Antigorite

Lamellated antigorite occurs in tough, pleated masses. It is usually dark green in color, but may also be yellowish, gray, brown or black. It has a Mohs scale hardness of 3.5–4 and its lustre is vitreous to greasy. Antigorite has a specific gravity of 2.5-2.6. The monoclinic crystals show micaceous cleavage, a distinguished property of phylosilicates, and fuse with difficulty. Serpentinite rocks that consist of mostly antigorite are commonly mylonites. The antigorite grains that make up these rocks are very fine (on the order of 1 to 10 microns) and are fibrous, which defines a texture in the rock caused by lattice preferred orientation.

The magnesian serpentines (antigorite, lizardite, chrysotile) are trioctahedral hydrous phyllosilictes. Their structure is based on 1:1 octahedral-tetrahedral layer structures. Antigorite is monoclinic in the space group Pm. Although the magensian serpentines have similar compositions, they have significantly different crystallographic structures, which are dependent on how the SiO4 tetrahedra sheets fit in with the octahedral sheets. Antigorite's basic composition has a smaller ratio of octahedral to tetrahedral cations (relative to lizardite and chrysotile), allowing the structure to compensate for the misfit of sheets through periodic flipping of the curved tetrahedra layers, and subsequently their polarity. Polysomes of antigorite are defined by the number of individual tetrahedra (denoted as the value m) which span a wavelength of the direction of curvature. The sheets of tetrahedra allow the platy, fibrous crystals to separate parallel to the 001 (basal) plane, giving antigorite its perfect cleavage.

There are two varieties of antigorite, Bowenite and Williamsite. Bowenite is the state emblem of Rhode Island, and features strongly in US history, being given to Mrs. Lady Bird Johnson, the First Lady in 1967, in the form of a brooch, famously called the "Our Mineral Heritage Brooch." It was also thought to have been used in historical carvings.