Rigrahamite
Chemical formula: Na<sub>2</sub>Ca<sub>2</sub>Th<sub>2</sub>(CO<sub>3</sub>)<sub>4</sub>(HCO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>(H<sub>2</sub>O)<sub>7</sub>
An extremely rare, radioactive sodium, calcium, and thorium carbonate, forming colorless, acicular crystals in pegmatite vugs.
Description
## Characteristics Rigrahamite is a very rare, hydrated thorium-bearing carbonate. It forms colorless, transparent, acicular crystals, typically occurring as small, radial or tangled aggregates. Due to its chemical composition, this mineral exhibits natural radioactivity. Specimens are usually microscopic (so-called microminerals) and fill small cavities in the host rock. ## Physical Properties Rigrahamite crystals have a vitreous luster and are transparent. This mineral is characterized by a relatively high density, approximately 3.45 g/cm³, which is a result of the presence of the heavy element thorium in its structure. Hardness on the Mohs scale has not been precisely determined. No cleavage has been observed. ## Colors and Varieties Rigrahamite is a colorless mineral. No colored varieties or trade names are known. ## History and Name The mineral was approved by the IMA in 2009. Its name honors Ronald P. Graham (1915–2014), a professor of chemistry at McMaster University in Canada, in recognition of his contributions to Canadian mineralogy. It was discovered at the world-famous mineralogical locality of Mont Saint-Hilaire in Quebec. ## Uses As an extremely rare mineral occurring only as microscopic specimens, rigrahamite has no industrial application. It holds purely scientific significance and is an object of interest for specialized collectors of rare minerals.
Diagnostic features
## Identification The key diagnostic feature of rigrahamite is its mode of occurrence – radial aggregates of fine, colorless, acicular crystals. Its radioactivity, detectable with a Geiger counter, is a decisive identification test. Relatively high density also aids in identification. ## Distinguishing from Similar Minerals At the type locality (Mont Saint-Hilaire), rigrahamite can be confused with other white or colorless acicular minerals, such as natrolite, eudialyte, or other zeolites. However, none of these minerals are radioactive. Reaction with acid (effervescence) distinguishes it from silicates, and radioactivity distinguishes it from other carbonates. ## Crystal Forms Rigrahamite crystallizes in the form of very thin, acicular crystals, elongated along one axis. These crystals form characteristic, radially divergent aggregates, as well as irregular, tangled masses resembling felt.
Geological environment
## Genesis Rigrahamite is a hydrothermal mineral, crystallizing in the late stage of magmatic evolution. It forms in miarolitic cavities and vugs within nepheline syenite xenoliths, which are embedded in an intrusive complex of alkaline gabbros and syenites. ## Mineral Associations This mineral occurs in association with many other rare alkaline minerals. The most common associations include: aegirine, analcime, calcite, catapleiite, microcline, natrite, natrolite, rhodochrosite, sodalite, and villiaumite. ## Localities Rigrahamite is known exclusively from one location in the world – the Poudrette quarry at Mont Saint-Hilaire, in the Montérégie region, Quebec province, Canada. This is its type locality.
Rarity
Extremely rare
Collector aspects
## Quality Criteria The quality of rigrahamite specimens is primarily assessed based on the richness and aesthetics of the crystal aggregates. The most desirable specimens are those with well-formed, large (for this mineral) radial clusters of acicular crystals. Samples in which colorless rigrahamite aggregates contrast with the colored host rock, such as dark green aegirine or pink rhodochrosite, are also highly valued. ## Popular Localities The only source of rigrahamite specimens is its type locality – the Poudrette quarry at Mont Saint-Hilaire in Canada. This site is known for the occurrence of over 400 different minerals, including many globally unique ones.
Care and storage
## Cleaning Rigrahamite specimens are extremely delicate and fragile. Mechanical cleaning, even with a soft brush, is strongly discouraged, as it can irreversibly damage the acicular crystals. To remove loose dust particles, a photographic air blower or a can of compressed air can be carefully used from a safe distance. ## What to Avoid Rigrahamite contains thorium and is a radioactive mineral. Prolonged exposure should be avoided, it should be stored away from permanent living areas, and dust that might be generated if it is damaged should never be inhaled. The mineral is soluble in acids. It should be protected from any impacts and vibrations. ## Storage It is recommended to store specimens in padded "micromount" boxes, which protect them from mechanical damage and dust. The container should be clearly labeled with the mineral name and the international radioactivity warning symbol.