Hendricksite

It mostly consists of oxygen (38.92%), zinc (18.56%), silicon (15.37%) and manganese (10.02%), but otherwise contains potassium (7.13%), aluminium (7.11%), magnesium (2.46%) and a negligible amount of hydrogen (0.41%). It has a barely detectable, 1% radioactivity measured by GRapi (Gamma Ray American Petroleum Institute Units) due to its potassium content, which gives the mineral its radioactive attributes.^[4] Zinc in hendricksite's formula can be replaced with manganese(II) or magnesium. Currently, this is the only mica known for its zinc dominancy, and it is the zinc analogue of phlogopite, annite, and shirozulite. It can have calcium, iron, titanium, barium, lithium, natrium and fluorine impurities. It has three known polytypes, which include 1M, 2M₁, and 3A.^[2] Hendricksite can be included in a solid solution series with zincohendricksite and manganoanhendricksite being the endmembers, a solid solution series meaning the three sharing a general formula but having a substitution of elements in at least one of the atomic sites.^[3] A part of the solid solutions series might be magnesium bearing hendricksite, the series perhaps being complete to phlogopite and partially to biotite.^[5] In the case of trioctahedral micas, the ellipsoids of the cationic sites have an uniaxial positive optical property, elongated to c. However, in hendricksite's case this is only typical for the two zinc-free sides. In the octahedras containing zinc, the ellipsoids are uniaxial negative and flattened to a.^[6]

It can appear only in metamorphosed stratiform zinc deposits, in irregular lens or sheet like skarn bodies. It can be found in the Franklin mine in New Jersey, US.^[2] The mineral can be found at the Sterling hill as well, although it is much rarer due to the higher iron and magnesium concentrations. It occurs with vesuvianite, bustamite and feldspars,^[5] additional associated minerals being minerals of the axinite group, calcite, rhodonite, willemite, hancockite, as well as andrasite.^[2]