Hydroxyledgrewite

Chemical formula: Ca<sub>9</sub>(SiO<sub>4</sub>)<sub>4</sub>(OH)<sub>2</sub>

Hydroxylellestadite is a rare mineral of the humite group, a hydrated calcium silicate, forming microscopic, colorless to pale blue crystals.

## Characteristics Hydroxylellestadite is a hydrated calcium silicate belonging to the humite group. It occurs as very small, tabular or elongated crystals, usually not exceeding 50 micrometers in length. It most often forms aggregates or occurs as inclusions in other minerals, such as calcite. It is a colorless, white, or pale blue mineral with a vitreous luster. ## Physical properties Due to the microscopic size of the crystals, most physical properties, such as hardness or density, have not been precisely measured, but only calculated based on crystallographic data. The calculated density is approximately 2.96 g/cm³. The mineral is transparent to translucent. ## Colors and varieties Observed colors are colorless, white, and very pale blue. No varieties of this mineral have been distinguished. ## History and name Hydroxylellestadite was first described in 2011 by S.N. Britvin and his team. The name refers to its chemical composition – the dominance of the hydroxyl (OH) group – and its structural similarity to ellestadite, another mineral of the humite group. It was approved by the International Mineralogical Association (IMA) under number 2010-048. ## Uses Hydroxylellestadite has no industrial application and is solely an object of scientific and collecting interest for specialists accumulating rare minerals and microminerals.

Properties

Luster
Vitreous
Streak
White
Density
2.96
Cleavage
Good on {100}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of hydroxylellestadite is possible almost exclusively using advanced laboratory methods, such as X-ray diffraction (XRD) and chemical analysis (e.g., EDS). Visually, under high magnification, characteristic, fine, tabular crystals with a vitreous luster can be observed, often in association with calcite and larnite. ## Distinguishing from similar minerals It can be confused with other minerals of the humite group (e.g., ellestadite, kumtyite) or other colorless silicates occurring in a similar environment. Certain distinction requires analysis of the chemical composition, in particular, confirmation of the dominance of the hydroxyl group over fluorine. ## Crystal forms It forms very small, tabular crystals, flattened parallel to {100}, or crystals elongated along the [001] axis. They often occur as aggregates or randomly oriented inclusions in calcite.

Geological environment

## Genesis Hydroxylellestadite forms under conditions of high-temperature contact metamorphism. It forms in skarns and marbles created at the contact of carbonate rocks (limestones) with magmatic intrusions. It is a product of the alteration of calcium silicates in a water-rich environment. ## Mineral associations This mineral co-occurs with calcite, larnite, sharyginite, kumtyite, ellestadite, and other minerals typical of high-temperature skarns. ## Localities The type and so far only confirmed locality of hydroxylellestadite is the Hatrurim Formation in Israel, also known as the "Mottled Zone". This is a complex of metamorphic rocks known for the occurrence of many rare minerals.

Rarity

Very rare

For collectors

## Quality criteria As a micromineral, hydroxylellestadite is valued primarily for its confirmed identity and the abundance of its occurrence on a given rock fragment (matrix). The most desirable specimens are those with the largest possible, well-formed crystals that can be observed under a microscope. A contrasting matrix and the presence of rare associated minerals increase the scientific and collecting value of the specimen. ## Popular localities The Hatrurim Formation in Israel is the only source of collectible specimens. Specimens from this locality are the standard for this mineral.

Care and storage

## Cleaning Hydroxylellestadite specimens are extremely small and delicate, often intergrown with other minerals (matrix). Mechanical cleaning is not recommended. If dust removal is necessary, compressed air can be used from a safe distance. Generally, specimens of this type should be left undisturbed. ## What to avoid Avoid contact with all chemicals, especially acids, which can react with the mineral or co-occurring calcite. Protect the specimen from ultrasound, high temperatures, and sudden temperature changes. ## Storage Micromineralogical specimens, such as hydroxylellestadite, are best stored in specialized, sealed "micromount" boxes, which protect them from dust, mechanical damage, and moisture. Display should only be done under a microscope.

Sources

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