Keiviite-(Yb)

Chemical formula: Yb<sup>3+</sup><sub>2</sub>Si<sub>2</sub>O<sub>7</sub>

Keiviite-(Yb) is a very rare ytterbium silicate, forming microscopic, colorless to white crystals with a vitreous luster.

## Characteristics Keiviite-(Yb) is an extremely rare mineral from the sorosilicate group, chemically classified as ytterbium disilicate. It occurs as very small, prismatic or tabular crystals, rarely exceeding 1 mm in length. It typically forms radial aggregates or appears as single, isolated crystals. It is colorless to white, sometimes with a pinkish or yellowish tint. Due to its microscopic size, it is a mineral of interest mainly to specialized collectors and scientists. ## Physical Properties The mineral is characterized by a hardness in the range of 4-5 on the Mohs scale. It has a vitreous luster and is transparent to translucent. Its density is relatively high, approximately 6.16 g/cm³, which is typical for minerals rich in rare earth elements, such as ytterbium. ## Colors and Varieties Keiviite-(Yb) is usually colorless or white. Some specimens may exhibit a pale pink or pale yellow hue. No named varieties of this mineral have been distinguished. ## History and Name The mineral's name refers to its discovery locality – the Keivy Massif on the Kola Peninsula in Russia. The "-(Yb)" suffix in the name indicates the dominant rare earth element in its composition, which is ytterbium. The mineral was described and approved by the International Mineralogical Association (IMA) in 1986. Its discoverers were Russian mineralogists A.V. Voloshin, Ya.A. Pakhomovsky, and F.N. Tyusheva.

Properties

Mohs hardness
4-5
Luster
Vitreous
Streak
White
Density
6.16
Cleavage
Good on {110}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of keiviite-(Yb) in the field is practically impossible due to its size and rarity. It requires advanced laboratory techniques, such as energy-dispersive spectroscopy (EDS/EDX) to confirm chemical composition (ytterbium dominance) and X-ray diffraction (XRD) for crystal structure analysis. Visually, it might be suspected as small, colorless, prismatic crystals with a vitreous luster in amazonite pegmatites. ## Distinguishing from Similar Minerals It can be confused with other colorless accessory minerals found in pegmatites, such as quartz, albite, phenakite, or topaz. Differentiation is almost exclusively possible based on chemical analysis. Keiviite-(Yb) is distinguished by its very high density compared to most visually similar silicate minerals. ## Crystal Forms It forms elongated, prismatic crystals, often terminated by pointed pyramids. Crystals are usually twinned, giving them a characteristic appearance. They occur as single crystals or form small, radial aggregates.

Geological environment

## Genesis Keiviite-(Yb) is a pegmatitic mineral. It forms in the late stages of crystallization in granitic alkaline pegmatites, especially in their zones rich in amazonite (a green variety of microcline). Its crystallization is associated with a high concentration of rare earth elements, particularly heavy lanthanides like ytterbium. ## Mineral Associations This mineral co-occurs with other minerals typical of rare earth element pegmatites. It is most often accompanied by albite, quartz, microcline (especially amazonite), fluorite, bastnäsite, xenotime, fergusonite, and other minerals from the keiviite group. ## Localities The most important and type locality (localitas typica) for keiviite-(Yb) is the Western Keivy Massif on the Kola Peninsula in the Murmansk Oblast, Russia. This is the primary and practically only location from which well-documented specimens of this mineral originate.

Rarity

Very rare

For collectors

## Quality Criteria The quality of keiviite-(Yb) specimens is primarily assessed based on the size and development of the crystals. The most valued specimens are those with clearly visible, sharply terminated prismatic crystals forming aesthetic, radial groups. Association with a contrasting matrix mineral, such as intensely green amazonite, is also important, enhancing the visual appeal of the specimen. The clarity and transparency of the crystals also affect their value, although most specimens are very small. ## Popular Localities The only source of collector specimens is the Keivy Massif on the Kola Peninsula in Russia. Specimens from this locality are considered classic for this mineral.

Care and storage

## Cleaning Keiviite-(Yb) specimens are typically microscopic and embedded in the host rock. Cleaning is not recommended to avoid damaging the delicate crystals. If absolutely necessary, compressed air can be used to remove dust. Contact with water and chemicals should be avoided. ## What to Avoid Avoid all acids and strong bases, which can react with the mineral. Due to its brittleness, it should be protected from impacts, vibrations, and ultrasonics. There are no specific recommendations regarding light or temperature, but standard protection from extreme conditions is advisable. ## Storage The safest way to store the mineral is to leave it in its original host rock and place it in a sealed "micromount" box, which protects it from dust and mechanical damage.

Sources

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