Probertite

Chemical formula: NaCaB₅O₇(OH)₄·₃H₂O

Probertite is a hydrated sodium and calcium borate, forming radial aggregates of colorless or white, acicular crystals.

## Characteristics Probertite is a mineral from the borate group, chemically a hydrated sodium and calcium borate. Its most typical and recognizable form of occurrence is radial or radiating-fibrous aggregates, resembling sea urchins or spheres composed of hundreds of thin, acicular crystals. Individual crystals are elongated, often flattened, and exhibit longitudinal striations. These aggregates can reach considerable sizes, even several tens of centimeters in diameter. The mineral is usually colorless, white, or grayish, and its aggregates have a silky luster. ## Physical Properties Probertite is characterized by a relatively low hardness of 3.5 on the Mohs scale. It has perfect cleavage in one direction, which is visible on larger crystals. Its luster is vitreous on crystal faces, and on fracture surfaces and in fibrous aggregates, it becomes silky. It is a light mineral, with a density of approximately 2.14 g/cm³. ## Colors and Varieties This mineral does not form colored varieties. It occurs almost exclusively in shades of white, gray, or is colorless. Impurities, such as clay inclusions, can impart a yellowish or brownish tint. ## History and Name Probertite was first described in 1929. Its name commemorates Frank H. Probert (1876–1940), Dean of the College of Mining at the University of California, Berkeley, in recognition of his contribution to the development of borate mining in that state. ## Uses Probertite, along with other borate minerals such as ulexite and colemanite, is locally mined as a boron ore. Boron is an element with widespread industrial applications, including in the production of glass (borosilicate), ceramics, enamels, cleaning agents, and in agriculture as a component of fertilizers.

Properties

Mohs hardness: 3.5
Luster: Vitreous, Silky
Streak: White
Density: 2.14
Cleavage: Perfect on {110}
Fracture: Uneven, Splintery
Transparency: Transparent to Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of probertite is its characteristic, radial aggregates of acicular or fibrous crystals, often forming spherical shapes. Silky luster in aggregates, low hardness, and reaction to moisture (partial solubility) are also key. Unlike many other borates, it does not exhibit fluorescence under UV light. ## Distinguishing from Similar Minerals Probertite is sometimes confused with ulexite, with which it often co-occurs. Ulexite forms similar fibrous aggregates, but they are usually more silky and exhibit the characteristic "television stone" effect (image conduction), which is not observed in probertite. Scolecite also forms radial aggregates, but it is a silicate, crystallizes in a different system, and is harder (5-5.5). Natrolite, another zeolite, has similar forms but is also harder. ## Crystal Forms Probertite crystals are strongly elongated in the form of needles or blades, often flattened and terminated by steep faces. They almost always occur as radial, spherical, or hemispherical aggregates. Single, well-formed crystals are rarely found.

Geological environment

## Genesis Probertite is an evaporite mineral. It forms as a result of the evaporation of saline lake waters (playa) in an arid, desert climate. It crystallizes directly from solutions rich in boron, sodium, and calcium, or through diagenetic processes (alterations) of earlier borate sediments, often at the expense of ulexite. ## Mineral Associations This mineral typically co-occurs with other borates, such as ulexite, colemanite, borax, kernite, and hydroboracite. It is also often accompanied by evaporite minerals, such as halite, gypsum, or thenardite. ## Localities The most important and classic localities for probertite are in the United States, in the Mojave Desert region of California (Kern and Inyo counties), particularly around Kramer, Boron, and in Death Valley. Significant deposits and beautiful specimens also come from Argentina (Salta and Jujuy provinces), Turkey (Bigadiç region), and Chile.

Rarity

Not very common

For collectors

## Quality Criteria The most prized by collectors are large, well-formed, spherical radial aggregates, known as "suns" or "sea urchins." An ideal specimen should be complete, undamaged, of a pure white color, and with a distinct silky luster. Contrastive placement on a small rock matrix enhances its attractiveness. Specimens from classic California localities are historically important and sought after. ## Popular Localities Among the most famous localities from which collector specimens originate are the borate mines in the Boron region of California, USA. Classic, large, spherical aggregates come from there. High-quality specimens are also found in borate deposits in Argentina and Turkey.

Care and storage

## Cleaning Probertite specimens should only be cleaned dry, using a soft brush or compressed air to remove dust. Contact with water, even distilled, is highly inadvisable, as the mineral is partially soluble in it. ## What to Avoid Contact with water, water vapor, and all chemicals must be strictly avoided. Probertite is sensitive to moisture and can become dull or partially dissolve. It should not be heated, as it contains water in its structure and can dehydrate, leading to its destruction. It is a soft and brittle mineral, susceptible to scratches and mechanical damage. ## Storage Probertite requires storage in dry conditions. It is best kept in a closed, airtight box (e.g., a "membrane box") or in a display case with a desiccant (silica gel). It should be protected from dust and mechanical damage, displayed away from specimens of greater hardness.

Sources

IMA Mineral List (CNMNC)