Hand Specimen identification: Phlogopite, an end member biotite, is identified by cleavage, color, and association. If not brown, it is indistinguishable from other biotites without additional analytical data.
Chemical Composition : KMg3(AlSi 3O10)(OH)2
Hardness : 21/2 to 3
Cleavage/Fracture : perfect basal (001)/elastic
Luster/Transparency : pearly/transparent
Color : yellow-brown
Streak : white
Optical Properties : Phlogopite is similar to other micas in thin section. It has a smaller 2V than muscovite and is often col-ored light brown. It may be pleochroic, but not as much as the Fe-rich biotites. Biaxial 1- 2, a = 1.56, b = 1.60, g = 1.60, d = 0.04, 2V = 0° to 20°.
Crystallography : Monoclinic, a = 5.31, b = 9.19, c = 10.15, b = 95.18°, Z = 2; space group C2/m; point group 2/m.
Habit : Coarse books of pseudohexagonal crystals are distinctive but uncommon. More frequently, phlogopite is disseminated as irregular grains or flakes or foliated masses.
Structure and composition : The basic phlogopite structure is similar to that of talc and pyrophyllite: Two tetrahedral SiO layers surround an octahedral Mg1O,OH26 layer. 4 Unlike talc and hyllite, however, the three-layer sandwiches are linked by K+ ions occupying large sites between them. Fe often substitutes for Mg, leading
to complete solid solution with annite, KFe 1AlSi O 21 2 3 3 10 OH . Al substitutes for both Mg and Si, creating
solid 2solutions with siderophilite, K1Fe,Al2 1Si,Al2
1 2 3 4 O OH . Limited solid solutions with muscovite,
10 1 2 21 2
KAl2 AlSi3O10 OH 2, are also possible. Mn, Ti, and a number of alkalis and alkaline earths may also be present, and F may replace some OH
Occurrence and associations : Phlogopite is a common mineral in marbles where it associates with calcite, dolomite, quartz, diopside, and tremolite; less commonly, it is found in highly magnesium-rich igneous rocks.
Related Minerals : Phlogopite has several different polymorphs, which are difficult to tell apart without detailed X-ray studies. It is isostructural with muscovite, KAl 1AlSi O 21 2 2 10
OH 2, and isotypical with other micas, forming complete or limited solutions with most.