Hydroxylwagnerite

Chemical formula: Mg<sub>₂</sub>PO<sub>₄</sub>(OH)

Hydroxylwagnerite is a rare magnesium phosphate, distinguished by its wine-yellow color and occurrence in high-pressure metamorphic rocks.

## Characteristics Hydroxylwagnerite is a basic magnesium phosphate belonging to the wagnerite group. It is the hydroxyl (OH) analogue of wagnerite. It most often forms irregular, anhedral grains intergrown in the rock or occurs as massive aggregates. Well-formed crystals are very rare and usually take the form of short, prismatic columns. The mineral is characterized by a vitreous luster and colors in shades of yellow and brown. ## Physical Properties The Mohs hardness of hydroxylwagnerite is 5. The mineral is relatively dense, with a specific gravity ranging from 3.14 to 3.21 g/cm³. It exhibits good cleavage in one direction. It is transparent to translucent, with a vitreous luster. ## Colors and Varieties The typical coloration of hydroxylwagnerite includes shades from wine-yellow, through honey-yellow, to yellowish-brown and pale reddish-pink. There are no named color varieties or commercial varieties. ## History and Name The name of the mineral refers to its chemical composition – the presence of the hydroxyl (OH) group – and its close relationship to wagnerite. The mineral was first described in 1964, but its status was debated. It was finally recognized as a distinct mineral species by the International Mineralogical Association (IMA) as the hydroxyl analogue of wagnerite. ## Applications Due to its rarity, hydroxylwagnerite has no industrial applications. It is solely an object of scientific interest and a valued acquisition for advanced systematic and regional mineral collections.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: White
Density: 3.14-3.21
Cleavage: Good on {010}, distinct on {110}
Fracture: Uneven to Subconchoidal
Transparency: Transparent to Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Helpful features in identifying hydroxylwagnerite include its characteristic wine-yellow to honey-yellow color, vitreous luster, and hardness (5 on the Mohs scale). However, a key indicator is its specific environment of occurrence – high-pressure metamorphic rocks, such as eclogites or whiteschists. ## Distinguishing from Similar Minerals Hydroxylwagnerite is sometimes confused with minerals from the humite group (e.g., clinohumite, chondrodite), which occur in similar geological conditions and have similar colors. However, these minerals often have a more orange hue and different optical properties. It can also be confused with titanite, which is slightly harder and crystallizes in a different system. Definitive differentiation most often requires advanced studies, such as chemical analysis or X-ray diffraction (XRD). ## Crystal Forms Well-formed crystals are extremely rare and take the form of short, thick prisms. Much more often, this mineral is found as irregular grains without distinct faces, intergrown in the rock matrix, or as massive aggregates.

Geological environment

## Genesis Hydroxylwagnerite is a mineral typical of rocks that have undergone metamorphism under very high pressure and moderate to high temperatures. It forms in eclogites, whiteschists, and in serpentinites associated with dolomitic marbles. ## Mineral Associations This mineral often coexists with other high-pressure minerals, such as kyanite, talc, ellenbergerite, clinohumite, as well as rutile, magnesite, and apatite. ## Localities The most important occurrences of hydroxylwagnerite are in the Alps. The type locality is Caselette in the Susa Valley (Piedmont, Italy). Other known occurrences include the Dora-Maira Massif in the Western Italian Alps, the Zermatt-Saas Zone on the border of Switzerland and Italy, and the Kokchetav Massif in Kazakhstan.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized hydroxylwagnerite specimens are those with well-formed, even if small, prismatic crystals. The intensity and purity of the color are important – vivid, wine-yellow shades are desirable. Specimens where hydroxylwagnerite occurs in association with other rare metamorphic minerals, forming aesthetic associations, are also highly valued. ## Popular Localities The Alpine localities, particularly the Dora-Maira Massif in Italy, are considered the source of the best quality collector specimens. Specimens from this region are sought after due to their good crystal development and mineral associations.

Care and storage

## Cleaning Hydroxylwagnerite specimens should be cleaned very carefully, using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they can damage the specimen, especially if it has internal fractures. ## What to Avoid The mineral is sensitive to strong acids, which can etch it. Despite a hardness of 5, it should be protected from contact with harder minerals (e.g., quartz) to avoid scratches. It should not be heated or exposed to rapid temperature changes. ## Storage It is recommended to store specimens in separate, padded boxes or display cases to prevent abrasions and mechanical damage. Stable, dry conditions, away from chemicals, should be ensured.

Sources

IMA Mineral List (CNMNC)