Mineral Species: Phlogopite

Chemical Formula: KMg3AlSi3O10(OH)2

Chemical Name: Potassium Magnesium Aluminum Silicate Hydroxide

Mineral Class: Phyllosilcate

Mineral Notes: Phlogopite is one of the rarer members of the Mica group. It has been mined for its heat and electrical insulating properties which are considered superior to other micas. The typical light brown colour of Phlogopite is characteristic although it is sometimes difficult to distinguish from Biotite. 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 colour 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 aluminium silicate sheets weakly bonded together potassium ions. These potassium ion layers produce the perfect cleavage. Single large plates or "books" of phlogopite can grow to considerable size.  

Description: Elongated, flattened sheets of Phlogopite laying on a pale-grey matrix. The mineral has the characteristic light-brown colour and a very strong glassy luster. The contrast agansit the matric makes this an attractive display specimen.

Location: Vezna, Czech Republic