Karupmøllerite-Ca

Chemical formula: (Na,Ca,K)₂Ca(Nb₅+,Ti₄+)₄(Si₄O₁₂)₂(O,OH)₄·₇H₂O

Karupmøllerite-Ca is a rare silicate mineral, forming acicular or fibrous aggregates with a silky luster, found in nepheline syenite pegmatites.

## Characteristics Karupmøllerite-Ca is a complex silicate of niobium, titanium, sodium, and calcium, belonging to the labuntsovite group. It occurs as aggregates of small, acicular or fibrous crystals, which can form radial aggregates, rosettes, or felt-like mats. Individual crystals are usually very small, rarely exceeding 1 mm in length. The mineral is brittle. ## Physical Properties Crystals exhibit a silky luster, and in the case of transparent forms, a vitreous luster. It is a relatively soft mineral, with a hardness of about 5 on the Mohs scale. Its density is approximately 2.86 g/cm³. It is transparent to translucent. ## Colors and Varieties Karupmøllerite-Ca is most commonly colorless or white. It can also take on pale yellow, cream, or grayish hues. No distinct color varieties or commercial names are recognized. ## History and Name The mineral was discovered in a syenite pegmatite of the Ilímaussaq igneous complex in Greenland. It was described in 2002 by Ole V. Petersen and collaborators. Its name honors the Danish mineral collector Svend Karup-Møller (born 1936), and the "-Ca" suffix indicates the dominance of calcium in its chemical composition. It is the calcium analog of gutkovaite-Mn.

Properties

Mohs hardness: 5
Luster: Vitreous, Silky
Streak: White
Density: 2.86
Cleavage: Perfect on {100}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification A characteristic feature of karupmøllerite-Ca is its habit – it forms aggregates of very fine, acicular or fibrous crystals, often in the form of radial aggregates. The silky luster of these aggregates is also helpful in identification. Its occurrence in the specific environment of nepheline syenite pegmatites is a key indicator. ## Distinguishing from Similar Minerals It can be confused with other minerals from the labuntsovite group or other acicular zeolite-like minerals, such as natrolite. Final differentiation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS), due to visual similarities and co-occurrence with other rare silicates. ## Crystal Forms Crystals are prismatic, elongated, and acicular, often flattened. They usually occur as radial or tangled aggregates. More rarely, they form small, individual crystals in vugs.

Geological environment

## Genesis Karupmøllerite-Ca is a hydrothermal mineral, crystallizing in the late stages of the evolution of pegmatites associated with nepheline syenites. It forms in rock cavities and fractures, often on the surface of other, earlier-formed minerals. ## Mineral Associations This mineral co-occurs with other rare minerals of alkaline pegmatites, such as aegirine, microcline, albite, natrolite, eudialyte, steenstrupine-(Ce), neptunite, lorenzenite, sérandite, and other minerals from the labuntsovite group. ## Localities The most important and type locality for karupmøllerite-Ca is the Ilímaussaq igneous complex in southern Greenland, from which the best-formed specimens originate. It is also known from the Khibiny Massif on the Kola Peninsula in Russia. These are the only confirmed localities for this mineral.

Rarity

Very rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, radial aggregates of acicular crystals, forming aesthetic "hedgehog" or "sunburst" patterns on a contrasting rock matrix. The size and richness of these aggregates, as well as the absence of damage to the delicate needles, are important. Purity (lack of coatings) and association with other rare minerals also increase the specimen's value. ## Popular Localities By far, the most sought-after specimens come from the type locality – the Ilímaussaq complex in Greenland. Material from the Kola Peninsula is much rarer on the collector's market.

Care and storage

## Cleaning Cleaning with compressed air is recommended to remove dust. Due to the delicate and fibrous nature of the crystals, contact with water and brushes, which could mechanically damage the specimen, should be avoided. ## What to Avoid All chemicals, especially acids, should be avoided. The mineral is susceptible to mechanical damage, so it should be protected from impacts and vibrations. It should not be heated or exposed to prolonged light. ## Storage Karupmøllerite-Ca specimens should be stored under stable conditions, in closed "micromount" boxes that protect them from dust, moisture, and physical damage. They are best displayed under magnification.

Sources

IMA Mineral List (CNMNC)