Actinolite is an inosilicate mineral with the chemical formula:

Ca2(Mg,Fe)5Si8O22(OH)2

  • Mineralogy
  • Actinolite is an intermediate member in a solid-solution series between tremolite (Mg-rich) and ferro-actinolite (Fe-rich) amphiboles. As seen from the chemical formula above, Mg and Fe ions can be freely exchanged in the crystal structure.

  • Occurrence
  • Actinolite is commonly found in metamorphic rocks, such as contact aureoles surrounding cooled intrusive igneous rocks. It also occurs as a product of metamorphism of magnesium-rich limestones.

    The old mineral name uralite is at times applied to an alteration product of primary pyroxene by a mixture composed largely of actinolite. The metamorphosed gabbro or diabase rock bodies, referred to as epidiorite, contain a considerable amount of this uralitic alteration.

    Some forms of asbestos are formed from fibrous actinolite, the fibres being so small that they can enter the lungs and damage the alveoli.

  • Gemology
  • Some forms of actinolite are used as gemstones. The first is called nephrite, which is one of the two types of jade (the other being jadeite a variety of pyroxene).

    Another gem variety of actinolite is the chatoyant form known as cat's-eye actinolite. This stone is translucent to opaque, and green to yellowish green color. This variety has had the misnomer jade cat's-eye. Transparent actinolite is rare and is faceted for gem collectors. Major sources for these forms of actinolite are Taiwan, and Canada. Other sources are Madagascar, Tanzania, and the US.

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