Description:
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. |
