Isoclasite

Chemical formula: Ca<sub>2</sub>PO<sub>4</sub>(OH)·2H<sub>2</sub>O

Isoclasite is a rare, hydrated calcium phosphate, forming colorless or white, acicular crystals and brittle aggregates.

## Characteristics Isoclasite is a hydrated calcium phosphate, occurring extremely rarely. It forms colorless to white, prismatic or acicular crystals, which often arrange themselves into radial or fibrous aggregates. Individual crystals are usually very small, rarely exceeding a few millimeters in length. This mineral is brittle and fragile. ## Physical Properties Isoclasite crystals exhibit perfect cleavage in one direction, causing them to easily break into fine fibers or lamellae. It has a hardness of about 1.5 on the Mohs scale, making it a very soft mineral. Its density is low, approximately 2.2 g/cm³. The luster is vitreous, and on cleavage surfaces, it can be pearly. ## Colors and Varieties Isoclasite is typically colorless or white. No named color varieties or commercial varieties are distinguished. ## History and Name The name isoclasite comes from the Greek words *isos* (equal) and *klasis* (to break), referring to its perfect cleavage in one direction. The mineral was first described in 1870 by Alfred Lewis Oliver Legrand Des Cloizeaux based on samples from the mine in Jáchymov, Czech Republic. ## Uses Due to its extreme rarity, small crystal size, and poor physical properties, isoclasite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and for scientists.

Properties

Mohs hardness
1.5
Luster
Vitreous, Pearly
Streak
White
Density
2.205
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Isoclasite can be identified by its characteristic white, acicular or prismatic crystals forming radial aggregates. A key feature is its extreme softness (hardness 1.5) and perfect cleavage, causing easy separation into fibers. It is one of the most delicate minerals. ## Distinguishing from Similar Minerals It can be confused with other white, fibrous phosphates or sulfates, such as brushite or gypsum. It differs from gypsum by its crystal form and occurrence environment. From brushite, which is harder (2.5), it differs primarily by its extreme softness. However, final confirmation requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms Crystals are elongated, prismatic, often acicular in habit. They usually occur as radial, stellate, or fibrous aggregates, as well as thin crusts and coatings.

Geological environment

## Genesis Isoclasite is a secondary mineral, forming in the oxidation zones of ore deposits, especially those containing uranium. It forms as a result of the interaction of phosphate-rich solutions with calcareous rocks or other calcium-rich minerals. It is a low-temperature mineral. ## Mineral Associations It most often co-occurs with other secondary phosphate and uranium minerals. Typical associated minerals include brushite, hydroxyapatite, strengite, as well as gypsum and calcite. ## Localities The most important and classic locality, from which the best specimens originate, is Jáchymov (St. Joachimsthal) in the Czech Republic. This mineral has also been reported in small quantities in the Wheal Owles mine in Cornwall (United Kingdom) and in several locations in Germany (e.g., Thuringia).

Rarity

Very rare

For collectors

## Quality Criteria The most prized isoclasite specimens are those with well-formed, radial aggregates of acicular crystals, contrasting with a dark matrix. Due to the small size of the crystals, specimens are usually of the "micromount" type. Purity (lack of damage to delicate needles) and richness of aggregates are key to the specimen's value. ## Popular Localities By far the most sought-after specimens by collectors come from the historic type locality - Jáchymov in the Czech Republic. Material from other places is extremely rare and is a great find for specialized collectors.

Care and storage

## Cleaning Isoclasite specimens are extremely delicate and sensitive to water. Mechanical cleaning is highly risky. If necessary, a very soft brush can be used to remove loose dust, with extremely careful movements. ## What to Avoid Contact with water, which can dissolve or damage the mineral, must be absolutely avoided. All chemicals, ultrasonic cleaners, and temperature changes should be avoided. The mineral is very soft and brittle, susceptible to mechanical damage from the slightest touch. ## Storage Isoclasite requires storage in stable conditions, in a closed, padded "micromount" box, away from vibrations, dust, and moisture. It is not suitable for display in open showcases.

Sources

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