Hydroxylellestadite

Chemical formula: Ca₅(SiO₄)₁.₅(S⁶⁺O₄)₁.₅(OH)

Hydroxyl-ellestadite is a rare mineral from the apatite group, being a calcium silicate and sulfate, most often forming massive aggregates or small, hexagonal crystals.

## Characteristics Hydroxyl-ellestadite is a complex silicate-sulfate belonging to the apatite supergroup. It rarely forms well-developed, hexagonal, prismatic crystals, which are usually small. Most often, it occurs as granular, massive, or radial aggregates. Its appearance is inconspicuous, and identification requires advanced research methods. ## Physical properties The mineral is characterized by a hardness of 4.5 on the Mohs scale. It has a vitreous to resinous luster. It is brittle, and its density is approximately 3.05 g/cm³. It is a relatively light mineral. ## Colors and varieties Hydroxyl-ellestadite is usually colorless, white, pale gray, or pale yellow. No colored or commercial varieties are distinguished. ## History and name The mineral's name refers to its chemical composition – the dominance of the hydroxyl (OH) group – and its relation to another mineral from the same group, ellestadite. It was recognized by the International Mineralogical Association (IMA) in 1971. It was originally described from Crestmore, California, USA. ## Applications Due to its rarity and usually small crystal sizes, hydroxyl-ellestadite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or minerals from the apatite group.

Properties

Mohs hardness: 4.5
Color: Light orange; Pink, Purple-grey
Luster: Vitreous to resinous
Streak: White
Density: 3.068
Cleavage: Imperfect on {0001}
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Identifying hydroxyl-ellestadite based on visual characteristics is practically impossible and requires specialized analyses, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). A clue might be the hexagonal habit of crystals (if visible) and co-occurrence with other skarn minerals. ## Distinguishing from similar minerals It can be confused with other minerals from the apatite group (e.g., fluorapatite, hydroxyapatite), as well as with quartz or calcite, if it occurs in massive aggregates. It differs from them in hardness, density, and reaction with acids (unlike calcite, it does not react violently). However, definitive differentiation requires laboratory analysis. ## Crystal forms It forms short, prismatic crystals with a hexagonal outline. It is more commonly found in the form of granular, massive aggregates or as radial aggregates.

Geological environment

## Genesis Hydroxyl-ellestadite is a mineral typical of contact metamorphic environments, especially skarns. It forms as a result of the interaction of hot, silica- and sulfur-rich solutions with carbonate rocks (limestones, marbles). ## Mineral associations It often co-occurs with other skarn minerals, such as calcite, diopside, wollastonite, vesuvianite, garnets (mainly grossular and andradite), as well as other minerals from the apatite group. ## Localities The most important and classic localities for this mineral are Crestmore, California (USA), where it was discovered. It is also known from the Commercial mine in Tooele County, Utah (USA), as well as from several sites in Japan (e.g., Fuka mine, Okayama Prefecture) and Romania (Ocna de Fier mine).

Rarity

Rare

For collectors

## Quality criteria The most valued by collectors are specimens with well-formed, sharp, hexagonal crystals, even if they are small. Transparency and lack of damage are important. A contrasting placement on a rock matrix, especially in the company of other rare skarn minerals, significantly increases the value of the specimen. ## Popular localities Specimens originating from the type locality, Crestmore, California, have particular historical value. Well-formed crystals from Japan are also highly prized.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Distilled water may be used, but prolonged soaking should be avoided. After wet cleaning, the specimen should be thoroughly dried. ## What to avoid Contact with acids, which can damage it, must be absolutely avoided. The mineral is brittle and sensitive to impacts and sudden temperature changes. It should not be cleaned in ultrasonic cleaners. ## Storage It is recommended to store specimens in separate, padded collector boxes to prevent scratching by harder minerals. It should be protected from dust and moisture.