Chain Silicates, Amphibole Group
Anthophyllite (Mg,Fe)7 [Si8O22](OH)2

Anthophyllite is the magnesian end-member of the orthorhombic anthophyllite-gedrite series. The R.I. of the series varies between 1.623 and 1.676, with the lower values being typical of anthophyllite. Some of the colored varieties show faint pleochroism. Most anthophyllite crystals are prismatic or acicular. However, truly fibrous (asbestiform) anthophyllite has been mined for centuries, notably in Finland and parts of the U.S. and Canada. This type of asbestos is rare in building materials. Fibers of anthophyllite appear to be extremely flat and thin. The characteristic shape resembles that of a knife blade, coming to a point at one end. Fibers from a particular sample tend to be relatively uniform in size. The parallel extinction of the mineral distinguishes it from tremolite or actinolite.