Occurs in Pinal County. Common in Arizona.
Chemical Formula: Ca2(Mg,Fe)5Si8O22(OH)2
Specific Gravity: 2.9-3.3
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.
- Chemical Formula: The general formula for actinolite is Ca2(Mg,Fe)5Si8O22(OH)2. This formula reflects its composition, which includes calcium, magnesium, iron, silicon, and hydroxide ions.
- Crystal System: Actinolite crystallizes in the monoclinic crystal system.
- Color: Actinolite can exhibit a range of colors, including various shades of green and gray. It is often translucent to opaque.
- Luster: Its luster can be vitreous to silky, depending on the quality of the crystal surfaces.
- Hardness: It has a hardness of around 5 to 6 on the Mohs scale, making it moderately hard.
- Cleavage: Actinolite has two cleavage directions intersecting at approximately 56 and 124 degrees.
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:
- 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.
- 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.