Actinolite: Ca2Mg4.5Fe2+0.5Si8O22(OH)2 - Ca2Mg2.5Fe2+2.5Si8O22(OH)2
Sources: Actinolite was seldom mined commercially. It is typically found as a contaminant such as in the Vermiculite mines of Libby, MT and the talc mines of Norway and Sweden.
Actinolite is part of a complete solid solution series from Tremolite to Ferroactinolite. All members differ by their relative amounts of Magnesium and Iron. Tremolite is at one end of the scale with a higher Mg to Iron ratio. When the total atomic Fe content to total Fe + Mg, exceeds 10% the species is no longer called Tremolite and is now Actinolite. As the iron content increases to > 50% the mineral is now classified as Ferroactinolite. In this solid solution series only Tremolite and Actinolite are regulated.
Actinolite is monoclinic and can exhibit inclined extinction by PLM. Actinolite can exhibit some poleochroism, the strength of which is related to its iron content. Actinolite is pale to dark green in plane polarized light. Random orientation SAED of actinolite shows a layerline spacing of 5.3 Å with twinning being uncommon. Actinolite is a member of the amphibole double chain silicate group.
Uncommon in building materials except as a contaminant, contaminant in talc and some vermiculites, present in some soils as is the case in California.