Phlogopite is a rarer member of the mica group and is not well known even by mineral collectors. It has been mined however for its heat and electrical insulating properties which are considered superior to other micas. The typical light brown color of phlogopite is characteristic although it is difficult to distinguish brown biotite from dark brown phlogopite. The two are actually end members in a series that is dependent on the percentage of iron. Phlogopite is iron poor and biotite is iron rich. The darker color and density increase with an increase in the iron content. Biotite tends to form in a wider range of conditions than phlogopite which is limited mostly to ultramafic rocks and magnesium rich marbles and pegmatites.
Phlogopite, like other micas, has a layered structure of magnesium aluminum silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage. Phlogopite is rarely considered a valuable mineral specimen, but well formed crystals are rare and some are now on the market showing nice crystals. These come from the Kola Pennisula area of Russia. Single large plates or "books" of phlogopite can grow to considerable size.