium borate. It most often occurs in the form of well-formed, tabular or prismatic crystals, which can reach several centimeters in length. It also forms radial, rosette-like, or massive aggregates. It is a colorless or white mineral, often transparent with a strong, vitreous luster, which gives it an attractive appearance. ## Physical Properties Tunellite is characterized by a relatively low hardness of 2.5 on the Mohs scale, meaning it is soft and susceptible to scratching. Its density is approximately 2.4 g/cm³. It is transparent to translucent. It exhibits perfect cleavage in one direction, which is visible in some specimens. The fracture is uneven. ## Colors and Varieties This mineral is typically colorless or white. No colored varieties or trade names are distinguished. Any coloration, if present, is most often the result of impurities or inclusions of other minerals. ## History and Name Tunellite was first described in 1961. Its name honors George Tunell (1900–1996), an American professor of geochemistry at the University of California, Santa Barbara, in recognition of his contributions to the study of borate minerals. The type locality, from which the first described specimens originated, is the U.S. Borax & Chemical Corp. Mine in Boron, Kern County, California, USA. ## Uses Due to its rarity and small deposits, tunellite has no industrial significance. However, it is a mineral sought after and valued by collectors specializing in mineral systematics, especially rare borates.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
White
Density
2.39-2.41
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Tunellite can be identified by its characteristic, colorless, and vitreous crystals with a tabular habit. Key features include low hardness (2.5 on the Mohs scale), perfect cleavage in one direction, and occurrence in a specific geological environment – evaporitic borate deposits. It reacts with hydrochloric acid. ## Distinguishing from Similar Minerals Tunellite is sometimes confused with other colorless borates, such as colemanite or ulexite, with which it often co-occurs. It differs from colemanite in crystal form (colemanite forms more isometric, prismatic crystals) and slightly lower hardness. From fibrous ulexite, it is distinguished by its distinctly crystalline, tabular form. Selenite (a variety of gypsum) is much softer (hardness 2) and has a different crystal habit. ## Crystal Forms Tunellite crystals are monoclinic and most often take the form of flattened, tabular crystals with outlines ranging from hexagonal to rectangular. They often form fan-shaped or radial aggregates, as well as rosettes composed of intersecting crystals. Granular or massive aggregates are less common.

Geological environment

## Genesis Tunellite is a mineral of sedimentary origin. It forms as a result of the evaporation of saline lakes rich in boron, under dry and hot climatic conditions. It crystallizes directly from solutions or through diagenetic processes occurring in borate sediments. ## Mineral Associations This mineral typically co-occurs with other borates, such as ulexite, colemanite, kernite, borax, and hydroboracite. It is often accompanied by clay minerals, calcite, as well as realgar and orpiment, which can impart characteristic coloration to the surrounding rocks. ## Localities The most important and well-known tunellite localities worldwide are in the United States, in the Boron area of Kern County, California (U.S. Borax mines). Significant borate deposits where tunellite occurs are also located in Turkey (Balıkesir, Eskişehir, and Kütahya provinces) and Argentina (Salta province).

Rarity

Rare

For collectors

## Quality Criteria The most prized tunellite specimens by collectors consist of large, well-formed, transparent, and undamaged crystals. Aesthetic compositions, such as radial aggregates (rosettes) on a rock matrix, enhance their attractiveness. Specimens from classic localities, such as Boron, California, are particularly sought after. Purity (absence of inclusions and dulling) and a strong, vitreous luster are key factors influencing value. ## Popular Localities Undoubtedly, the most desirable specimens come from the mines in the Boron area of California, USA, which is the type locality for this mineral. High-quality crystals, often in the form of large rosettes, also come from Turkish borate deposits, especially from the Bigadiç and Emet regions.

Care and storage

## Cleaning Tunellite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its partial solubility in water, prolonged contact with liquids should be avoided. If liquid is necessary, a brief wipe with a cloth dampened with alcohol, which evaporates quickly, is recommended. ## What to Avoid Contact with water (especially hot), acids, and other chemicals should be absolutely avoided. The mineral is very soft (2.5 on the Mohs scale), so it must be protected from scratching by harder minerals. It is also sensitive to changes in humidity and temperature, which can lead to dehydration and dulling. ## Storage Tunellite should be stored in stable conditions, preferably in a closed, dry container or display case, to protect it from dust and humidity changes. It should be separated from harder minerals to prevent mechanical damage.

Sources

Read more