Annite
Annite is a relatively rare iron-rich endmember of the phlogopite-annite series within the mica group of phyllosilicate minerals. It is chemically defined as KFe2+3AlSi3O10(OH)2. Annite is often found in iron-rich igneous rocks, particularly those that are alkaline, as well as in metamorphic rocks that have undergone iron-rich alteration.
Properties: Annite is typically black or dark green in color and exhibits a perfect basal cleavage, characteristic of micas, which allows it to be easily separated into thin, flexible sheets. It has a vitreous to pearly luster and a Mohs hardness of 2.5-3, meaning it can be scratched by a steel knife.
Occurrence: Annite is found in a variety of geological settings. It is commonly associated with granites, syenites, and other alkaline igneous rocks. It also occurs in contact metamorphic zones and hydrothermal veins. Notable occurrences include localities in the Ilímaussaq intrusive complex in Greenland, and various locations within the United States, such as Colorado and New York.
Distinguishing Features: Annite can be difficult to distinguish from other dark-colored micas without chemical analysis. However, its strong iron content often leads to increased magnetic susceptibility. Its association with alkaline igneous rocks can also provide a clue to its identification.
Health and Safety: Like other micas, annite is generally considered non-toxic and safe to handle. However, it is always advisable to wash hands after handling any mineral specimen.