serpentine - hand specimen from the Melones fault zone
Serpentine is a mineral formed by the metamorphism and igneous activity associated with subduction zones, where an oceanic plate is diving below a continent. It is derived from the metamorphism of magnesium-rich peridotite from the mantle. It's the state "rock" of California. Its chemical formula is Mg6(Si4O10)(OH)8
Serpentine's structure is sheets of silica tetrahedrons sandwiching layers of magnesium hydroxide. The magnesium hydroxide layers imperfectly stack with the silica tetrahedron sheets, causing bending of the layers. In most serpentines, these layers form convoluted sheets, well represented in these examples. When the sheets bend into tubes, fibers of asbestos are formed.
These textbook specimens were collected from the Melones fault zone in the American River Canyon, Placer County, California, right in the Forty-Niner gold belt. They are glossy dark green and say "squeezed" to a student.
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