What is the Difference Between Pyroxene and Amphibole

The main difference between pyroxene and amphibole is that pyroxenes have a single-chain structure, whereas amphiboles have a double-chain structure.

An inosilicate is a type of silicate mineral structure characterized by chains of linked tetrahedral silicate groups. These chains are connected through shared oxygen atoms, resulting in a single or double-chain arrangement. Pyroxenes and amphiboles are both subcategories of inosilicate minerals.

Key Areas Covered

1. What is Pyroxene
     – Definition, Properties, Composition
2. What is Amphibole
     – Definition, Properties, Composition
3. Similarities Between Pyroxene and Amphibole
     – Outline of Common Features
4. Difference Between Pyroxene and Amphibole
     – Comparison of Key Differences

Key Terms

Pyroxene, Amphibole

What is Pyroxene

Pyroxene is a group of rock-forming minerals that are part of the larger group of inosilicates and have a wide range of compositions and properties. Pyroxenes are essential constituents of various igneous and metamorphic rocks. Pyroxene minerals have a general chemical formula (Mg, Fe, Al)2Si2O6, where M represents divalent cations like magnesium (Mg) and iron (Fe). Depending on the specific cation composition, pyroxenes can be divided into two major subgroups: orthopyroxene and clinopyroxene. Orthopyroxenes have a higher percentage of magnesium and iron, whereas clinopyroxenes contain more calcium, sodium, and aluminum.

Single chains of linked silicon-oxygen tetrahedra characterize the crystal structure of pyroxene. These chains are connected by metal cations occupying the gaps between the tetrahedra. This structure results in the formation of elongated, prismatic crystals with two planes of cleavage at nearly 90-degree angles.

Pyroxene minerals are widespread in various geological settings. They are commonly found in igneous rocks, such as basalts and gabbros, as well as in metamorphic rocks, like amphibolites and eclogites. Pyroxenes can also be present in some rare sedimentary rocks. These minerals form during the solidification of molten rock (magma) deep within the Earth’s crust and mantle. The composition of the parent magma and the cooling rate significantly influence the types of pyroxene that crystallize. The presence of pyroxenes in metamorphic rocks can result from the recrystallization of existing minerals under high pressure and temperature conditions.

Industrial Applications of Pyroxene

Pyroxenes, particularly certain varieties like jadeite and diopside, have various industrial applications and commercial value. Jadeite, a rare and valuable form of jade, is useful in jewelry and ornamental carvings. Its vibrant green colors and exceptional hardness make it highly prized by collectors and artisans. Certain pyroxene minerals, such as enstatite, are used in the manufacture of refractory materials due to their high melting points and resistance to heat and chemical corrosion. In some regions, certain pyroxene-bearing rocks are used as construction materials for roads, buildings, and monuments.

What is Amphibole

Amphibole is a group of rock-forming silicate minerals that are part of the inosilicate group and encompass various compositions and properties. Amphiboles are present in a wide range of geological settings and are essential components of many igneous, metamorphic, and even some sedimentary rocks.

Amphibole minerals have a general chemical formula (Ca,Na)(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂, where M represents various cations like calcium (Ca), sodium (Na), and potassium (K), in addition to magnesium (Mg), iron (Fe), and aluminum (Al). This extensive range of cations leads to various amphibole species with distinct properties.

Double chains of linked silicon-oxygen tetrahedra characterize the crystal structure of amphibole. These chains are bonded together by metal cations and hydroxide groups (OH-). The unique arrangement of these chains gives amphibole minerals elongated, prismatic-shaped crystals, which exhibit two planes of cleavage at approximately 56 and 124 degrees.

Amphiboles can be in various colors, including dark green, black, brown, white, or colorless. The color depends on the specific cation composition and impurities within the crystal lattice. Amphiboles often exhibit a vitreous to dull luster, depending on the surface texture of the crystal. When an amphibole mineral is scratched on a streak plate, it leaves a streak that is usually white or light-colored.

The hardness of amphibole minerals ranges from 5 to 6 on the Mohs scale, making them relatively resistant to scratching. Amphiboles have higher densities compared to many other common rock-forming minerals.

Similarities Between Pyroxene and Amphibole

• Pyroxene and amphibole minerals belong to the group of silicate minerals.
• Both pyroxene and amphibole minerals have elongated prismatic habits due to their chain-like structure.

Difference Between Pyroxene and Amphibole

Definition

Pyroxene is a group of single-chain silicate minerals, often found in igneous and metamorphic rocks, whereas amphibole is a group of double-chain silicate minerals occurring in various rock types, including igneous, metamorphic, and sedimentary rocks.

Crystal Structure

Pyroxenes have a single-chain silicate structure in which the tetrahedral silicate groups are linked in a single-chain arrangement. Meanwhile, amphiboles have a double-chain silicate structure with tetrahedral silicate groups forming two interconnected chains.

Common Elements

Moreover, pyroxenes commonly contain elements such as magnesium, iron, and calcium, whereas amphiboles often contain calcium, sodium, iron, and aluminum as their prominent elements.

Composition     

Pyroxenes are composed of minerals represented by the chemical formula (Mg,Fe,Ca)₂Si₂O₆. However, amphiboles are composed of minerals represented by the formula (Ca,Na)(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂.

Occurrence        

Pyroxenes are abundant in igneous and metamorphic rocks, but amphiboles are abundant in a variety of rock types, including igneous, metamorphic, and sedimentary rocks.

Appearance      

While pyroxenes typically appear dark-colored, amphiboles come in a range of colors and often form prismatic crystals.

Common Minerals

Common pyroxene minerals include augite, enstatite, and diopside, whereas amphibole minerals include hornblende, actinolite, and tremolite.

Conclusion

Pyroxene is a group of single-chain silicate minerals, often found in igneous and metamorphic rocks, whereas amphibole refers to a group of double-chain silicate minerals occurring in various rock types, including igneous, metamorphic, and sedimentary rocks. Thus, this is the main difference between pyroxene and amphibole.

Reference:

1. “Pyroxene.” Wikipedia. Wikipedia Foundation.
2. “Amphibole.” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “Diopside-172005” By Rob Lavinsky, iRocks.com (CC-BY-SA-3.0) via Commons Wikimedia
2. “Tremolite-121232” By Rob Lavinsky, iRocks.com (CC-BY-SA-3.0) via Commons Wikimedia