Wightmanite

Chemical formula: Mg₅O(BO₃)(OH)₅·2H₂O

Wightmanite is a rare hydrated magnesium borate, forming colorless or white, silky fibrous aggregates.

## Characteristics Wightmanite is a rare mineral from the borate group, chemically a hydrated magnesium oxyborate. It most often occurs as asbestos-like, radial, or tangled aggregates with a silky luster. Individual "fibers" are actually elongated, bladed crystals. It can also be found as chalk-white, earthy masses. ## Physical Properties Wightmanite crystals are very soft, with a Mohs hardness of 3.5. This mineral is characterized by a silky luster in the case of fibrous aggregates or a dull luster in the case of earthy masses. It is translucent. Its density is approximately 2.56 g/cm³. ## Colors and Varieties Wightmanite is typically colorless or white. No colored or commercial varieties are distinguished. ## History and Name The mineral was discovered in 1961 at the Commercial Quarry in Crestmore, Riverside County, California (USA). Its name commemorates Randall H. Wightman (1899-1979), an American mineral collector who first found specimens of this mineral. ## Applications Due to its rarity and small crystal size, wightmanite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or minerals from specific localities.

Properties

Mohs hardness
5.5
Color
Colourless or light green
Luster
Silky
Streak
White
Density
2.60
Cleavage
{010} perfect, {100} fair; the two making an angle of 73.5 deg.
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification A characteristic feature of wightmanite is its silky, fibrous or asbestos-like aggregates of white color. Its occurrence in magnesian skarns in association with other borates is a key diagnostic indicator. ## Differentiation from Similar Minerals Wightmanite can be confused with other fibrous borates or silicates, such as tremolite or chrysotile (asbestos). Differentiation requires chemical analysis (XRD, EDS) or knowledge of associated minerals from the given locality. Unlike many silicates, wightmanite is very soft. ## Crystal Forms Wightmanite forms elongated, bladed crystals with a rhombic cross-section, which usually occur as radial, tangled, or fibrous aggregates. It is less commonly found in the form of earthy masses.

Geological environment

## Genesis Wightmanite is a mineral of metamorphic origin. It forms in contact zones (skarns) where magmatic intrusions interact with magnesium-rich carbonate rocks (dolomites or dolomitic marbles), supplying boron from hydrothermal solutions. ## Mineral Associations This mineral co-occurs with other magnesium borates, such as kotoite, fluoborite, szaibelyite, as well as with periclase, brucite, spinel, and calcite. ## Localities The most important and type locality, from which the best specimens originate, is the Commercial Quarry in Crestmore, California (USA). Its occurrence has also been confirmed in iron ore mines in Yakutia (Russia) and in several locations in China.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, silky radial aggregates, contrasting with darker host rock (matrix). The abundance of the mineral on the specimen and the absence of damage to the delicate fibers are important. Specimens from the type locality in Crestmore are the most sought after. ## Popular Localities By far the most known and valued source of collectible wightmanite specimens is its type locality – Crestmore in California, USA.

Care and storage

## Cleaning Wightmanite specimens are very delicate and fragile. They should only be cleaned using compressed air to remove dust. Any contact with water or a brush can permanently damage the fibrous aggregates. ## What to Avoid Avoid contact with water, acids, and other chemicals. The mineral's structure is sensitive to shock and mechanical pressure. It should not be heated or exposed to ultrasound. ## Storage It is recommended to store wightmanite specimens in closed, padded collector boxes to protect them from dust, moisture, and mechanical damage. Avoid placing other minerals on top of them.

External references

Sources

Read more