Hydroxylherderite

Chemical formula: CaBe(PO₄)(OH)

Hydroxylherderite is a rare calcium beryllium phosphate, prized by collectors for its well-formed, vitreous crystals of varied colors.

## Characteristics Hydroxylherderite is a hydrated calcium beryllium phosphate belonging to the gadolinite group. It typically forms well-developed, prismatic or tabular crystals, often with complex structures and numerous faces. Occasionally, it occurs as radial aggregates or spherulitic masses. It is a transparent to translucent mineral with a strong, vitreous luster, which gives it an attractive appearance. ## Physical Properties This mineral is characterized by a hardness of 5-5.5 on the Mohs scale. Its density ranges from 2.95 to 3.01 g/cm³. It exhibits weak cleavage in one direction and an uneven to conchoidal fracture. Some specimens may show weak, greenish-yellow fluorescence under ultraviolet light (both shortwave and longwave). ## Colors and Varieties Hydroxylherderite occurs in a wide range of colors. The most common crystals are colorless, pale yellow, greenish-yellow, and grayish-white. Less frequently, light blue, purple, or green specimens are observed. Its color is often pale and delicate. There are no named varieties, and classification is primarily based on color and crystal habit. ## History and Name The mineral's name refers to its chemical composition – the dominance of the hydroxyl (OH) group over fluorine (F) – and its close resemblance to herderite. It was first described as a distinct mineral in 1934 based on samples from the Epprechtstein mine in Bavaria (Germany), although the material was known earlier. ## Uses Due to its rarity and attractive crystals, hydroxylherderite is primarily a mineral of collector's interest. Clear and well-colored crystals are occasionally faceted for advanced gemstone collectors, but its relatively low hardness limits its use in jewelry.

Properties

Mohs hardness: 5-5.5
Color: Colorless, gray, brown, pale yellow, greenish white, light blue, purple; colourless in transmitted light.
Luster: Vitreous
Streak: white
Density: 2.95
Cleavage: On {110}, interrupted.
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Hydroxylherderite is most easily identified by its characteristic crystal shape – often prismatic, flattened, with a complex form and vitreous luster. Its hardness (approximately 5-5.5) and density (approximately 3.0 g/cm³) are helpful indicators. Its occurrence in granitic pegmatites in association with other phosphate minerals is also an important clue. ## Distinguishing from Similar Minerals Hydroxylherderite is difficult to distinguish from herderite without advanced chemical analysis (EDS, XRD), as they form an isomorphic series. Visually, they are practically identical. It can be confused with apatite, which, however, often has a hexagonal crystal habit and is slightly harder. Topaz is much harder (8 on the Mohs scale). Beryllonite, although similar in appearance, has perfect cleavage. ## Crystal Forms Crystals are typically short or long prismatic, often flattened (tabular). Twinning is common, forming complex, pseudo-rhombic shapes. Fan-shaped, radial aggregates, and massive, granular aggregates are also found.

Geological environment

## Genesis Hydroxylherderite is a hydrothermal mineral, typical of the late stages of crystallization in granitic pegmatites. It forms in rock cavities and fissures as a result of solutions rich in phosphorus and beryllium. It is one of the last minerals to crystallize in pegmatitic vugs. ## Mineral Associations It most commonly co-occurs with other pegmatitic minerals such as quartz (especially smoky quartz), albite (particularly cleavelandite), microcline, muscovite, lepidolite, tourmaline (elbaite, schorl), topaz, apatite, beryl (aquamarine, morganite), and other phosphates, e.g., euclase and montebrasite. ## Localities The most important and well-known localities for high-quality hydroxylherderite are in Brazil, in the state of Minas Gerais (e.g., in the Linópolis, Golconda, Itinga regions). Significant specimens also come from Afghanistan (Kunar and Laghman provinces) and Pakistan (Gilgit-Baltistan region). Historical localities include the Epprechtstein mine in Germany (type locality) and Oxford County in Maine, USA.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued hydroxylherderite specimens are characterized by sharp, well-terminated crystals with high transparency and strong luster. An intense, though usually pastel, color is desirable – especially shades of green, yellow, and blue. Large, single crystals or aesthetic groups on matrix (e.g., on quartz or albite) fetch significantly higher prices. The absence of damage and inclusions increases the specimen's value. ## Market Prices The prices of hydroxylherderite vary. Small, single crystals (1-2 cm) can cost from several tens to several hundreds of Polish zlotys. Better quality specimens, especially aesthetic groups on matrix or larger, clear crystals (over 3-4 cm), can reach prices from several hundreds to several thousands of Polish zlotys. Exceptional museum-grade specimens from the best localities are valued much higher. ## Popular Localities Collectors most seek specimens from Brazil (Minas Gerais), which are famous for large, often greenish-yellow and very clear crystals. Specimens from Afghanistan and Pakistan are also highly prized, often exhibiting attractive, light green or colorless forms. Older specimens from Maine (USA) are primarily of historical significance.

Care and storage

## Cleaning Hydroxylherderite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. If necessary, it can be rinsed in distilled water and then thoroughly dried with a soft cloth. Ultrasonic cleaners should be avoided, as they can cause fractures along cleavage planes. ## What to Avoid This mineral is sensitive to strong acids, which can damage it. Avoid sudden temperature changes and prolonged exposure to direct sunlight, which theoretically might cause some lighter specimens to fade. Due to its moderate hardness, it is susceptible to scratches from harder minerals (e.g., quartz). ## Storage Collector's specimens of hydroxylherderite are best stored in separate, padded boxes or on stands in a closed display cabinet to protect them from dust and mechanical damage. Avoid contact with harder minerals to prevent scratches.