Allanite-(Sm)

Chemical formula: CaSm(Al₂Fe₂+)(Si₂O₇)(SiO₄)O(OH)

A silicate from the epidote group, a rare mineral characterized by the dominance of samarium in its chemical composition.

## Characteristics Allanite-(Sm) is a rare mineral belonging to the allanite group and, more broadly, to the epidote group. It is a silicate of calcium, samarium, aluminum, and iron. Its name indicates samarium (Sm) as the dominant rare earth element in its structure. It typically occurs as irregular grains or aggregates embedded in rock. Well-formed, tabular or prismatic crystals are rare. It is a dark mineral, ranging in color from brownish-black to black, with a resinous or pitchy luster. ## Physical Properties Its Mohs hardness is 5.5 to 6. It has a luster described as resinous, pitchy, and sometimes vitreous. It is a brittle mineral, with a conchoidal to uneven fracture. It is usually opaque, and only in very thin fragments can it be translucent. Due to the presence of rare earth elements, and sometimes thorium and uranium, it often exhibits radioactivity. This leads to the gradual destruction of its crystal lattice, known as metamictization, which manifests as a glassy appearance and loss of regular internal structure. ## Colors and Varieties This mineral is monochromatic – it occurs in shades from dark brown to black. No color or commercial varieties are distinguished for it; its classification is based solely on chemical composition, specifically on the dominant rare earth element. ## History and Name The name "allanite" honors the Scottish mineralogist Thomas Allan (1777-1833), who first described a mineral from this group. The prefix "-(Sm)" was added according to nomenclature rules to indicate that samarium is dominant in this particular mineral. Allanite-(Sm) was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2003 (IMA2003-064) based on material from Mongolia. ## Applications Due to its extreme rarity, allanite-(Sm) has no industrial applications. It is solely an object of scientific interest and a prized acquisition for specialized collectors of rare earth minerals.

Properties

Mohs hardness: 5.5-6
Luster: Resinous to pitchy, vitreous
Streak: Grayish brown
Density: 3.4-4.2
Cleavage: Poor on {001}
Fracture: Conchoidal to uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Helpful identification features include: black or dark brown color, resinous to pitchy luster, relatively high density, and occurrence in granitic pegmatites. It often exhibits measurable radioactivity. Metamict specimens have a characteristic, glassy appearance and conchoidal fracture. ## Distinguishing from Similar Minerals Final differentiation of allanite-(Sm) from other minerals of the allanite group (e.g., the more common allanite-(Ce)) is impossible without advanced chemical analyses, such as X-ray microanalysis (EDS, WDS), which allow determination of the dominant rare earth element. It can be confused with black tourmaline (schorl), which is harder and has different crystal forms, and also with gadolinite, which has a similar environment of occurrence but usually a greenish streak. ## Crystal Forms It is most commonly found as irregular, anhedral grains embedded in rock or as granular aggregates. Well-formed crystals are exceptionally rare and take on tabular or prismatic forms, typical of minerals from the epidote group.

Geological environment

## Genesis Allanite-(Sm) is an accessory mineral, crystallizing in the late stages from magmas rich in rare earth elements. Its typical environment of formation is granitic and syenitic pegmatites. ## Mineral Associations This mineral coexists with minerals typical of pegmatites, such as quartz, feldspars (microcline, albite), biotite, zircon, and other rare earth element-bearing minerals. ## Localities The only confirmed and described locality of allanite-(Sm), which is also its type locality, is the Ulyn Khuren (Ulan Khuren) deposit in Khovd Aimag in western Mongolia.

Rarity

Very rare

For collectors

## Quality Criteria The greatest value for collectors is the mere fact of possessing such a rare mineral. Specimens with well-formed, even if small, crystals are exceptionally prized. Samples originating directly from the type locality in Mongolia are also highly valued, especially if they coexist with other minerals from that site. ## Popular Localities The only source of collector specimens is the type locality in Mongolia. Material from this location is extremely difficult to acquire on the market.

Care and storage

## Cleaning Allanite-(Sm) specimens should be cleaned very carefully, using a soft brush and, if necessary, distilled water. Strong rubbing, which could damage the surface, especially in the case of metamict specimens, should be avoided. ## What to Avoid This mineral is sensitive to strong acids. Ultrasonic cleaners should not be used, as they can cause brittle specimens to crack. Due to potential radioactivity, prolonged, close contact and storage of large amounts of material in direct proximity should be avoided. It is recommended to wash hands after contact with a specimen. ## Storage Specimens should be stored in separate, padded boxes to prevent scratching by harder minerals or damage to softer minerals in the collection. It is advisable to label the specimen as potentially radioactive.

Sources

IMA Mineral List (CNMNC)