Roymillerite
Chemical formula: Pb<sup>2+</sup><sub>24</sub>Mg<sub>9</sub>(Si<sub>10</sub>O<sub>28</sub>)(CO<sub>3</sub>)<sub>10</sub>(BO<sub>3</sub>)(SiO<sub>4</sub>)(OH)<sub>13</sub>O<sub>5</sub>
A complex lead-magnesium silicate-carbonate, an extremely rare mineral forming colorless, hexagonal crystals, known only from the Kombat Mine in Namibia.
Description
## Characteristics Roymillerite is a very chemically complex mineral from the silicate and carbonate groups, containing lead, magnesium, boron, and hydroxyl groups. It occurs as small, hexagonal, prismatic crystals, not exceeding 0.3 mm in length. The crystals are colorless and transparent, exhibiting a strong, adamantine luster. They typically form small, radial or nearly parallel aggregates in rock fissures. Due to the microscopic size of the crystals, it is a typical micromount mineral. ## Physical Properties This mineral is characterized by a high density of approximately 5.55 g/cm³, which is typical for lead-rich minerals. It is transparent and has a strong adamantine luster. Its hardness has not yet been precisely determined. Cleavage has not been observed, although theoretical calculations indicate its potential occurrence. ## Colors and Varieties Roymillerite is a colorless mineral. No colored or commercial varieties are distinguished. ## History and Name The mineral was officially approved by the International Mineralogical Association (IMA) in 2017. Its name honors Roy Miller (born 1949), a renowned American micromineral collector from Arizona, in recognition of his contributions to mineralogy. The type locality, meaning the only known place of its occurrence, is the Kombat Mine in Namibia. ## Applications Roymillerite has no industrial applications. It is solely of scientific and collector's interest as a unique and newly discovered mineral species.
Diagnostic features
## Identification Characteristic features of roymillerite include its form – hexagonal, prismatic crystals, colorlessness, strong adamantine luster, and very high density. The key to identification is also its origin (Kombat Mine) and co-occurrence with manganese ore minerals. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Differentiation from Similar Minerals Roymillerite can be confused with other colorless, hexagonal minerals found in the same locality, such as hydrocerussite or kentyoreite. Distinguishing it based on visual characteristics is extremely difficult and often impossible without specialized equipment. It differs from them in chemical composition and crystallographic parameters. ## Crystal Forms It forms elongated crystals with a hexagonal prismatic habit, often terminated by flat basal pinacoid faces. Crystals occur individually or form small, divergent (radial) or nearly parallel aggregates.
Geological environment
## Genesis Roymillerite forms under hydrothermal conditions within a metamorphosed, stratiform manganese, copper, and lead ore deposit, hosted in dolomites. It occurs in small vugs (druses) within manganese-rich ore. ## Mineral Associations This mineral occurs in association with other manganese minerals and ore minerals, primarily jacobsite, hausmannite, barite, and calcite. ## Localities The only confirmed locality of roymillerite in the world is the Kombat Mine in the Grootfontein District, Otjozondjupa Region, Namibia. This is its type locality.
Rarity
Extremely rare
Collector aspects
## Quality Criteria The quality of roymillerite specimens, being a micromount mineral, is assessed based on several criteria. Specimens with sharply terminated, well-formed, and undamaged crystals of high transparency and luster are most highly valued. A contrasting matrix and aesthetic arrangement of crystals are also important. Associations with other rare minerals from this locality are particularly desirable. ## Popular Localities The only source of roymillerite specimens is the Kombat Mine in Namibia. Specimens from this locality are the only ones available on the collector's market.
Care and storage
## Cleaning Roymillerite specimens should be cleaned with utmost care, preferably using compressed air to remove dust. If necessary, a soft brush and distilled water can be used, followed by gentle drying of the specimen. Ultrasonic cleaners should be avoided, as they can damage delicate crystals. ## What to Avoid As a carbonate, roymillerite is sensitive to acids, which cause its rapid decomposition. Contact with any chemicals should be avoided. Due to the microscopic size and delicacy of the crystals, it is very susceptible to mechanical damage. Given its lead content, inhalation of dust should be avoided, and hands should be washed after contact with the mineral. ## Storage The safest storage method is to place the specimen in a sealed micromount box, which protects it from dust, shocks, and direct handling. Exposure to extreme temperatures should be avoided.