Mimetite

Greetings everyone; this month we will examine the mineral Mimetite. Mimetite is a secondary mineral found in the oxidation zones of primary lead deposits where the elements Lead and Arsenic occur together. Chemically Mimetite is a lead chloroarsenate with a chemical formula of: Pb5(AsO4)3Cl and is the arsenate analogue of Pyromorphite and Vanadinite. In fact Mimetite is also isostructural with Promorphite and Vanadinite and forms a chemical solid solution series with Pyromorphite on one end Mimetite in the middle and Vanadinite on the other end. This chemical solid solution series go this one is a little different. Instead of substituting cations (typically positively charged metal elements) such as calcium for magnesium as is done in most mineral chemical solid solution series, the Pyromorphite, Mimetite, Vanadinite series substitutes the anion groups (Negatively charged groups of atoms) Phosphate (PO4), Arsenate (AsO4) and Vanadate (VO4). The term Isostructural means that the three minerals all share the same crystal structure and crystal shape. This is why Mimetite is sometimes confused with Pyromorphite. In fact, the name Mimetite is from the Greek”Mimethes” (Which means “imitator) because of its resemblance to the mineral Pyromorphite. It is interesting to note the mineral Endlichite is chemically intermediate between Mimetite and Vanadinite. Color of the mineral Mimetite is quite variable ranging from pale yellow to yellow-orange to orange red, brownish, greenish, white and colorless. One of the reasons this mineral is a collector favorite is because of its color and crystal morphology variability. From a handling point of view, care must be taken when handling specimens of mimetite as it is quite soft with a Mohs hardness of 3.5 to 4 (Roughly the same as the mineral Calcite) and can be easily damaged. Mimetite crystallizes in the Hexagonal system, Dipyramidal class with a symmetry of 6/m. Typically Mimetite occurs as small hexagonal prisms. Occasionally it occurs as diverging groups parallel acicular crystals giving the appearance of a sheaf of wheat. I have seen some Mimetite as rounded nodules associated with Wulfenite. I have seen some Mimetite at mineral shows labeled as “yellow Vanadinite.” Notable mineral occurrences for Mimetite are: Mapimi, Durango, Mexico; The lead mines of Arizona and Tsumeb, Africa. Be prepared to pay a princely sum for the Mapimi and tsumeb material. Shown below are some photomicrographs of Mimetite





References:
www.webmineral.com/data/Mimetite.shtml
www.mindat.org/min-2714.html
www.galleries.com/minerals/phosphat/mimetite/mimetite.htm
Mason, B and Berry, L.G.; Elements of Mineralogy, 1968, Freeman and Co., San Francisco, Page 390 -391.
Dana, E.S and Ford, W.E.; A Textbook of Mineralogy, 1951, John Wiley and Sons, New York; Page 708.