Dumortierite is a complex aluminum boro-silicate mineral with the chemical formula Al7BO3(SiO4)3O3. It typically presents in colors ranging from deep blue and violet to brownish-pink and green, often found as fibrous aggregates or inclusions within host rocks. Dumortierite is valued for its attractive color and durability in the lapidary and jewelry trade. Its ability to fire to a pure white also makes it highly useful in the manufacture of high-grade porcelain and ceramics.
Geological Context of Dumortierite Formation
The presence of dumortierite indicates high-energy geological processes in the Earth’s crust. Formation requires a unique combination of high aluminum content and the presence of boron. The mineral typically crystallizes under high-temperature and high-pressure conditions associated with regional or contact metamorphism.
Dumortierite is frequently discovered in aluminum-rich metamorphic rocks, such as schists and gneisses, where intense heat and pressure have altered the original material. It also forms within boron-rich pegmatites, which are coarse-grained igneous rocks. In these environments, boron-rich fluids allow aluminum-silicate compounds to recrystallize into the characteristic fibrous structure. The resulting material exhibits a hardness between 7 and 8.5 on the Mohs scale, making it durable for industrial and ornamental applications.
Primary Commercial Deposits
The largest commercial supplies of dumortierite are sourced from deposits in Brazil, the United States, and India. Brazil produces some of the finest gem-quality material, particularly the variety known as “dumortierite quartz.” This material consists of minute blue dumortierite fibers intergrown within clear quartz, creating a striking blue stone often called “Bahia Blue Quartz.”
Major Brazilian localities include the states of Minas Gerais and Bahia. The Vaca Morta Quarry in Bahia is a well-known source for quartz specimens. Minas Gerais is also a rare source of transparent, facetable dumortierite crystals. In the United States, the mineral is commercially mined in Arizona and Nevada, such as the historically significant deposit near Limerick Canyon in Nevada’s Rochester Mining District.
Material from the Nevada deposits often occurs in a schist matrix and displays colors including pink, lilac, violet, and blue. This raw material is processed for the ceramics industry, utilizing the mineral’s high aluminum content to be fired into pure white porcelain. India also functions as a major global supplier, contributing a substantial volume of dumortierite for both the industrial and lapidary sectors.
Notable Secondary Localities
Other localities worldwide yield dumortierite, though often in smaller volumes that lack commercial scale. These secondary finds are important for their geological significance or for providing unique mineral specimens for collectors. Madagascar, for instance, is known for producing distinct specimens, sometimes featuring unique colorations and crystal habits.
European occurrences include sites like the Gföhl unit in Austria, which has been the subject of extensive scientific investigation into the mineral’s formation under high-grade metamorphic conditions. Finds are also located across Poland and Russia, often yielding small, localized pockets. These diverse locations confirm that the mineral is distributed across several continents.