Stillwellite-(Ce)

Chemical formula: Ce³⁺BSiO₅

Stillwellite-(Ce) is a rare borosilicate of cerium, forming characteristic barrel-shaped crystals with a vitreous or resinous luster.

## Characteristics Stillwellite-(Ce) is a mineral belonging to the silicate group, containing cerium and boron in its composition. It typically forms well-developed, hexagonal crystals with a barrel-shaped or prismatic habit, often terminated by pyramids. Crystals can be striated transversely to the main axis. It also occurs in granular or massive aggregates. Its appearance is distinctive, and its vitreous to resinous luster gives it a specific look. ## Physical Properties This mineral is characterized by a hardness of 6.5 on the Mohs scale, making it relatively resistant to scratching. It has a vitreous to resinous luster. It is a rather heavy mineral, with a density of approximately 4.57-4.7 g/cm³. It is transparent to translucent. It does not exhibit cleavage, and its fracture is conchoidal to uneven. ## Colors and Varieties Stillwellite-(Ce) most commonly occurs in colors ranging from honey-yellow, through various shades of brown, to reddish-brown and greenish. Colorless, gray, or pink specimens are also found. The coloration is often unevenly distributed, forming streaks or patches within a single crystal. No named commercial or color varieties are distinguished. ## History and Name The mineral's name honors Frank Leslie Stillwell (1888-1963), an Australian geologist who first identified this mineral in 1953 during studies of uranium ores at the Mary Kathleen mine in Queensland, Australia. The formal description was published by J. McAndrew and T. R. Scott in 1955. The suffix "-(Ce)" in the name indicates the dominance of cerium in the chemical composition, in accordance with rare-earth mineral nomenclature rules. ## Uses Stillwellite-(Ce), due to its rarity and small accumulations, has no industrial significance. However, it is a sought-after and valued collector's stone, especially well-formed and transparent crystals.

Properties

Mohs hardness: 6.5
Color: Pale violet-gray, light pink, brownish yellow, orange
Luster: Vitreous to resinous
Streak: White
Density: 4.57
Cleavage: One imperfect
Fracture: Conchoidal
Transparency: Transparent,Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Stillwellite-(Ce) can be identified by its characteristic crystal habit – hexagonal, often barrel-shaped prisms. Important features also include high hardness (6.5), vitreous to resinous luster, relatively high density, and typical brown, yellow, or greenish coloration. The absence of cleavage and conchoidal fracture are also helpful in identification. ## Distinguishing from Similar Minerals Stillwellite-(Ce) is sometimes confused with certain minerals of similar habit and color. It is distinguished from quartz by its significantly higher density and slightly lower hardness. From apatite, which forms similar hexagonal prisms, it is much harder (apatite has a hardness of 5). Tourmaline, although sometimes similar, usually has distinct striations along the crystal, whereas in stillwellite, striations are transverse or absent. It is distinguished from zircon by its lower hardness and different crystallographic system. ## Crystal Forms Crystals are trigonal but exhibit pseudohexagonal (six-sided) symmetry. They most often form short or elongated prisms, often barrel-shaped, tapering towards the ends. Prism faces can be horizontally striated. Crystals are terminated by pyramidal faces. It also occurs in granular aggregates and radial aggregates.

Geological environment

## Genesis Stillwellite-(Ce) is a hydrothermal or metasomatic mineral, associated with rocks rich in rare earth elements. It forms in skarns, carbonatites, and altered alkaline pegmatites. Its crystallization occurs under conditions of elevated temperature and pressure, in the presence of solutions rich in boron, silica, and rare earth elements, primarily cerium. ## Mineral Associations This mineral often co-occurs with other rare earth minerals and minerals typical of metasomatic environments. Its most common associations include: allanite-(Ce), uraninite, garnets (especially andradite), epidote, diopside, calcite, quartz, titanite, fluorite, and various amphibole group minerals. ## Localities The most important and historical locality, from which large and well-formed crystals originate, is the Mary Kathleen mine in Queensland, Australia. Other significant localities worldwide include deposits in the Baikonur region of Kazakhstan, alkaline complexes on the Kola Peninsula in Russia (e.g., Khibiny massif), the Mountain Pass deposit in California (USA), as well as occurrences in Sweden (e.g., in the Bastnäs region mines) and Canada (Quebec).

Rarity

Rare

For collectors

## Quality Criteria The most valued stillwellite-(Ce) specimens by collectors are those with well-formed, sharp, and undamaged crystals. Intense, honey-yellow or reddish-brown color and good transparency are highly prized. Large, single barrel-shaped crystals are much more sought after than granular aggregates. The attractiveness of a specimen is also enhanced by its aesthetic placement on a rock matrix, especially in the company of contrasting minerals, such as white calcite or quartz. ## Popular Localities Classic and most desirable specimens, considered exemplary for this mineral, come from the Mary Kathleen mine in Queensland, Australia. Crystals from there reach significant sizes and are characterized by excellent development. Specimens from Kazakhstan and Russia are also valued in the collector's market, although they often have a slightly different habit or color.

Care and storage

## Cleaning Stillwellite-(Ce) specimens can be safely cleaned with lukewarm water and a mild soap, using a soft brush to remove dirt from crevices. After washing, it should be thoroughly rinsed, preferably with distilled water, and allowed to dry completely or gently blotted with a soft cloth. ## What to Avoid The mineral is sensitive to strong acids, which can damage it. Ultrasonic cleaners and steam cleaning should be avoided, as temperature fluctuations and vibrations can cause internal fractures. It should not be exposed to prolonged intense sunlight, which theoretically may affect its color. ## Storage Stillwellite-(Ce) is relatively hard, but to avoid scratches or damage, it should be stored separately from other minerals, especially harder ones (like quartz, topaz, or corundum). It is best kept in individual display boxes with soft lining. Protect from dust and sudden temperature changes.