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Picture Gallery of Amphibole Minerals


Actinolite hand specimen from Ward Creek, Sonoma County, California. (more below)
A classic locality
Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)
Actinolite is one of the amphibole group of minerals. Because its molecular structure is made up of long double chains of silicon atoms, actinolite forms long, thin crystals, sometimes even fibers. (Thus actinolite is sometimes classified as asbestos.) Its green color reflects a high iron content—the white variety without iron is called tremolite. A microcrystalline variety of actinolite is called nephrite, a major jade mineral.

The general formula of actinolite is Ca2(Mg,Fe)5(OH)2[Si8O22]. The silica structure signified by Si8O22 is a double row of connected SiO4 tetrahedra, around which the other atoms are arranged.

Actinolite forms from the olivine in many igneous rocks under conditions of high pressure and fairly low temperature. This specimen is from northern California, where actinolite forms many bodies of greenschist.

Other picture galleries:
Geologic Features and Processes
Glaciers and Ice
Geology and Society

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