Lizardite is a serpentine group mineral of the phyllosilicate class of silicates. It is a 1:1 trioctahedral phyllosilicate (clay) mineral. The tetrahedral sheet is mainly dominated by Si and the octahedral sheet is dominated by Mg in the lizardite end member. The structure of serpentines is similar to that of kaolinite, except that the octahedral sheet is occupied by octahedraly coordinated Mg and all octahedral sites are filled.
Some substitutions of Al for Si in the tetrahedral sheet and for Mg in the octahedral sheet can occur, making the mineralogical properties more complex than those of the kaolin minerals. Typically the charges are balanced within the layer structure. Lizardite has identical composition to antigorite and chrysotile, two other serpentine group minerals. However, they have different structures, making them polymorphs. Lizardite occurs in flat sheets, Antigorite has a wavy structure due to inversion of the tetrahedal sheets at regular intervals, and chrysotile has a tubular, asbestiform shape due to rolling of the layers. Differences are attributed to the degree of misfit between the lateral dimensions of the Mg-octahedral and the Si-tetrahedral sheets.
Serpentines are particularly found in ultrabasic parent rocks such as dunites, pyroxenites and peridotites, and in some basic rocks that have been affected by Mg-rich hydrothermal fluids. Serpentinite rocks are common in low temperature, high pressure metamorphic regions, typically found in subduction zones. Serpentinite rocks are common in California, Oregan and around the Pacific Rim.
Serpentines are found in soils in very limited regions and are inherited from the parent materials. They are relatively unstable under soil conditions, particularly in more humid regions, and commonly weather to form trioctahedral smectites, notably saponite. Soils derived from serpentine rocks may have elevated concentrations of Cr, Ni, Co and Mg that may cause toxicity problems to plants. The high Mg to Ca ratios are also problematic for most plants; consequently, serpentine rich areas often support a flora that has evolved characteristics that allow it to survive in this environment.
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