Hydroxylchondrodite

Chemical formula: Mg₅(SiO₄)₂(OH)₂

Hydroxylchondrodite is a rare magnesium silicate from the humite group, forming small, vitreous crystals that are colorless to yellow or brown.

## Characteristics Hydroxylchondrodite is a mineral belonging to the humite group, being the hydroxyl analogue of chondrodite. It usually occurs in the form of small, granular aggregates or single, poorly formed crystals. Its appearance is often inconspicuous, and identification requires advanced research methods. Crystals, when well-formed, tend to be flattened or elongated. ## Physical Properties This mineral is characterized by a hardness of 6-6.5 on the Mohs scale, which makes it relatively resistant to scratching. The luster is typically vitreous, and the mineral can be transparent to translucent. The density ranges from 3.16-3.21 g/cm³. ## Colors and Varieties Hydroxylchondrodite is most often colorless, white, pale yellow, yellow, or brown. No named varieties are distinguished, and color variability depends on trace element impurities. ## History and Name The mineral's name refers to its chemical composition – the presence of hydroxyl (OH) groups – and its structural similarity to chondrodite. It was formally described and recognized as a distinct mineral species relatively recently, reflecting the difficulty in distinguishing it from other members of the humite group without detailed chemical analysis. ## Applications Hydroxylchondrodite has no industrial applications. Its significance is purely scientific and as a collector's item, as a rare member of the humite group.

Properties

Mohs hardness: 6-6.5
Luster: Vitreous
Streak: White
Density: 3.16-3.21
Cleavage: Poor on {001}
Fracture: Uneven to conchoidal
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of hydroxylchondrodite in the field is practically impossible without specialized equipment. In a collection, it can be suspected based on its co-occurrence with other metamorphic minerals (e.g., in skarns), granular form, vitreous luster, and color. Definitive identification requires chemical analysis (e.g., EDS) or diffraction methods (XRD) to confirm its composition and structure. ## Distinguishing from Similar Minerals Hydroxylchondrodite is extremely difficult to distinguish from other minerals of the humite group, such as chondrodite, humite, clinohumite, or hydroxylclinohumite. They differ mainly in the ratio of (OH) and fluorine (F) groups and subtle differences in crystal structure, which are impossible to determine visually. It can also be confused with olivine, from which it differs by slightly lower physical parameters and occurrence environment. ## Crystal Forms Crystals are rare and usually small, with a tabular or short prismatic habit. It most often occurs as grains or granular aggregates within the rock mass.

Geological environment

## Genesis Hydroxylchondrodite is a mineral typical of metamorphic rocks that have undergone contact metamorphism. It forms in magnesian skarns, which originate at the contact of magmatic intrusions (e.g., granitoids) with carbonate sedimentary rocks, such as dolomites and limestones. It forms under conditions of relatively high temperature and pressure, in an environment rich in magnesium and silica, with low fluorine content. ## Mineral Associations This mineral often co-occurs with other minerals typical of skarns, such as calcite, dolomite, spinel, phlogopite, diopside, forsterite (olivine), as well as other minerals from the humite group (chondrodite, clinohumite). ## Localities The most important and well-documented localities of hydroxylchondrodite worldwide include the Tilly Foster mine in Brewster, New York (USA), where it was first thoroughly studied. It also occurs in the Kovdor carbonatite complex on the Kola Peninsula in Russia and in iron ore mines in the Värmland region of Sweden (e.g., Långban).

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of hydroxylchondrodite is limited due to its rarity and generally inconspicuous appearance. The most prized specimens are those with well-formed, sharp crystals of distinct color (intensely yellow or brown) and good transparency. Association with other, contrasting minerals and precise locality designation, confirmed by analysis, are also crucial. ## Popular Localities Specimens from classic localities, such as Tilly Foster (USA) or Kovdor (Russia), are most sought after by specialized collectors of rare minerals and minerals from the humite group.

Care and storage

## Cleaning Hydroxylchondrodite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Always dry the specimen thoroughly after cleaning. ## What to Avoid Avoid contact with acids and strong chemicals, which can damage the mineral's surface. It should also not be subjected to ultrasonic cleaning. Due to the possible presence of inclusions and internal stresses, sudden temperature changes should be avoided. ## Storage Hydroxylchondrodite specimens, especially those with well-formed crystals, should be stored in separate boxes or display cases to prevent abrasion and mechanical damage. It is not sensitive to light, but as with most minerals, it is advisable to avoid prolonged exposure to direct sunlight.

Sources

IMA Mineral List (CNMNC)