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Occurs in Pinal County. Common in Arizona.

Actinolite. Al Copley photo.

Chemical Formula: Ca2(Mg,Fe)5Si8O22(OH)2

Specific Gravity: 2.9-3.3

Luster: Vitreous to silky

Hardness: 5.5-6

Actinolite is a green-to-gray amphibole mineral that is a member of the hornblende group. It forms prismatic crystals and is often found in metamorphic rocks.

Next Pinal County mineral: Ajoite


It is a silicate mineral with complex crystal structures and is known for its elongated, needle-like crystal habit. Actinolite has a range of colors, from green to gray, and is commonly found in metamorphic rocks. It holds significance in both mineralogy and geology due to its presence in various rock formations and its role as an indicator mineral for certain geological conditions.

Key Characteristics:

Occurrence and Formation:

Actinolite forms primarily in metamorphic environments, where rocks are subjected to high pressure and temperature. It commonly occurs in schists, gneisses, and other metamorphic rocks, often alongside other minerals like garnet and chlorite. Actinolite can also be found in certain igneous rocks and is associated with certain types of hydrothermal mineral deposits.

Varieties and Similar Minerals:

  1. Tremolite: Tremolite is a closely related mineral to actinolite and belongs to the same amphibole group. They share similar chemical compositions but have different crystal habits and properties.
  2. Asbestos: Some varieties of actinolite, such as “byssolite,” are part of the asbestos mineral group. Asbestos minerals have long, fibrous crystals and were historically used in various industrial applications due to their heat resistance and durability. However, asbestos minerals are now recognized as health hazards due to their potential to cause respiratory diseases.

Uses and Significance:

Actinolite itself is not widely used for commercial purposes due to its relative rarity and limited availability. However, it is a valuable mineral for understanding geological processes, particularly those related to metamorphism and the formation of certain rock types. Actinolite’s presence in certain rock formations can also indicate specific geological conditions that were prevalent during their formation.

MinDat Actinolite