Hydroxylgugiaite

Chemical formula: (Ca<sub>3</sub>&#9744;)<sub>Σ4</sub>(Si<sub>3.5</sub>Be<sub>2.5</sub>)<sub>Σ6</sub>O<sub>11</sub>(OH)<sub>3</sub>

Hydroxylgugiaite is a very rare mineral of the melilite group, being the hydroxyl analogue of gugiaite, found in vesuvianite skarns.

## Characteristics Hydroxylgugiaite is an extremely rare silicate mineral of the melilite group. It occurs as very small, colorless to pale yellow grains, usually less than 0.5 mm in size, forming aggregates in skarns. Due to the small size of the crystals, its visual features are difficult to observe without specialized equipment, such as a microscope. ## Physical Properties This mineral has a hardness of approximately 5 on the Mohs scale. It has a vitreous luster and is transparent. Its density, calculated based on the chemical formula and unit cell parameters, is approximately 2.96 g/cm³. ## History and Name The name "hydroxylgugiaite" refers to its chemical composition – it is the hydroxyl (OH) analogue of gugiaite, in which the hydroxyl group dominates over fluorine. It was first described by Japanese mineralogists in 2014 based on material from the Fuka mine in Okayama Prefecture, Japan.

Properties

Mohs hardness
5
Luster
Vitreous
Streak
White
Density
2.96
Cleavage
Good on {001}
Fracture
Uneven
Transparency
Transparent
Crystal system
Tetragonal

Diagnostic features

## Identification Identification of hydroxylgugiaite is impossible without advanced analytical techniques. This requires the use of methods such as X-ray diffraction (XRD) and chemical analysis (e.g., electron microprobe) to confirm the crystal structure and chemical composition, particularly the presence of beryllium and the dominance of the hydroxyl group over fluorine. ## Distinguishing from Similar Minerals This mineral is visually indistinguishable from other minerals of the melilite group, such as gugiaite, leucophanite, or melilite. Differentiation relies solely on detailed chemical and structural analysis. ## Crystal Forms Hydroxylgugiaite forms irregular, anhedral (lacking well-formed faces) grains not exceeding 0.5 mm in size. It occurs as aggregates of grains within the host rock.

Geological environment

## Genesis Hydroxylgugiaite forms in skarn environments, which develop at the contact of granitic intrusions with limestones. It is a product of metasomatic processes occurring at high temperatures. ## Mineral Associations This mineral coexists with other minerals typical of skarns, such as vesuvianite, wollastonite, diopside, grossular (garnet), as well as other rare beryllium minerals like gugiaite and leucophanite. ## Localities The only confirmed occurrence of hydroxylgugiaite in the world is its type locality – the Fuka mine, near Takahashi City, Okayama Prefecture, on Honshu Island, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, hydroxylgugiaite is not evaluated according to typical criteria such as crystal size or form. The collector's value of a specimen lies in its extreme rarity and the richness of the mineral association on the rock fragment. Scientific confirmation of the mineral's presence on the specimen is crucial. ## Popular Localities The only source of specimens is the Fuka mine in Japan, which is known for the occurrence of many rare skarn minerals. Specimens from this locality are highly prized by collectors specializing in rare minerals and microminerals.

Care and storage

## Cleaning Due to its extreme rarity and the small size of crystals embedded in the host rock, mechanical cleaning is not recommended. Any attempts at cleaning may lead to damage or loss of material. Specimens should be left undisturbed. ## What to Avoid Avoid contact with chemicals, acids, and ultrasonic cleaners. Do not subject the specimen to sudden temperature changes. As a hydroxyl mineral, it may be sensitive to conditions causing dehydration. ## Storage Hydroxylgugiaite specimens, typically in the form of microscopic grains within rock, should be stored under stable conditions, away from dust and moisture. The safest way to store them is in a sealed "micromount" box or a specialized mineralogical display case.

Sources

Read more