Chain Silicates, Amphibole Group
Tremolite-Actinolite Ca₂(Mg,Fe)₅ [Si₈O₂₂](OH)₂

Members of the tremolite-actinolite series are among the most common rock-forming minerals. They are frequently found as cleavage fragments in cements, plasters, roofing, and other building materials. Tremolite and actinolite may occur as contaminants in fillers, or as sand and gravel components of plasters or roofing materials. Tremolite and actinolite may also be associated with vermiculite, in vermiculite insulation samples. Asbestiform tremolite and actinolite are relatively uncommon in building materials, but may be found in significant quantities in rock and soils in some areas (e.g. El Dorado County, CA.)

The division between tremolite and actinolite is defined by the relative amounts of magnesium and iron. Tremolite is at the magnesian end of the series and is white in the hand sample; manganiferous varieties are pink or pale violet. Actinolite (10-50% Fe/[Fe + Mg]) is higher in iron content and is pale green to dark green in the hand sample. Ferroactinolite (iron-rich end member of the series) is technically not regulated as asbestos.

Because of the solid solution between the tremolite-actinolite and hornblende series, intermediate compositions are possible. Some crystals formed during metamorphism may, for example, have hornblende cores and actinolite rims. The morphology of tremolite and actinolite ranges from prismatic or bladelike to acicular (needlelike). The blades are often pointed at both ends. Bundles commonly exhibit a radial arrangement.