Dumortierite

Chemical formula: AlAl₆BSi₃O₁₈

A borosilicate of aluminum, valued for its intense blue or violet color and occurrence in fibrous or radial aggregates.

## Characteristics Dumortierite is a borosilicate of aluminum that most often forms fibrous, radial, or felted aggregates. It rarely occurs as well-formed, prismatic crystals. Its most characteristic feature is an intense blue color in various shades, from lavender to navy blue. It often occurs as inclusions in quartz, forming a visually attractive material known as dumortierite quartz. ## Physical Properties This mineral is relatively hard and dense. Its Mohs hardness is 7, making it scratch-resistant. The luster is typically vitreous, and in the case of compact, fibrous aggregates, it can be dull or silky. It is a translucent to opaque mineral, and well-formed crystals (which are rare) can be almost transparent. ## Colors and Varieties The dominant color of dumortierite is blue, often with a violet tint. Brown, reddish-brown, pink, or greenish specimens are also found. The most prized variety is intensely blue dumortierite, especially as inclusions in transparent quartz, which is a popular ornamental and collectible stone. ## History and Name The mineral's name, given in 1881 by the French mineralogist M. F. Gonnard, honors the French paleontologist Eugène Dumortier (1803-1873), who first discovered this mineral in the French Alps near Chaponost. ## Applications Due to its high melting point and chemical resistance, dumortierite is used in the production of high-quality ceramics, including insulators for spark plugs and laboratory porcelain. As an ornamental stone, it is cut into cabochons, beads, and carvings.

Properties

Mohs hardness: 7
Luster: Vitreous
Streak: White
Density: 3.3-3.4
Cleavage: Good on {100}
Fracture: Uneven, Splintery
Transparency: Translucent to opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Key features for identifying dumortierite are its characteristic blue or violet color, fibrous or radial habit, and high hardness (7 on the Mohs scale). It often occurs as inclusions in quartz, which is also a strong diagnostic indicator. The streak on an unglazed porcelain plate is white. ## Distinguishing from Similar Minerals Dumortierite is sometimes confused with several other blue minerals: - **Sodalite:** It is significantly softer (hardness 5.5-6) and often occurs with nepheline and calcite in igneous rocks. It also has a more greasy luster. - **Lapis Lazuli:** It is also softer (hardness 5-5.5) and almost always contains pyrite inclusions. It occurs in calcitic marbles. - **Kyanite:** It has a characteristic variable hardness depending on the direction (4.5-7) and forms flat, bladed crystals. Its cleavage is perfect. ## Crystal Forms Most commonly found in fibrous, radial, spherulitic, or massive aggregates. Well-formed, columnar or prismatic crystals belonging to the orthorhombic system are rare. Very often found as randomly oriented inclusions in quartz crystals.

Geological environment

## Genesis Dumortierite is a typical metamorphic mineral, forming under regional metamorphic conditions in aluminum-rich rocks such as gneisses and crystalline schists. It can also form in pegmatites and quartz veins cutting these rocks. Its formation is associated with the presence of boron in hydrothermal solutions or in the composition of the primary rock. ## Mineral Associations It often co-occurs with minerals such as quartz, kyanite, andalusite, sillimanite, muscovite, as well as tourmaline and topaz. ## Localities Significant deposits and occurrences of dumortierite are found worldwide. The most important include: - **France:** Lyon area (discovery locality). - **USA:** Nevada (Oreana - famous inclusions in quartz), Arizona, and California (Dehesa). - **Brazil:** Bahia and Minas Gerais states (beautiful dumortierite quartz). - **Madagascar:** Numerous localities providing ornamental material. - **Namibia:** Usakos area. - **Poland:** Occurs in the Karkonosze Mountains, near Szklarska Poręba and Karpacz, in hornfelses and pegmatites.

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued dumortierite specimens are characterized by an intense, pure blue or violet-blue color. In the case of fibrous aggregates, well-formed, radial aggregates are desired. Dumortierite quartz is particularly popular, where the value of the specimen depends on the color saturation, the transparency of the quartz, and the aesthetic arrangement of the inclusions. Well-formed, freestanding crystals are extremely rare and command high prices. ## Popular Localities The most sought-after specimens of quartz with dumortierite inclusions by collectors come from Brazil (Vaca Morta Quarry) and from Nevada and Arizona in the USA. Material of beautiful, uniform color, suitable for processing, often comes from Madagascar and Namibia.

Care and storage

## Cleaning Dumortierite is a hard and chemically resistant mineral. It can be safely cleaned with lukewarm water, mild soap, and a soft brush. After washing, it should be thoroughly rinsed with clean water and dried. ## What to Avoid Avoid contact with strong acids, especially hydrofluoric acid, which can damage its surface. Despite its high hardness, specimens should be protected from impacts and contact with harder minerals (e.g., diamond, corundum) to prevent scratches. Ultrasonic cleaning is not recommended, especially for specimens with inclusions in quartz, as vibrations can cause cracks. ## Storage Dumortierite specimens can be stored under standard conditions. It is best to keep them in separate boxes or pouches to prevent abrasion with other minerals in the collection. It is resistant to sunlight and does not fade under its influence.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FDumortierite