Stillwellite-(La)

Chemical formula: LaBSiO<sub>5</sub>

Stillwellite-(La) is a rare lanthanide silicate, valued for its well-formed, hexagonal, honey-colored crystals.

## Characteristics Stillwellite-(La) is a rare mineral from the silicate group, containing lanthanum, boron, and silicon. It forms well-developed, hexagonal crystals with prismatic or tabular habits, often terminated by pyramids. It also occurs as granular aggregates or radial clusters. Its appearance is distinctive, and specimens can be partially transparent. ## Physical Properties This mineral is characterized by a Mohs hardness of 6.5, making it relatively scratch-resistant. It has a vitreous to resinous luster. It is quite dense, with a specific gravity of approximately 4.6-4.7 g/cm³. It exhibits no cleavage, and its fracture is conchoidal to uneven. ## Colors and Varieties Stillwellite-(La) most commonly occurs in colors ranging from yellow, through honey-brown, reddish-brown, to greenish or almost colorless. Its name with the -(La) suffix indicates that lanthanum is the dominant rare-earth element in its structure, distinguishing it from Stillwellite-(Ce) with dominant cerium. ## History and Name The name "stillwellite" honors Frank Leslie Stillwell (1888–1963), an Australian geologist. The suffix "-(La)" was added according to the rules for naming minerals containing rare-earth elements, to indicate lanthanum as the dominant element in that specific site within the crystal structure. The mineral was first described in 1955. ## Uses Stillwellite-(La) has no industrial applications. Due to its rarity, attractive crystals, and interesting chemical composition, it is a sought-after mineral valued by collectors specializing in rare minerals and pegmatite minerals.

Properties

Mohs hardness
6.5
Luster
Vitreous to Resinous
Streak
White
Density
4.6 - 4.7
Cleavage
None
Fracture
Conchoidal to Uneven
Transparency
Transparent to Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Stillwellite-(La) can be identified by its characteristic hexagonal crystal shape, high hardness (6.5), vitreous to resinous luster, and relatively high density. It often occurs in characteristic mineral associations in pegmatites and skarns. ## Distinguishing from Similar Minerals It can be confused with other hexagonal minerals of similar color, such as some varieties of apatite, beryl, or quartz. It is distinguished from apatite by its greater hardness. From beryl and quartz, it differs significantly by its much higher density (it feels noticeably heavier for the same size) and often resinous luster. ## Crystal Forms Crystals are typically trigonal, with a habit of short or long hexagonal prisms. They are often terminated by trigonal pyramid faces. It also occurs as radial, fibrous, or granular aggregates.

Geological environment

## Genesis Stillwellite-(La) is a mineral of hydrothermal or metasomatic origin. It forms in skarns, carbonatites, and syenite pegmatites, i.e., in environments enriched in rare-earth elements, boron, and silica. ## Mineral Associations It often co-occurs with other rare-earth element-bearing minerals. Typical associated minerals include allanite, magnetite, garnets (andradite), epidote, calcite, diopside, titanite, and various minerals from the pyrochlore group. ## Localities The most important and well-known occurrences of Stillwellite-(La) are in Russia, particularly in the Khibiny Mountains region on the Kola Peninsula. Other known localities include the Mary Kathleen mine in Queensland, Australia (type locality), as well as occurrences in Canada (e.g., Mont Saint-Hilaire in Quebec) and the USA (e.g., in New Jersey).

Rarity

Rare

For collectors

## Quality Criteria The most prized by collectors are well-formed, sharp, single crystals with a distinct hexagonal outline and smooth faces. Specimens with an intense, honey-brown color and good transparency are highly valued. Crystal size and aesthetic placement on the rock matrix, especially in association with contrasting minerals, are also important. ## Popular Localities Specimens from the Kola Peninsula in Russia and from the Mary Kathleen mine in Australia are considered classic and most desirable in the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. For heavier soiling, distilled water and a soft brush can be used, followed by thorough drying of the specimen. Ultrasonic cleaners should be avoided, as they can cause fractures. ## What to Avoid Avoid contact with strong acids and other aggressive chemicals. The mineral is sensitive to rapid temperature changes. Prolonged exposure to direct sunlight is not recommended, although there is no evidence of fading. ## Storage Stillwellite-(La) specimens are best stored in individual, padded boxes or on stands in a closed display cabinet to protect them from dust and mechanical damage. Due to its hardness, it should not be stored in contact with softer minerals that it could scratch.

Sources

Read more