Augite
Chemical formula: (Ca,Mg,Fe)₂Si₂O₆
Augite is a common mineral from the pyroxene group, a calcium, magnesium, and iron silicate, frequently found in igneous and metamorphic rocks.
Description
## Characteristics Augite is a mineral belonging to the pyroxene group, characterized by its dark color, usually black, dark green, brownish-green, or purplish-brown. It forms columnar or tabular crystals, often with an octagonal or square cross-section. Augite aggregates can be granular or massive. It is a common mineral in many rock types, making it an important component for identifying geological environments. ## Physical Properties Augite has a Mohs hardness of 5.5-6. It exhibits good vitreous to resinous luster. It is a translucent to opaque mineral. Its density is approximately 3.19 g/cm³. It shows good cleavage in two directions, intersecting at an angle close to 90 degrees. ## Colors and Varieties Typical colors of augite include black, dark green, brownish-green, brown, and purplish-brown. The coloration depends on the content of iron and titanium ions in the crystal structure. There are no specific varieties with distinct trade names, but differences in chemical composition can influence subtle color changes. ## History and Name The name "augite" comes from the Greek word "augites," meaning "brightness" or "luster," referring to its vitreous appearance. The mineral was described in 1792. It is one of the first minerals from the pyroxene group to be identified and named, which highlights its commonality and importance in mineralogy. ## Applications Augite has no significant industrial or jewelry applications due to its commonality and lack of exceptional aesthetic properties. Its primary value lies in its petrological significance as an indicator mineral for identifying types of igneous and metamorphic rocks. It is valued by collectors for well-formed crystals and as a representative of the pyroxene group.
Diagnostic features
## Identification Augite can be identified by its dark color (black, dark green, brown), vitreous or resinous luster, and characteristic cleavage in two directions at an angle of approximately 90 degrees. Crystals often have a columnar or tabular habit with a cross-section similar to a square or octagon. ## Distinguishing from Similar Minerals Augite is sometimes confused with hornblende, another dark mineral. The key difference is the cleavage angle: augite has cleavage at approximately 90 degrees, while hornblende exhibits cleavage at approximately 56 and 124 degrees. Furthermore, augite crystals often have a more massive, short-columnar habit compared to the typically elongated hornblende crystals. ## Crystal Forms Augite forms columnar, tabular, or isometric crystals. Twinning is common. Aggregates can be granular, massive, or form radiating clusters.
Geological environment
## Genesis Augite is an igneous and metamorphic mineral. It forms under high temperatures and pressures, typical of magma crystallization processes and regional and contact metamorphism. It is a common component of many igneous rocks, such as basalts, gabbros, andesites, and diabases, as well as some metamorphic rocks, e.g., amphibolites and eclogites. ## Mineral Associations Augite often coexists with plagioclase, olivine, hornblende, magnetite, ilmenite, and biotite. In metamorphic rocks, it can be associated with garnets and epidote. ## Localities Augite is a common mineral and occurs worldwide. Significant localities include Vesuvius in Italy, where well-formed crystals have been found. It also occurs in many volcanic regions, such as Iceland, Japan, and the United States (e.g., Colorado, California). In Poland, augite is present in the basalts of Lower Silesia.
Rarity
Very Common
Collector aspects
## Quality Criteria The most prized augite specimens are those with well-formed, distinct crystals, with intense, uniform color and strong luster. Large, undamaged crystals, especially those with visible faces and edges, are particularly sought after. Specimens with rare twinning also make an attractive addition to a collection. ## Popular Localities Popular localities from which prized augite specimens originate include Vesuvius in Italy, where large and well-formed crystals are often found. Other important locations include volcanic areas in Japan and Iceland, as well as some localities in the United States.
Care and storage
## Cleaning Augite can be cleaned using a soft brush and lukewarm water. For heavier dirt, mild soap can be used. After washing, rinse the specimen thoroughly with clean water and dry with a soft cloth. ## What to Avoid Avoid using strong acids and bases, which can damage the mineral's surface. Augite is relatively hard but still susceptible to impacts and scratches. Prolonged exposure to intense sunlight does not negatively affect its color, but it is advisable to protect specimens from sudden temperature changes, which can lead to cracks. ## Storage Augite specimens are best stored in a dry, dust-free place. They can be displayed in closed display cases to prevent the accumulation of contaminants. When storing in drawers or boxes, it is advisable to separate them from other minerals to avoid mutual scratching.