Pyroxene

Chemical formula: ADSi₂O₆

Pyroxenes are an important group of rock-forming silicates, crucial for the classification of igneous and metamorphic rocks, with diverse compositions and colors.

## Characteristics Pyroxenes are a large and complex group of silicate minerals, being among the most important rock-forming constituents in the Earth's crust and upper mantle. They rarely form well-developed, collectible crystals, most often occurring as granular or fibrous aggregates within the rock. Typical crystals, if present, have a prismatic or tabular habit with a cross-section close to square. This group includes several dozen minerals, and its name comes from the Greek words "pyr" (fire) and "xenos" (stranger), which erroneously suggested that they were "foreign" to igneous rocks. ## Physical Properties Due to their wide range of chemical compositions, the physical properties of pyroxenes are variable. Hardness on the Mohs scale ranges from 5 to 7, and density from approximately 3.0 to 4.0 g/cm³. Luster is usually vitreous, sometimes dull or silky in the case of fibrous varieties. These minerals are most often opaque or translucent, rarely transparent. ## Colors and Varieties The color of pyroxenes is very diverse – from almost colorless, through white, gray, green (most common), brown, to black. Color differences are mainly due to the content of iron, chromium, titanium, and other elements. Important members of the group include diopside (often green), augite (dark green to black), enstatite (brownish-green), hedenbergite (dark green), jadeite (intensely green, valued in jewelry), and spodumene (colorless, pink kunzite, green hiddenite). ## History and Name The name "pyroxene" was introduced by the French mineralogist René Just Haüy in 1796. He observed these minerals in volcanic lavas and mistakenly interpreted them as impurities that survived the rock melting process, hence the name "stranger in fire." Today, it is known that pyroxenes are among the first minerals to crystallize from magma. ## Uses Beyond their rock-forming significance, some pyroxenes have practical applications. Jadeite is a highly valued ornamental and gem material, especially in Asian culture. Spodumene (in its kunzite and hiddenite varieties) is also used as a gemstone. Furthermore, spodumene is an important source of lithium, a key metal in battery production.

Properties

Mohs hardness: 5-7
Luster: Vitreous
Streak: White to grayish green
Density: 0
Cleavage: Good, two planes intersecting at angles of ~87° and 93°
Fracture: Uneven
Transparency: Transparent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of pyroxenes, especially visible in larger crystals or cross-sections, is the cleavage angle between cleavage planes, which is approximately 87° and 93° (i.e., close to 90°). This distinguishes them from the very similar amphibole group, where these angles are approximately 56° and 124°. Pyroxene crystals often have a cross-section close to square. ## Distinguishing from Similar Minerals - **Amphiboles (e.g., hornblende):** The main difference is the cleavage angle (nearly 90° for pyroxenes, approx. 60/120° for amphiboles). Amphibole crystals often have a hexagonal cross-section, while pyroxenes have a tetragonal cross-section. - **Tourmalines:** Tourmalines have a similar prismatic habit, but usually exhibit characteristic striations along the crystal faces and have a rounded triangular cross-section. They are also harder (7-7.5). - **Epidotes:** Often occur in similar environments and have a green color, but crystallize in the monoclinic system and have perfect cleavage in one direction. ## Crystal Forms Pyroxene crystals are typically short, stout prisms with a tetragonal or octagonal cross-section. They also occur as granular, fibrous (as in pyroxene asbestos), radial aggregates, or as irregular grains within the rock.

Geological environment

## Genesis Pyroxenes are key minerals in igneous rocks, both intrusive (gabbro, peridotite, pyroxenite) and extrusive (basalt, andesite). They crystallize at high temperatures directly from magma. They also form under conditions of high-pressure and high-temperature metamorphism, creating rocks such as eclogites (composed of omphacite and garnet) or granulites. They can also occur in some sedimentary rocks as heavy minerals. ## Mineral Associations Pyroxenes are associated with a wide range of minerals, depending on the environment. In igneous rocks, these are most often olivine, plagioclase, amphibole, and magnetite. In metamorphic rocks, they are accompanied by garnets (especially in eclogites), epidote, vesuvianite, wollastonite, and calcite (in skarns). ## Localities As rock-forming minerals, pyroxenes occur worldwide. However, specimens of collector significance come from specific localities: - **Diopside:** Zillertal Valley (Austria), Ala (Italy), Outokumpu (Finland), De Kalb (USA), Siberia (Russia). - **Spodumene (kunzite, hiddenite):** Minas Gerais (Brazil), Afghanistan, Pakistan, California (USA). - **Augite:** Eifel region (Germany), Vesuvius (Italy), Ontario and Quebec (Canada). - **Jadeite:** Myanmar (Burma - the most important source of gem-quality jadeite), Guatemala, California (USA). - **Hedenbergite:** Serifos Island (Greece), Dalnegorsk (Russia), Broken Hill (Australia).

Rarity

Common

For collectors

## Quality Criteria The collector appeal of pyroxene specimens depends on the specific mineral within the group. Generally, the following are valued: - **Well-formed crystals:** Sharp, undisturbed prismatic or tabular forms are much more desirable than amorphous aggregates. - **Color:** Intense and pure color is key, e.g., emerald-green chromian diopside, purplish-pink kunzite, or deep green jadeite. - **Transparency:** Transparent crystals (e.g., spodumene, diopside) are rare and highly valued, especially if suitable for faceting. - **Size:** Large, well-formed crystals are rare and command high prices. - **Associations:** Aesthetic combinations with other minerals (e.g., diopside with garnet, hedenbergite with quartz) increase the specimen's value. ## Popular Localities The most prized specimens come from classic localities that yield minerals of exceptional quality. For diopside, these are the Italian and Austrian Alps; for spodumene, Afghanistan and Brazil; and for hedenbergite, Serifos Island in Greece. Augite specimens from volcanic bombs in the Eifel region of Germany are also popular among collectors.

Care and storage

## Cleaning Pyroxene specimens are relatively hard and chemically resistant. They can be safely cleaned with lukewarm water with a mild soap and a soft brush. After washing, they should be thoroughly rinsed, preferably in distilled water, and left to dry completely. ## What to Avoid Avoid contact with strong acids, especially hydrofluoric acid, which can damage the mineral surface. Despite their relatively high hardness, specimens should be protected from impacts and scratching by harder minerals (e.g., quartz, corundum). Some varieties, such as kunzite (pink spodumene), may fade with prolonged exposure to intense sunlight. ## Storage Pyroxene specimens are best stored in separate, padded boxes or on stands to prevent mutual scratching with other minerals in the collection. When displaying light-sensitive varieties, direct sunlight should be avoided.