Hilgardite-4M

Chemical formula: Ca₂B₅O₉Cl·H₂O

Hilgardite-4M is a rare calcium borate, forming characteristic, triclinic crystals with a vitreous luster, prized by collectors of rare minerals.

## Characteristics Hilgardite-4M is a hydrated calcium borate with chlorine, belonging to the hilgardite group. This mineral typically forms small, but well-formed crystals with a characteristic, wedge-shaped habit. Crystals are most often colorless or white, exhibiting a vitreous luster. It is a transparent to translucent mineral, which, combined with its rarity, makes it an interesting object for advanced collectors. ## Physical Properties The mineral is characterized by a hardness of 5 on the Mohs scale. Its luster is vitreous, and its streak is white. It is brittle and exhibits an uneven fracture. The density has not been precisely determined in sources. ## Colors and Varieties Hilgardite-4M occurs mainly as colorless crystals, less frequently white or with a delicate, yellowish tint. No named color varieties or commercial varieties are distinguished. ## History and Name The name "hilgardite" honors Eugene Woldemar Hilgard (1833-1916), an American geologist and agricultural chemist. The suffix "-4M" refers to the specific monoclinic polytype of this mineral, described by S. Ghose and C. Wan in 1979. The mineral was approved by the IMA in 1978. ## Uses Due to its extreme rarity and small crystal sizes, hilgardite-4M has no industrial application. Its significance is solely scientific and collectible.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: White
Density: 0
Cleavage: {010} Good, {100} Good
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification A characteristic feature of hilgardite-4M is its small, colorless, wedge-shaped crystals with a vitreous luster. A hardness of 5 on the Mohs scale and solubility in acids are also important diagnostic clues. However, identification often requires advanced research methods. ## Distinguishing from Similar Minerals It can be confused with other colorless borates or evaporite minerals occurring in similar environments. It is distinguished from calcite by its higher hardness and lack of reaction with cold, dilute hydrochloric acid. Precise differentiation from other hilgardite polytypes (e.g., hilgardite-1Tc) is almost exclusively possible using X-ray diffraction (XRD) analysis. ## Crystal Forms It forms tabular or wedge-shaped crystals, often gathered in small, radial or chaotic aggregates. Crystals rarely exceed a few millimeters in size.

Geological environment

## Genesis It is a secondary mineral, formed within salt domes (salt diapirs). It forms in evaporite rocks, in paragenesis with other borates and salt minerals, under conditions of low temperatures and pressures. ## Mineral Associations Hilgardite-4M co-occurs with minerals such as boracite, halite, anhydrite, sylvite, and other rare borates. ## Localities The most important and classic locality for this mineral is the Choctaw salt dome in Iberville Parish, Louisiana, USA. It has also been reported in the Penobsquis salt dome in New Brunswick, Canada.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens of hilgardite-4M are those with sharp, well-formed, transparent crystals, set on a contrasting matrix. Due to the small size of the crystals, their perfect form and lack of damage are important. The size of the crystals, even if only a few millimeters, significantly increases the value of the specimen. ## Popular Localities Specimens most desired by collectors definitely come from the type locality – the Choctaw salt dome in Louisiana, USA. This is the classic and practically only commercial source of this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to the water content in its structure and solubility in acids, contact with water and any chemicals should be avoided. Dry cleaning is the safest method. ## What to Avoid The mineral is sensitive to acids, in which it dissolves. Contact with water, cleaning agents, and sudden temperature changes should be avoided. The crystals are brittle and susceptible to mechanical damage. ## Storage It is recommended to store specimens in stable conditions, away from moisture and direct sunlight. It is best to keep them in closed display boxes or cabinets to protect them from dust and physical damage.

External references

https://www.mindat.org/min-9763.html