silicates
Tremolite
Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2
- Also known as
- hexagonitenephrite
- Crystal system
- monoclinic
- Hardness
- 5–6 on the Mohs scale
- Lustre
- vitreous
- Colour
- White to pale grey or lavender
- Where it forms
- In metamorphosed dolomitic limestones and in ultramafic rocks.
- Chiefly from
- New York, United StatesItalyTanzaniaMyanmar
Why it is that colour
Essentially iron-free, so nearly colourless. Manganese gives the lavender variety hexagonite.
Safety
Asbestiform tremolite is a regulated asbestos mineral and is considered particularly hazardous. Its most consequential occurrence is as a contaminant: tremolite in the vermiculite from Libby, Montana caused hundreds of deaths and thousands of illnesses among workers and residents, and tremolite contamination of talc has been central to major product liability litigation. Massive tremolite is safe to handle; fibrous material is not.
Where it comes from
Not traded as a stone in its own right beyond specimens and nephrite. Its importance is as an unwanted passenger in other materials.
Historical use
Named in 1789 after the Tremola valley in Switzerland. Along with actinolite it constitutes nephrite jade.
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
Libby, Montana is the case that makes tremolite worth an entry. A vermiculite mine there supplied insulation sold across North America for decades, and the vermiculite was naturally contaminated with asbestiform tremolite. Miners, their families, and townspeople who had never been near the mine developed asbestosis and mesothelioma. The site was declared a public health emergency in 2009 — the first time the US Environmental Protection Agency had ever done so.
The mechanism is worth noting because it recurs. Nobody was selling asbestos. They were selling insulation, and the asbestos was in the rock the insulation came from, and nobody had looked.
Identity checked against IMA-CNMNC master list, June 2025.