silicates
Actinolite
Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2
- Also known as
- smaragditebyssolite
- Crystal system
- monoclinic
- Hardness
- 5–6 on the Mohs scale
- Lustre
- vitreous
- Colour
- Green to greyish green
- Where it forms
- In metamorphosed basic igneous rocks and impure limestones. The felted variety is one of the two minerals that make up nephrite jade.
- Chiefly from
- TaiwanCanadaRussiaItaly
Why it is that colour
Iron. It sits in a series with tremolite, which is the iron-free end, and more iron means more colour.
Safety
Actinolite occurs in both a blocky form and an asbestiform one, and asbestiform actinolite is one of the six regulated asbestos minerals. Massive material is safe to handle; fibrous material is not, and telling them apart by eye is not always possible. Do not cut, grind or disturb fibrous specimens.
Where it comes from
Not commercially traded except as nephrite and as specimens. Its significance is as a contaminant: actinolite and tremolite occur in talc deposits, and talc contaminated with asbestiform amphibole is the subject of extensive litigation.
Historical use
Named in 1794 from the Greek for ray stone, after its radiating crystal habit.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
Worth knowing
Actinolite matters chiefly for two things it is part of. Felted together with tremolite it is nephrite, one of the toughest natural materials known and one of the most culturally significant stones in the world. Grown as fibres it is regulated asbestos.
Same mineral. The variable is habit — the shape it grew in — and this collection records crystal system and habit for exactly this kind of reason.
Identity checked against IMA-CNMNC master list, June 2025.